ABS Problem MG ZT-T
 

Hi -- I have an MG ZT-T CDti Diesel. Developed an ABS brake pedal judder problem recently and struggling to clear it.
ABS light came on 2 weeks ago and speedo failed. Unplugged ABS fuse and cleaned it and reinstalled it - worked OK, then did it again. Same procedure cleared it.
Since then the ABS light has stayed off but experienced brake pedal judder on braking, as the ABS is coming in erroneously. I confirmed it was an ABS problem by removing fuse again and braking went back to normal, no judder.

Replaced sensor on front right, thinking this would be the cause of an intermittent fault causing the speedo to go off in the first place.
Still had judder at various braking speeds, so took it to garage to be tested. £40 lighter, there was no definite conclusion. Static test showed fault on front right and one on rear right. Cleared faults and took it for a test. Retested it and faults didn't show. Also tried a wheel by wheel test rotating the wheels and recording the variation in voltage - no abnormalities found, they all showed voltages that rose and fell pretty much the same.

Next step - took off rear hubs (that have integrated ABS reluctor rings), cleaned up the rusty back plates (told these could cause the problem), cleaned stub axles and the hubs. Reassembled and it was much improved, ABS not kicking in on general driving, however, it still kicks in at very low speed, almost a few miles per hour.

I noticed on the rear Hubs, one hub had what looked like a brass ring on the outside with oval holes cut out at intervals - I assumed this was the reluctor ring, but there wasn't one on the rear nearside hub.

Does this make a difference, i.e. is this brass ring the actual magnetic reluctor ring or is it actually embedded into the hub? As it's been working fine since hubs were last changed in October 2014, I'm at a loss to know what to do next and what could be causing this to judder at low speeds, and yet no ABS light is coming on and no faults recorded.

I guess the ABS is working OK but the signals going back to the ABS control unit are such that it thinks the wheel is locking, but only at low speeds.

Any help welcome.

Hi Heddy, is the mag ring visible on the outside of the back of the hub when you take it off, or is it embedded inside the hub and you can't see it?

I'm still confused as to why the rear right hub has a brass ring visible on the outside with regular oval cut outs that coincide with position of the sensor on the back plate, yet the hub on the rear near side doesn't have this ring at all. The system has been working for 2 years with both these hubs on until now, so completely confused as to what's what.

LR

Depending on the manufacturer , some hubs have the cover ring you describe and some don't . Not a problem ; I'm running one of each kind :}

Thanks Chris, that's one off the list -- any idea what could still be causing the ABS to kick in at low speed. I also took the battery and casing out to check the ABS computer plug was dry. I still couldn't get to it but it was as dry as a bone around that area, and the plenum's are dry. Also I guess if there was an electrical fault, this should trigger the ABS warning light. Since mine is not lighting up, I'm still looking toward a reluctor ring problem. I'm hoping it's not the front bearings as they are not fun to take off and replace -- the rear hubs are a doddle in comparison.

Reading that, suggests you are misunderstanding how to carry out the test...

It is NOT just a matter of checking the voltages, but of marking the tyre where the voltage changes. A light coloured crayon, with a brick or similar as a fixed marker works well. That ensures that a full continuous set of changes of voltages/ pulses appear around the circumference of the wheel/tyre. You may need to repeat it several times to be sure the pulses appear without fail.

The judder happens when pulses signals fail to appear, when they are expected. Most likely they will be missed at lower speeds, hence the low speed judder you are experiencing.

T4 and other diagnostics will not see it as a fault, because it is doing more or less what it is supposed to be doing - generating mostly regular pulses.

When I had a similar situation I bought a cheapish rear hub to use as a test piece . If I had to bet I would say that one of your rear hub reluctors is duff :shrug: Try it on both rears in turn and see if it fixes the problem , and if it does then leave it on :} aapoldham on ebay do a decent one for a good price .http://stores.ebay.co.uk/aapoldham?_...p2047675.l2563

This sounds very familiar to me as it follows my own experiences , and is good advice , methinks:}

Follow Harry's advice below and make sure you rotate the wheel slowly to check each magnetic reluctor segment.

Have done this a few times now and it is a very good way to test all the ABS sensors and magnetic reluctor rings. :D

Chris, Thanks -- sounds like good advice, I'm going to go ahead and order one today and try swapping each of the rears out see what happens.

Regards Larry

Thanks Harry -- I relied on the guys at the garage to run the test. I don't recall any chalk marking. This was the first time I'd witnessed this kind of test, so wasn't aware of the full procedure, despite having read up the basics of how the ABS system works.

I have a low cost cable, and engine test software, but nothing that tests the ABS. I've run the test at the garage twice so far, so will avoid that since I doubt the guys understand what the effect of placing chalk marks on the tyres will do. They did rotate each wheel individually by hand, as well as trying a drive test with the test rig plugged in and monitoring the readings.
The only real way to measure this I suppose would be on an oscilloscope and look at the uniformity of signal sent back to the ABS control unit.

I'll have a go at a new hub swap, as I think the absence of a scope I'm not going to get anywhere -- and it's cheaper than another inconclusive ABS test.
I did notice on the rear nearside hub, that a lot of the green gel had leaked, so I cleaned it up before reinstalling it - perhaps this was a factor in the reluctor ring deteriorating. It still puzzles me why one has a visible brass perforated ring on the outside and the other doesn't.

Regards Larry -- I'd happily do without the ABS (it's a bit of a pain), as I don't drive like a lunatic and don't use the car to commute -- I won't be selling though as I've had this since new, even if I get another car.

Best to get hold of a multimeter and carry out the checks before changing anything else.

It is easy to add faults to the ABS system when replacing parts without a correct initial diagnosis.

On my car the fault was a slightly too large air gap between one of the rear sensors and the magnetic reluctor - both aftermarket parts. Checked the mounting was rust free and clean and then filed 1.5mm off the back of the bracket to move the sensor a bit nearer and it worked perfectly.

I agree, any attempt to fix things by substitution has the risk of adding even more faults and can work out to be expensive. Do a proper diagnosis first and save yourself a lot of time and expense.

It is not easy to mark the tyre wall whilst watching a meter, so I devised a better method. The better method uses a piezo earphone connected to the output terminal of the sensor. That produces a click in the earphone at each segment. A piezo earphone has to be used, because they present a tiny load on the output - they are very high impedance, but they are not so common/easy to source as the low impedance earphones. They produce a sharp and very precise click at each segment.

I have not tried it, but I suspect the click might be able to be picked up if a bit of wire is connected to the sensor output pin and a medium wave radio tuned to a quiet point on the dial placed near the wire.

Mike, Harry, -- I have a multimeter, will give it a go, bit it's nigh on impossible to connect to the tiny terminals pins inside the connector either on the sensor side or up to the ABS circuit. It's the output from the sensor we are aiming to look at here right?

Only option is to cut into the sheathing on the leads to get a contact and I'm a tad reluctant to do that.
I'm curious as to why changing a part would introduce an new ABS fault. At the moment I don't have any reported faults and a new part wouldn't do any damage to the system would it?

Mike -- how did the sensor move further away from the reluctor ring which is fixed in the hub? - The sensors are up tight against the hub backplate and held in with an 8mm bolt. Unless you had a bang on the backplate, buckling it away from the hub somehow? I'm going to check both these distances on both rear backplates to see if there is any variance, but both looked normal on the last inspection.

You can use a (drawing) pin to go into the wire and connect the meter to that (french mikes trick).

With the help of folk on here I was able to get hold of two spare blue plugs and make up a loom which I could plug into the sensor circuit . It had two extra wires to provide an output to my multimeter , so no wires were harmed during the test :}
For the sake of £28 , I still support getting a new rear hub for a substitution test ! The world would be boring if we all agreed on everything ......

You guys are great - well ahead of the garage. I ordered the hub this morning, for the sake of £25 I'll change the back rear one anyhow, given that it had green lubricant leaking when I took it off.

Jakg, thanks for the drawing pin, it made me laugh, but it ain't such a bad idea. I'm going to make up a rig with spare plug -- I have the old one from when I took it off the from right wheel. Chris I'll make this up and give it a go on the wheel test.

That way I'll have it covered from both angles.

I'll post back when I've done -- cheers -- have a great weekend.

Games -- I couldn't possibly sell the old girl now after all this hassle, who knows it might be a future classic -- well you can dream can't ya!!

I also made up a cable a few years ago from an old ABS plug and socket, some speaker wire and a bit of chocblock. The original idea was to use it as an extension cable so that any ABS sensor could be plugged into the OSF and the speedometer used as a diagnostic.

Here it is being used to check a hub before fitting to the car:

http://i1108.photobucket.com/albums/...ps40be8806.jpg


Very easy to damage the magnetic reluctor rings if drilling out old ABS sensors, or get a fault as I did when I replaced first the bearing due to it wearing out, and then a couple of years later, the sensor when it packed up, with non original parts.

There was no damage to the hub or backplate and no rust build up around the mounting, so likely the combination of the two aftermarket items needed a slightly smaller air gap to function correctly together.

As French Mike says: Measuring is knowing. ;) :D

Status so far
 

OK - quick update.
Ran the tests on the back two wheels, by adopting the drawing pin in the leads to the sensors. Steadily made simple marks at each progressive change in the signal voltage with my multimeter by placing bluetack on the wheel. Also monitored the changes in the voltages whilst spinning the wheel slowly and a little quicker and the changes +ve and -ve of the voltage were pretty uniform. Also tested that there was a voltage present on the other lead to the sensor from the ABS control unit.
So sadly it doesn't look like a back wheel problem. my new hub still sits in it's brand new box.

I then tested the front right wheel that I had already replaced the sensor in. This was a different story -- again used drawing pins in the leads to get a connection for my meter.
This was the wheel I suspected originally as the speedo went a couple of times.
On this wheel there were large areas (about 1/6 of the wheel circumference) where the voltage did not change at all, which leads me to believe there may be a reluctor damage in this wheel bearing. They are supposed to be fitted for the life of the car according to the Haynes Manual.

The problem still only resides at very low speeds, a few miles per hour, and not on general braking and I still have no warning lights.

I tried the rock test to see if there was play in the bearing (I think there is a little but I may be imagining it) and there is no rumbling grinding sound on the bearing when I rotate the wheel freely.

Before I think about the nasty job of replacing the front wheel bearing, can anyone offer a view as to whether I might be barking up the wrong tree here?

I will assume you meant - absolutely uniform, rather than pretty uniform?

There should be no absent pulses at all..

It is obvious your problem is with the front off side hub or sensor, but before swapping the hub, I would try moving the sensor a bit deeper/ nearer the reluctor to be sure it is not just the distance causing the issue....

File the lug, so the sensor is a fraction nearer.

Harry, Thanks, I'll give it a try. One thing that does concern me is that when I took out the old sensor I was forced to drill it, which was a long job. I was careful not to drill too deep for fear of taking the drill through to the hub. Unfortunately, a piece of the old sensor fell through the gap. I'm hoping this will not affect the operation.

Larry

Ok car won't start now at all. Just took it out of the garage this morning and it started fine. early evening, nothing. It turns over (like a clicking sound) but won't fire up.
Tried jump start, admittedly from a small Fiat Panda -- possibly battery is too weak to make a difference.

Any thoughts on where to start. I did notice early a whirring sound when turning on the ignition to test the ABS sensors with my meter, but it just sounded like the lifter fuel pump trying to activate.

Any idea where to start looking?

T4 will see ABS problems as a fault, and anything causing low speed ABS actuation will be logged.

I had cause to sort out exactly the same type of fault on a V8 recently (I know I know) and T4 reported incorrect toothed wheel fitted to one or more wheels.......the cause, well how the owner chortled after he had replaced both front wheel bearings, both front discs and pads, and both rear hubs in an attempt to cure this issue........I said to him have you had any tyres fitted recently? yes came the reply, why's that?

On inspection there was a different profile tyre on one corner :getmecoat:

It's not always about what information is presented to you, it's how you interpret that information ;)

Brian :D

Yep, stick your battery on charge ;)

Were the headlights flashing when you attempted to start the car? this is the fabulous way these cars tell you the battery is discharged :)

If not, have a look for starter motor solenoid contact replacement ;)

Brian :D

Hi Brian.
I had the same problem pulsing no light but T4 still gave a reading and indicated rear right plausible, turned out to be the bearing and a very nasty back plate corroded, which was cleaned for the time being :o this still as to be rectified when I can sort out the time.

It also showed other possibilities as you can see from the pics below. ;)
Arctic

PS Operator I am not yet, as I am still in training :D

Marina, Arctic,

Duuurr -- It was the battery, I'd left the ignition on too long when running the ABS sensor tests. Charged and OK now.

Thanks for the responses on the tyre option. I just had all 4 tyres changed, all the same profile, all Firestone with the same pattern, so I'm quietly confident that one hasn't caught me out.
The front right sensor I changed as it was the speedo that went first, but miraculously came back on.

When I ran the tests with my meter on the two rear sensors (after taking the back seat out - stupid to hide the sensors under the seat but there you go), they both registered very uniform changes in the voltage on the signal wire when rotating the wheel slowly and marking each change in the voltage - all were equidistant and no breaks or missed changes in the voltage.
I also had the back hubs off and cleaned everything up including the back plates and the stubs and around the bearings.

The test of the front right was markedly different with no responses to voltage on about 1/6th the circumference of the wheel. This seems consistent with the original fault (speedo dying and ABS warning light on).

I'm going to double check the length of the new sensor as suggested by Harry, but I am coming to the conclusion it is the front right wheel bearing that needs changing.

Anyone got a youtube link for this on an MG ZT-T?
The bearing, with integrated reluctor magnetic ring is about £40, but I don't have a press to get the old one out of the hub or the new one in and I don't want to bodge it with bolts and nuts as indicated for fear of damaging the part.

Regards Larry

No further forward with ABS problem
 

This is driving me crazy, so much so I'm thinking of junking my MG ZT-T due to what appears to be some kind of design fault with the braking system.
Why?
Well I've checked what seems to be everything and I'm still getting the judder on the brake pedal at low speeds.
I've checked the voltages on the back hubs, and cleaned up the back plates to make sure nothing is blocking the senors.
I've replaced the front offside sensor, as it was the speedo that went initially. I did a voltage check on all the wheels and the voltage changed equidistantly on all wheels except the front offside which had no response on about a 6th of the wheel circumference so I thought that's it, change the wheel bearing. Did that, same problem.
Put new front discs and pads on at the recommendation of the garage, as no ABS faults were showing on the test after the old one's were cleared and I test drove it braking 20 times with judder.

So now I have no ABS light, and no ABS faults, yet still I'm getting the judder after replacing a sensor, a bearing and front discs and pads.

Where does one go from here?

HI Larry.
You really need to get this on a T4 in my opinion to move forward, whom is closest to you ( larryr123 ) ? where is that place.

A few weeks back a nano member had the same trouble, he changed the sensor after drilling it out, all was ok passed the initial test but he still had pulsing at the brake at low speeds.

It lasted for a day and the ABS then reverted back to no speedo etc, we decide to remove the hub and replace it with one we had spare and hey presto all was ok, what we did notice was the debris left behind from drilling may have damaged the magnectic ring

So therefore you replacing a new wheel bearing it should have been ok, is the speedo working ok, if so then it could be one of the other bearings ie the rear OS etc

Remember it is the side of the senor which reads the ring in the bearing not the tip of the senor on the front, the rear sensors use the tip of the senor.


Note the magnet ring missing
http://i66.tinypic.com/28rfv5t.jpg1

http://i66.tinypic.com/iwua7n.jpg2


You can see the black magnet ring on these below,
http://i67.tinypic.com/a0ky1v.jpg3

http://i68.tinypic.com/wtxi7q.jpg4

You can see now how if you drill out the sensor how easy it is to damage the ring, when dealing with the front ABS I would remove the diss etc to make sure all is clean, do this even if you are only changing the sensor.

It is best if you have to change the front sensor that you try and remove with out drilling first, if you do need to drill it out snap the sensor off flush once the bolt as been removed, drill down the centre about 12mm with a 3mm drill then screw in a wood screw with a flat head & domed top, you can then prise out the section of the sensor using a claw hammer or pry bar with a fork cut into it, but this will only do you any good if it is the senor in the first place.

To be honest most of these sensors are robust and most of the time it is the magnetic rings that fail, or pic up rust particles etc

As I said above at the beginning T4 will pinpoint your problem for sure.

Arctic thanks for the input
 

Arctic thanks for the description and the graphics.
Originally the speedo went and the ABS light came on, that's were it all started.
I had T4 run at a garage, which pinpoints faults to each wheel but doesn't tell you what the problem is.
I diligently tested the changes in the voltage with my voltmeter and pin connections in each sensor lead and the break in continuity was on the front offside.
I had already drilled out the sensor and replaced it. I then replaced the wheel bearing making sure to get the magnetic ring in the right way round as per your photos.

I have a working speedo and all old ABS faults were cleared. I tested it with 20 or so brakes (with judder) on a run and plugged in the T4 again and no faults came up.

Judder is still there.
I changed the rear hubs at end of 2014, and they all had equidistant markers on the wheels as I tested the voltage readings. I also cleaned up the back plates to make sure there was no blockages between the rear sensor tips and the rear hubs, and cleaned around the hubs generally.
Also put new discs and pads on the front so it's not a disc deformity issue.

Mystery -- done everything I can think of.

Larry

Where am I
 

"whom is closest to you ( larryr123 )"

Arctic, I'm in Bedford.

Regards Larry

Did you mark the tyre, where the pulses from the sensor occurred?

That is crucial, the whole point of testing it with a meter!

Test for the pulses, mark the tyre and make sure you get the pulses without any pulses being missed for several revolutions. The marks should be evenly spaced around the tyre. If even one pulse is missed out when on the road, it will trigger your ABS and T4 cannot detect it.

This is the first test which should have been done when the judder was noticed and better still - it costs nothing at all apart from a bit of time.

In your case, it sounds as if there might be an issue with the sensor, either its sensitivity or its positioning.

Marked tyre
 

Hi Harry, Thanks -- Yes I marked the tyre with blue tac at every change in the voltage for each wheel. The rear wheel sensors are stupidly hidden under the back seats, so accessing them is pain.
All the pulses were uniform and equidistant on both rear wheels, and on the passenger front wheel. There was a big break in the drivers side wheel and this is the wheel I have changed the bearing on and installed a new sensor.

Unfortunately, this, or the change of the front discs and pads have not solved the problem.
Just for surety, I also installed a new rear passenger hub this afternoon, as there had been an old reported fault on that wheel and I noticed some of the green grease fluid had leaked from the hub.
So this has been changed, all the back plate re-cleaned and surface of the sensor is clean and clear to interact with the new reluctor ring in the new hub.

The car also has 4 new Firestone tyres with correct pressures.

It seems strange that there are no proper automated electronic tests that can properly detect the problem. Two garages I've taken it to seemed to understand very little about ABS and the workings despite one being a MG Rover specialist.

Larry

"""In your case, it sounds as if there might be an issue with the sensor, either its sensitivity or its positioning."""

Harry - The sensor was installed new on the front drivers side. Some have commented about the distance between the sensor and the reluctor ring, and here you have two possibilities.
On the front there is no possibility to control the distance between the sensor and the embedded reluctor ring in the hub, since it's part of the wheel bearing and it takes the reading from the side of the sensor, not the tip, as in Arctic's photos.

For the rear wheels, the sensor tip is taking the reading and yes there is a possibility of the distances changing if the back plates get warped or bent. This does not appear to be the case on my two rear wheel back plates and I've cleaned them both up and made sure there is no obstructions at the end of the sensor. Both rear wheels were providing regular pulses and there are now no recorded faults in either rear wheel.

I don't have an ABS warning light and I don't have a brake light warning, there are no ABS faults after a road test, so the system things all is OK.

Is there another, non ABS, cause for judder on the brake pedal.
The sound it makes is like a springing noise.

Larry

The ABS cutting in at low speed is very distinctive. You should hear the noise and definitely feel it at the pedal. There is only one possible cause, if it is the ABS cutting in - that the ABS is missing pulses from a wheel when compared to the other three wheels, so it reacts.

The ABS will not show a fault, because it thinks everything is behaving perfectly, except it thinks a wheel is locking up under braking at low speed, so it does what it is designed to do.

There are only two failure modes which bring the ABS light on.
1. The static test at switch on where the ABS spots faulty wiring or a sensor.
2. As you pull away and a sensor fails to provide pulses.

Several have found exactly the same issues when replacing the sensors, I appreciate you cannot alter the air gap between the rotor and the sensor, but you can file down the bracket a bit so the sensor protrudes further through the hole.

My own assumption is that the Hall Effect sensor inside some of the after market units, may not be as deep inside the housing as the OEM sensors. Which explains why filing the brackets fixes them.

That though, should not be needed if you are certain you are seeing all the pulses without fail.

Just for completeness...

The Hall Effect sensors, trigger via a change in magnetic field. They should 'click over' if you bring a magnet near them.

Harry, I'm going to run the pulse test again on all 4 wheels to double check.

If that doesn't identify anything, it's up for sale :)

Is there also a potential for a connector problem, such as the one under the battery that some say can suffer from corrosion?
I guess any break in the circuitry due to wiring or contact contamination would trigger the ABS warning light on the dash?

Larry -- OK that's Saturday taken care of -- will report back.

That should generally cause the light to come on, it would be a complete loss of a sensor, rather than just a loss of pulses.

Stick at it, it will be something simple and when you solve it you can give yourself a gold star. Most garages struggle with ABS systems and really there is not that much to them if you study them.

OK back from a trip around Vietnam on a motorbike
 

3 weeks away and almost forgotten about my blessed ZT-T ABS problem.
Now that I'm back it's staring me in the face again.

I said on last post I'd try and recheck the pulses on each wheel with a rotation test and a multi-meter on the sensor leads.

Just to recap, I changed the rear left hub, the front right hub and the front right sensor. No warning lights on so it thinks ABS is working OK.

Started to test the front left wheel this time, thinking perhaps the reluctor ring might be as worn as the front right.
This time, after breaking into both sensor wires and connecting up the multimeter (and turning on the ignition) I'm getting no change in voltage at all when rotating the wheel?

Is this for real? Surely with no signal generated from the reluctor ring to the hall effect sensor -- the warning light would come on, no?

Or is it still thinking the ABS is working but not receiving ABS pulses?

Anyone want to buy a clean, good condition MG ZT-T - 2005, silver metallic with 129K on the clock and 12 months MOT?
Complete with a Krypton Factor puzzle of how to get rid of ABS low speed judder!!!!!

Make me an offer I'm seriously thinking of getting a Jag :)

Larryr123

Yes, the ABS would come on with no signal, so I would suspect your test procedure as being at fault, rather than the sensor.

What voltage value do you get on the sensor signal ?

Mike, Harry, thanks for responding.

Mike I agree it's unlikely the sensor is open circuit (failed altogether) as the ABS warning light would be on.
Harry -- I made sure I had exposed the two wires leading to the sensor, the ignition was switched on and the meter was set to 2.5 volts setting.
Note I did not disconnect the plug, but left it connected, just hacked into the existing wires -- same test as adopted previously.
Also very careful not to set the meter to meaure resistance a this can blow the circuitry in the Hall effect sensor leading to the magnetic detection element.

When the wheel was rotated nothing responded at all on the meter.

This is exactly the same test I adopted on the other wheels. I used Mike's drawing pin method on all the other sensors.

I just got back from the MOT station, having fixed a bunch of things and the car has just passed with no advisories -- so it's up to scratch with 124K on the clock, new brakes all round, new high pressure diesel pump, new belts and all and sundry.

Just this annoying ABS.

So annoyed with this, I'm thinking of sitting down and devising a new braking system that dispenses with the total time wasting ABS system. Nearly everyone I speak to has had problems and many have taken weeks and thousands of £'s to resolve.

Larry

Mike -- no voltage at all.

An healthy sensor well powered (around 12 volt) must give 0.6 / 1.6 volt
on the output signal unless it's dead :shrug:

Mike, if it was dead, wouldn't the ABS warning light be on?

Mike, is in not conclusive that if no ABS warning light appears, all of the sensors, ABS control unit etc are all functional and that the ABS judder is due to erroneous feedback via the sensors to the control unit?

In essence this implies that the system thinks everything is OK, and that it just keeps reading the wrong data, or non uniform wave forms from one or more of the wheels?

And when you disconnect one sensor ,does the warning light appear ?

If you get the same bad value on the four sensor ,there is something wrong
in your measurements :shrug:

The easy way to work on ABS sensor issues, is by making what is called a break-out unit/ box....

You get an ABS plug and socket from a scrap car, with a bit of wire attached, then join the wires back to back, with something like a choc block, to give you easy access to the wires.

I agree lots of money can be wasted trying to fix ABS faults, but usually it is needless. They just need to be understood and diagnosed properly.

The ABS system uses two diagnostic phases for faults....
1. At ignition switch on it, it puts on the ABS light checks out the ABS unit and the four sensors to confirm all is as it should be, then switches the ABS light off, to indicate no problems found - the static test.

2. When the car is driven, it ensures it is receiving proper pulse signals from all four sensors. Then it enables use of the ABS system, should it be needed - the dynamic test.

If the system fails either test, or contact with a sensor is lost on the road - the ABS warning light comes on.

I cannot therefore understand how your warning light is not appearing, if you know for certain one sensor is providing no signal.

Is the light definitely coming on briefly, when the ignition is turned on?

Does the light come on if a sensor is unplugged, with the ignition on?

It is not unknown for the ABS system to be faulty and for some unscrupulous person to have taped the light over. Connecting the light to another source, so it comes on and goes off, has also been known. This just to get it through an MOT, so a car can be sold.

MG John, Thanks for your reply, however, I don't have any problems with the ABS warning light coming on so my problem is not the same as yours I guess, even though you also had some low speed ABS brake judder.

Harry, going back to your point about the ABS warning light. I have had the car from new, so no one has tampered with light for MOT purposes.

I can't get hold of an old sensor (plug-socket) to build a test jig as suggested. I have tried this, no one has one. Even then, I would need the socket (or plug side) of the connector to connect the sensor into it.

I will check tomorrow (today) that the ABS warning light is coming on temporarily when I start up the car ignition. This will prove that the system is completing the static test OK.

I will also check (with the ignition on), that when I disconnect one of the sensor plugs, that the ABS warning light does come on.

After that I will then try a test on each of the wheels with the each sensor unplugged, starting with the front nearside, placing my voltage meter across the two wires connected only to the sensor, not back to the car.

I notice that you guys are testing the sensor in it's unplugged state to test the sensor voltage generated by the movement of the wheel over the hall effect sensor magnet. Previously I had kept the plugs connected.

Does this all make sense?

And going back to my earlier question -- If there is no warning light, is it safe to conclude that the only cause of low speed brake judder is a break in one of the reluctor rings or a gap too big between the sensor and one of the reluctor rings?

i.e. --- There are "NO" other causes of low speed brake judder when the warning light is "NOT" on?

Larry

I test mine with everything connected. As you can see from the photo of a hub being tested prior to fitting. The meter is connected to the output from the sensor and ground.

http://i1108.photobucket.com/albums/...ps40be8806.jpg


I haven't found any other cause. With my car it was a slightly too wide air gap. As everything was positioned correctly, maybe it was due to both sensor and bearing being aftermarket parts?

Either way, after around 1.5mm was filed off the sensor bracket (rear sensor) everything worked fine.

Interestingly, when tested, every magnetic segment was detected until the rotation speed was slowed right down - only then did it start missing random segments.

Larry it might be worth adding your location - you never know someone may be living nearby who can help, or has diagnostic equipment. :}

The voltage tests are carried out with the sensors plugged in and ignition turned on... That is why you need to break into the wiring to check the voltages, there is no voltage or pulses generated, unless plugged in and switched on.

There are other potential causes of brake judder, but most often it is a poor signal or missing pulses from the sensor. It could even be mechanical as in disk warp. The ABS cutting in is very distinctive, a very pronounced hammering at the brake pedal.

HI Larry.
Perfect sense, answer to your earlier question on no warning light and low speed judder at about 10mph down to 5mph and stopping, this was how my car displayed the problem, yes it will most likely be the magnetic ring damaged or rust flakes getting between the sensor & reluctor ring, I would plump for the OS rear, again I say this because the last four members cars I have looked at all turned out to be the same rear OS ring, why I do not have the answer for that ? I had access to T4 and also used the manual method using the made up ABS sensor kit and meter connected.

You could always take the rear hub off and check it for damage, also check the back plate at the same time for corroded rusting, flaking etc.

when I used my tester I plugged it into the cars abs system and used the rear door stay as the earth.

A few photo's below I did take lots of pictures when I did mine from testing with the meter, right through to removing the disc hub and replacing the sensor, which turned out to be ok it was the hub that was damaged by the corroded back plate, I keep meaning to get round to doing a full how to but real life gets in the way. Arctic

I would be interested in a spare abs plug and socket, don't seem to have time to go round scrapyards these days:shrug:
Thanks in advance,
Martyn

Hi Arctic, this statement confused me a tad :-

"""when I used my tester I plugged it into the cars abs system and used the rear door stay as the earth. """

On testing the pulses I simply applied my voltmeter (standard AVO multimeter srt to meaure voltage) across the two wires on the sensor. Is that the wrong approach then?

Should I be looking for a voltage on each wire individually between the wire and ground?
E.G. -- 12volts on one wire from the ECU, and 0.6 to 1.6 volt signal back from the second wire, as the wheel is rotated?

Larry

Mike I'm in Bedford

Harry, brand new discs on the front, new hubs on the rear and 4 new Firestone tyres.
This is (or seems to be) an ABS judder, which is a distinctive twanging (almost like the sound of a spring) on low speed braking.
It also goes if I unplug the ABS fuse.

On testing for signal variation, I had kept everything plugged in and ignition on on previous tests, but given that the sensor is a hall effect type, it should generate a small variable voltage when rotating the wheel, even if the sensor is unplugged (not powered).

Larry

You're not a million miles from Phil-T4, might be worth popping over to see him and get it on the T4

It is not a coil, a coil would produce a variable voltage as the wheel is rotated, it is a Hall Effect switch. A coil's output is dependent on the speed of the rotation as well. An Hall Effect switch is an electronic switch activated by a magnetic field. They have to have power to them to operate and they are either open or closed - only the two states, snap. Turn the wheel as slow as you like, the switch will still snap from one state, to the other - no middle ground.

I have just had another potential idea - I don't suppose the volt meter is one of those where it has battery test positions on it and you are testing with that selected. They apply a small load, which might be enough to kill any output voltage. The other thing is, you are testing on DC volts are you not? An AC volts setting will show zero, apart from a voltage glitch as the Hall sensor switches.

Harry, agreed - just reread the theory on Hall Effect sensing and yes it must be powered. So testing it without the appropriate current applied to the semiconductor circuitry of the sensing device will not provide a large enough voltage at the output of the sensor, when the magnet is continually passed over it.

For my meter, it's a standard AVO, used in DC mode, as the Hall Effect sensors used here provide a digital out put methinks.

I've had this working OK on the other wheels, it's just when I applied it to the front nearside wheel I got no response.

Unfortunately I had no time to try again yesterday as I had to strip off the front bumper of the wife's Audi A3 to get access to the leaking windscreen washer bottle.

Why do the eejits design cars that provide no basic access to the consumable serviceable elements?

Possibly £95+VAT hourly labour charges at Audi garages is the answer to my own question.

Larry -- I'm not bitter of course -- no not me!!!

Harry -- reporting back after a long break decorating the bedroom and en-suite. Getting back to the MG seemed like a break in some ways.
Anyhow I decided to start from scratch again on the testing. Ripped the back seats out again and retested both back wheels and the markings were equidistant all the way around the edge in coordination with the change in voltage -- so all looks OK on the backs - both newish hubs.

On testing the front, I finally got a proper reading on the front near side and the change in the voltages from 0.25v to 1.25v were uniform and equidistant.

Now to the front offside -- this was the one I had a new hub bearing fitted and a new ABS sensor. Equidistant all the way round, so the new reluctor ring is OK, however, the voltage, whilst also changing in a uniform manner, went from 0V to 0.5V each time.

Are we getting closer?
Why would the voltage be lower on just this wheel?

Low speed ABS judder still present, still no warning lights.

Regards Larry

Nothing been done to the hydraulics of late no. Think this is a factor?

I'm not an expert, but I wouldn't imagine the ABS system would be affected by either low brake fluid (checked that, it seems OK) or air in the fluid itself. ABS triggering is predominantly initiated by the electronic signal (or lack therof) from the wheel sensors. Although I guess you are considering the effect on the solenoid/pump at the master ABS control unit?

Regards Larry

Is there a link that shows how to bleed the ABS.

Is this done with the system closed, and the mechanical actions of applying the brakes at each point of the wheel turn on each wheel.
So you jacked up one wheel at a time and had someone rotate the wheel a little bit at a time while you applied the brake pedal hard?

How many times, part rotations on each wheel?

Without opening the bleed nipples, this seems more like forcing the air around the system, rather than bleeding the air from the system completely.

Or am I interpreting this incorrectly?

Any views on the discrepency (0v to 0.5v) I found on the voltages at the front offside wheel, compared to the other 3 wheels (0.25v to 1.25V)?

Regards Larry

I would say this sensor is faulty :shrug:
(however check its power voltage on the other wire)

MG John, Thanks for the detailed description. I'd be a bit nervous about banging the car into gear with one of the front wheels up on an axle stand :)

Actually John, I should highlight the original fault started with the speedo going off and the warning light coming on. I changed the front wheel bearing-hub, after detecting missing pulses on testing, and the front right sensor. Since then I haven't had a warning light. I confirmed it was an ABS problem because I removed the ABS fuse and the normal braking was resumed.

So I think I should take Mike's advice and look again at the front right sensor. It's possible I was just unlucky and the aftermarket part is faulty. Anyone know a reliable source of sensors?

Regards Larry

Without knowing what the actual spec. voltage should be, I would agree that the sensor is at fault. You can test a replacement without going as far as installing it, just plug it in, connect your meter and wave a magnet past the sensor.

Mike, Testing the voltage on the other lead with the meter connected to earth, we have about 9 volts.

The test lead (the signal lead, again compared to earth) reads between 0.0 volts and 0.5 volts on front offside wheel. The front nearside wheel binary switches between 0.5 volts and 1.25 volts as the wheel is slowly rotated (similar to 2 rear wheels).

Both front wheels are picking up regular signals, just one is a lot lower voltage.

I'm going to try another sensor, about £11-12 on eBay. Actually I'm not too bothered about the cost, I just want to stop having to deal with it.
Incidentally, question more for Mike and Harry -- "Have either of you had this false ABS problem ever diagnosed down to air in the ABS brake fluid?"

Or have they always been a combination of sensors, reluctor rings, air gaps in the sensors, and dirt corrosion on the back plates?

Regards Larry

In order to rule out a possible controller issue,can you swap this sensor
with a known one ?
no need to be fitted into the hub ...

Mike, not really possible as old sensors don't come out unless you drill them which is very time consuming and destroys the sensor.
I have to order another one.

One thing that struck me last night though, is why the ABS warning light hasn't come on if the sensor is causing the problem.
Or is it that the system thinks it's working at between 0 volts and 0.5 volts, but this is not sufficient to stop the low speed problem?

Also -- any chance this could be dirty contacts in the plug under the battery. I took the battery out and looked down at that area and it was dry so I don't see that as a problem although can't be sure. It looked very difficult to remove the battery holder and all the stuff around it to get to the plug, so stopped at that point as there was no sign of moisture or water sitting around that area. Also, if there was corrosion in the contacts on the plug, wouldn't that trigger the ABS warning light?

This is becoming a bit of nightmare -- lol!!!

Regards Larry

You don't need to fit the new sensor in the hub Larry - just plug it in.

Mike - No problem but I have to wait for the new one to arrive, should be here by Friday.

I had the judder last thing as coming to a stop, particularly when also turning the steering wheel, but on a Granada. Which was when I first studied how the ABS systems worked. No issues at all so far with the 75, but I like to study systems so I am ready and prepared for issues to develop - a bit of insurance if you like.

To answer your direct question, no I have never had an issue solved by bleeding the air out, but I can see how that might work - a circuit with air in it, might not produce the same braking effect at the wheel, which would cause imbalance and the ABS to be triggered.

Active Wheel Speed Sensors
 

This a useful article that describes both passive and active wheel speed sensor systems.

http://www.brakeandfrontend.com/diag...speed-sensors/

The active (Hall Effect) sensors are described and the indication in this example gives a digital output between 0.9 volts and 1.6 Volts.

A weak signal from one of the sensors (as described in my example) could be due to a faulty semiconductor device in the sensor electronics, or it's the wrong specification sensor (always a risk with after market crxp).
Alternatively the circuit could have higher resistance, causing a fall off in the current (low milliamps).
In my case the voltage seems radically different at 0volts to 0.5Volts and the other three sensors at 0.25Volts to 1.25volts.

The active wheel speed sensors are fitted to later model cars, because they give higher accuracy at low speeds (the measurement is frequency change with speed increase or decrease).

Larry -- interesting topic, despite not having a resolution as yet.

As I have pointed out several times - The sensors need to pass two stages of ABS diagnostics, the static at switch on test, then the dynamic test when the car is driven at more than a few mph.

That is interesting as my ABS low speed pedal kicking started after having both front wheels balanced. The tyre fitter mentioned that one tyre was wearing unevenly.

Should I try swapping tyres around?

Hi MG John, My problem doesn't include an ABS warning light.

The car thinks the ABS system is working OK, i.e. it doesn't see any faulty (completely defunct) sensors.

What I'm now hoping is that when I get the new sensor it operates with the same "Signal" voltage output range (0.25v to 1.25v) as the other 3 on the car.

If the new one reports the same lower (0 volts to 0.5 voltage) range, then there must be something else causing the sensor to give a low reading.

If this doesn't work, I have no idea what is causing this ABS brake judder.

I will report back when the new sensor arrives, possibly Friday.

Larry

I do hope you get it sorted
had the very same problem occur with mine realy sharpe judder at low speed on brake pedel no abs , i was lucky in as much as i removed rear drums on both sides both back plates were rusting up ,cleaned with wire brush and sprayed wd40 round sencor alls been good for 6 months , will only change them when the bearing needs doing.

The ABS is not interested in values, only whether or not it 'sees' proper, regular pulses at appropriate intervals. The voltage pulses are either in spec., borderline, or none.

"""The ABS is not interested in values, only whether or not it 'sees' proper, regular pulses at appropriate intervals. The voltage pulses are either in spec., borderline, or none."""

Harry, are you implying that me buying a new sensor will not be useful?
My pulses are all regular, but the voltage range on this one wheel is much lower 0v to 0.5V instead of 0.25 v to 1.25 v on the other three wheels.

If what you are saying is true, then all of my reluctor rings are OK (providing regular pulses); all of the sensors are working (no ABS warning light) and the ABS system is OK.

So what are the alternative things that can cause low speed ABS judder (and it is ABS judder, as proved by removing the ABS fuse and returning it to normal smooth braking)?

Regards Larry

I'm saying nothing of the kind!

The voltages you are seeing are almost certainly out of spec., though I don't know what the actual spec. is.

Each family of IC's (CMOS, TTL and etc.) has a different spec. for the digital input specification voltages - what voltages are considered to be 1's and what are considered to be 0's the high and low voltages which are acceptable as input to the device. Between these values, it is considered to be indeterminate, the device will not be able to make sense of the value, might provide the wrong output.

Hi Guys -- Got the new sensor this morning, changed it and test drove the car. It's made absolutely no difference.
I can't now believe that I am unlucky enough to have replaced the front right sensor twice and both new sensors are faulty, so I'm starting to conclude that this fault has nothing to do with broken reluctor rings, or sensors and therefore there must be some other cause of this ABS judder.

So far I have :-
1. Fitted 4 new tyres
2. Changed the front drivers side sensor (twice)
3. Cleaned up the rear hub back plates ensuring no rust blocking the rear sensors.
4. Changed the rear nearside hub.
5. Replaced the drivers side wheel bearing with integrated reluctor ring - probably unnecessarily
5. Tested all 4 wheels to confirm that there is a uniform set of pulses (marked with chalk) around each tyre when measuring the signal voltage changes at each small turn of the wheel.
6. Changed both front discs and pads to ensure there is no distortion on the front drive train.
7. The system has also been on a T4 tester twice - static and slow speed rotational, and the people in the garage don't seem to be able to interpret anything of significance.

That's it - I'm stumped - I have no idea why the ABS system is not recording a fault, yet the brake pedal judders like crazy when braking at low speed and does not when I remove the ABS fuse.

It seems incomprehensible that a manufacturer will make a product with no means of testing where, or how a fault occurs in the system - or design a system that does not report a fault to the user when it occurs.

Is it possible to deactivate the ABS system and still maintain a speedo function?
Clearly removing the ABS fuse kills the speedo as well.
Is it considered illegal to not have an integrated functional speedo in the car, when a SatNav can provide a more accurate speed measurement anyway?

Regards Larry

As there is no difference between your type of fault and the normal operation of the ABS - the ABS cannot differentiate between the two and flag up a fault.

Have you checked the high and low voltages coming from the new sensor?

Is the sensor from the same supplier or a different one?

Hi Harry, Yes I just tested the signal strength and it switches between 0.2 volts and 0.75 volts (actually that is the same as the other wheels now).

Unfortunately when rotating the wheel the signal only changes at a few points on the wheel. There are now whole dead zones where the voltage stays the same.

The other lead records a supply voltage of 10 volts so the sensor is getting a proper supply feed.

Somehow the sensor is not picking up most of the signals from the rotation of the reluctor ring segments.

Larry

Right Harry,
First thing to check ;if you find the same wrong values ,then see ABS PCB
controller ....
Or possible corrosion on the connector :

http://rovermg1.free.fr/rovermg/thum...Prd5VuEw9j.png

Mike

Hi Mike, I had entirely consistent (no breaks) voltages changes all of the way around the wheel with the old 1st new sensor and now not. As far as the connector is concerned, nothing has changed between the 1st and 2nd new sensor installations.

If the PCB connector had a corrosion problem it would have interfered with the pulses on the first new connector.
I assume this is the connector under the battery.

Larry

Larry,you must obtain the nominal voltages ...0.6 low AND 1.6 volt high.

These informations are crucial for the ECU

Dead zones are caused by the reluctor /gap /rust ...

Mike on the last sensor, (the first replacement one) I was getting oscillations from 0 volts to 0.5 volts and now with the 2nd new sensor I'm getting 0.2 volts to 0.75 volts.

0.2 Volts to 0.75 Volts is the same as the other 3 wheels, all which report uniform signal changes around the wheel at equal intervals.

The signal reading is taken between the signal lead and earth.

Regards Larry

IMO,it's not correct ...

Can you, on the sensor removed ,apply 12 volt on the power pin ;
earth on the brass part .
Then ,measure the output voltage (move a small magnet to simulate the reluctor)

Mike - nigh on impossible with the sensor removed, there is no access to the two tiny pins that sit deep inside the the plug-socket.

I just tested the accuracy of my meter. I have a transformer from mains to 12 volts and the reading on my meter is 10 volts on this. Which is an error of approximately 17% out.

I'm not now convinced this is a sensor issue at all, as all the readings on the 4 sensors are now within similar range.
The anomaly here is that I got regular pulses on the 1st new sensor, and irregular on the second new sensor, albeit at a higher voltage range.

Regards Larry

Mike - Is there a simple way to disable the ABS and maintain the speedo, and without any dash warning lights?

I don't want the ABS system at all any more, if possible as it's an unnecessary interference and looks like being prone to problems on an ongoing basis?

I'm not a crazy driver, so it's probably a relatively small risk for me not having ABS.

Regards Larry

I agree ,same error on the four values ; multimeter is implied .
irregular on the second new sensor.not normal ...check gap /rust /ring

DON'T give up ,thousands vehicles are on the roads with working ABS !

I missed this comment, sorry :-

"Dead zones are caused by the reluctor /gap /rust ..."

I have a new bearing fitted, and all rust cleaned up. The gap will be fixed as the bearing is integrated on the front wheel and the length of sensors ordered for the front wheel are longer and standard.
Also I can't have a reluctor ring problem, given that the 1st new sensor changed voltage at every single small turn of the wheel on the last test.

Larry

Anyways,you can't be sorted as long as one sensors delivers an incorrect
reading ....

Mike - That's true, at least all the signal levels are consistent. I can't understand any reason why the pulses are no longer appearing regularly when they were on the last newly fitted sensor. It's not as if there is any margin of error in installing the sensor, because it only fits in one way and is bolted down by a single bolt.

There seems to be an indeterminate situation here and no one knows how to solve it despite valiant and very generous help from your good self, from Harry, and others on this very good system.

Given this is the 105th post about this issue and it's dragged on for a few months and has cost me quite a lot of time and a lot of unnecessary expense, it would be much better if I could disable the ABS system.

Is there a way to disable it, maintain the speedo and not have a warning light on the dash?

Regards Larry

Fairly sure it's a gap issue ;
seems to remember that Mike Noc had to file an aftermarket sensor to obtain the correct (minimum) gap between active sensor side and magnetic ring ..

Go on,You don't regret :xmas-smiley-032:

Mike, just been filing away at the base plate for where the sensor fits in. Cleaned it up and vacuumed the hole out. filed a bit of the sensor metal ring. Popped it in and the pulses are still crazily irregular. m8 just called and wants a few beers.

I think I need a few myself - so it's down tools until tomorrow.

Where are you in France?
I used to travel to Grenoble, Toulouse, Paris, Montpelier and Nice, Sophia-Antipolis and Rousset on business.

Regards Larry

You're right ,make a little pause :Ball:

I live 25km Southeast of mont ventoux ...zero degree today :icon_lol:

Likely the error will be even greater - The unloaded output of a small transformer will likely be much more than 12v, to allow for voltage drop under load. The other thing is you are I think checking it on an AC range, best to test it on a DC range you are using...

The voltage across your car battery should be between 12.5 and 12.9v. A single AA or AAA battery in good condition should be around 1.5v.

Harry -- all checking at DC range as per a Hall Effect sensor technology. Basically a binary switch.

Larry

mont ventoux

Ahhh you are not too far from Aix En Provence, Peter Mayle country. Aix is charming. Ever eaten at the restaurants along the boulevard in the town. The tradition being you eat to your main course then walk around the fountain then come back for your desert -- Creme Brulee of course and Calvados?

Must go back sometime, with or without my blessed ABS :)

Well I've filed away at the surface of the metal on the hub, but I'm not sure this makes any difference, since getting the sensor in deeper into the hole doesn't seem to have any effect, given that the reading is taken from the side of the sensor on the front hubs. I could appreciate getting the tip of the sensor closer to the ring on the back wheel, but it's the side of the sensor that counts on the front.

Did it anyway and it's not making much difference.

The rotation of the wheel sometimes causes the needle on the meter to move and then not for parts of the wheel.

Any final thoughts -- at the moment I don't see any possible resolution to this and what's more puzzling is that there doesn't seem to be any method to test what is wrong.

What's completely strange is that the 1st new sensor gave unbroken changes all around the wheel, but a lower voltage -- this second one seems to pick up and move the signal sporadically.

Larry

Could be a damaged or displaced magnetic ring ? (be aware there are the equivalent of 46 tiny magnet into)
The gap must be small and regular ...

Hi Mike, I had the front hub wheel bearing replaced with a new one, which has the reluctor ring integrated into it. It didn't show any missing marks on the tyre with the first new sensor.

That's what makes this such a mystery, because the second new sensor is now not picking up all the reluctor elements like the first new one did.

Regards Larry

Have you an idea of the gap value before and after ?

"Have you an idea of the gap value before and after ?"

Hi Mike, I don't know what the gap value means.
All I know is the sensors are standard and the wheel bearing was replaced with a brand new and identical to the old wheel bearing.
To be honest I think this was an unnecessary, had I not carelessly drilled too deep to get the old one out.

The whole problem started with the speedo going off and the ABS warning light coming on a couple of months back. Since then I drilled out the old sensor and possible damaged the reluctor ring, hence the replacement was done. Since then I have no ABS warning lights and it passed the MOT with the brake judder.

All the sensors are the same length, there is no room for play in the receptor hole for the sensor, they fit really tight, and they are side facing onto the hub ring, so I can't see how the gap would vary from one sensor to the next - it's a standard fit.

Larry

Hum,if only few sectors are missing,i would say ,your reluctor ring is damaged !

Yes that's what I thought, but when I had a similar problem on a rear bearing I checked the signal with the bearing and sensor in my hand and all the magnetic segments were picked up.

Fitted back on the car random segments were missed when rotating the wheel slowly. It could only be the air gap, so I filed 1.4mm off the bracket and eveything has worked fine since.

At the time I put it down to the combination of an aftermarket sensor and bearing being not quite as compatible as they should be.

Mike, Your problem with the air gap was related to the rear hub/sensor. On the rear the tip of the sensor faces down onto the reluctor ring, so filing the bracket down will bring the tip closer to it, which solved your problem.

On the front hubs the reluctor ring slots in with the ring facing the side of the sensor magnet, not the tip. My assumption therefore is that filing won't make an iota of difference as this won't bring the sensor any closer to the reluctor ring.

Regards Larry

Also the reluctor ring is brand new and it tested well for all the pulses on the 1st new sensor.

I think this is a pretty indeterminate problem, since random problems keep arising no matter what parts are replaced or adjustments made.

And there are no tests that the manufacturer has devised to cover the proper function of the ABS system.

Personally I want it disabled, but I want to be able to maintain a speedo and no warning lights -- Is this possible?

Regards Larry

Air Gaps
 

Just picked this info up :-

I guess this is the most relevant statement in the paragraphs below :-
----Most sensors on newer vehicles, however, do not have an adjustable air gap."----

"SENSOR AIR GAP
The distance or "air gap" between the end of the sensor and its ring is critical. A close gap is necessary to produce a strong, reliable signal. But metal-to-metal contact between the sensor and its ring must be avoided since this would damage both. The air gap must not be too wide, on the other hand, or a weak or erratic signal (or no signal) may result.

The air gap on some wheel speed sensors is adjustable, and is specified by the vehicle manufacturer. The gap will vary from one application to another so always refer to the exact specifications for the vehicle when adjusting the sensor. Most sensors on newer vehicles, however, do not have an adjustable air gap."

Yes of course that is true Larry. But you could still have exactly the same problem with a slightly too large air gap between sensor and reluctor due to aftermarket parts not being compatible. If that is the case then a replacement part will be required.

Interestingly with the problem I had all the segments were read until the rotation was slowed right down, and then random segments were missed.

One other check you could do to eliminate a wiring fault is to plug the sensor into one of the other plugs and see if things improve. I made up an extension cable from some old speaker twin core with an ABS plug and socket from a scrapper so any sensor could be connected to the OSF and the speedometer used as a diagnostic. Here it is being used to check a hub prior to fitting.

[IMG]http://i1108.photobucket.com/albums/...ps40be8806.jpg[/IMG]

"But you could still have exactly the same problem with a slightly too large air gap between sensor and reluctor due to aftermarket parts not being compatible."

Mike on the front sensors, appreciate you had a rear sensor problem, there is no tolerance in the way the sensor fits into the hub.

If you filed the sensor side itself, you would in fact increase the size of the air gap, because the thing is reading side on when it is facing the reluctor ring.

Filing can not serve any purpose on a front sensor, does everyone else agree with this, or am I missing something?

Both the new sensors I have tried have been the same length and thickness and the positioning of the sensor inside the hole can not be varied because it is screwed into place with a single screw which is also in a fixed position and allows no play of movement to re position the sensor.

Larry

""One other check you could do to eliminate a wiring fault is to plug the sensor into one of the other plugs and see if things improve.""

Mike, if the sensor is applied to another wheel, I would need to drill out one of the old one's and this would just create more problems. Given that all the other 3 wheels seem to be giving regular pulses, I'd want to leave them alone. It takes at least an hour to remove an old sensor and there is every possibility I would damage one of the reluctor rings in the same way that I did with the front right sensor.

Disabling ABS
 

Anyone know if this can be done, maintaining the speedo function and no dash warning lights?

No need to do more than unplug one of the other sensors, plug the supposed faulty one in and run a magnet past it whilst measuring the voltage.

If you have not done so already, I would suggest sourcing an ABS plug and socket, connecting them back to back via a choc -block, so as to make a wiring break out. It would save you a lot of effort trying to tap into the existing wiring.

Thanks Harry. Spot on - I couldn't have put it better myself. :icon_lol:

"No need to do more than unplug one of the other sensors, plug the supposed faulty one in and run a magnet past it whilst measuring the voltage."

Thanks Harry, but I'm not sure what this proves. Of the two new sensors fitted - the first read all of the reluctor segments equidistantly (which seemingly proves there is not a problem with the wheel bearing/ magnet ring). This is hardly surprising as it was newly fitted a few weeks back.

The second sensor reads a higher voltage as indicated in the last messages. waving a magnet against the free floating sensor will just make the voltage change.

I think there must be something else in the ABS system causing the ABS judder as there is no definitive test to prove or disprove there is a problem at the wheel level.

Again - If anybody has a way to disable the ABS, maintain the speedo and without a warning light, that is my preferred option.

At the moment I don't see a resolution to this, since the garage using a T4 tester can not resolve it, the garage using a motion test can not solve it, individual slow turn wheel tests can not solve it and changing the relevant parts (sensors, hubs, integrated wheel bearings) can not solve it.

You guys have been very kind in helping, but this has been going on for months and I believe further testing just sends it around in circles.

Larry

MG John,

This definitely looks worth a shot, I'm going to give it a go.

Regards Larry

It is easy enough to do, but bodging a safety system to make it look as though it is working when it has failed isn't on. Apart from driving a vehicle in a dangerous condition, what if it is sold on and the new owner finds himself in a situation where he needs to rely on it?

I have already mentioned how to get the speedometer working in a previous post here.

Is there not a decent garage near you? One that can diagnose faults as opposed to just plugging a diagnostic machine in? As John has mentioned this could be a faulty plug connection, but if not then all the wiring should be checked for continuity and resistance to earth, and if everything checks out then the ABS modulator swapped to eliminate it.

Often with faults like these you need to get the multimeter out and go back to basics.

Might be worth adding your location - someone may be able to recommend a local good garage or specialist.




.

Mike I'm in Bedford.

Phil-T4 is in Olney, near Milton Keynes so not far down the road from you, and I would put money on him getting to the botttom of it Larry. ;)

http://www.the75andztclub.co.uk/foru...d.php?t=201841

Larry - I suggested a method for you to test a sensor's two voltage levels, avoiding the need to swap the sensor in the hub. As I see it, the wrong voltages you are seeing are a crucial part of the problem, something you need to get to the bottom of as a priority. The low voltage might indeed be due to to a poor connection at the multi-plug. A poor connection will produce resistance and voltage drop - which seems to be what you are seeing.

Hi Guys, Disconnected the battery and removed the battery tray and the body brackets to get easier access to the multi-plug.

I notice that the plug has a plastic bracket that I slowly started to lever out to the side to remove the plug.

Before I break anything here, can anyone confirm that this is how the plug comes out?

Larry

Ignore last message -- got the plug off (the retractor withdraws a long way before the plug is freed up).

Inside it looks as clean as a whistle, but I suppose I should clean each pin. Access to it is really difficult.
The pins are on the car side, not on the plug that has been withdrawn, which is a female connector.

Any advice on how and with what to clean each one of these tiny pins?

Would spraying electrical contact cleaner on it be sufficient?

Larry

Ok - cleaned all the contacts on the multiplug using wet and dry paper carefully on each connector pin , used electrical contact cleaner to spray both the plug and socket and reassembled the plug.

Rebuilt the battery compartment and tested it.

It made no difference at all -- back to square one.

Larry

Mike - I reached out to Phil just now with a detailed message of the history of this fault, including today's effort in cleaning the multi plug which unfortunately did not improve things.

Regards Larry

Hi Harry, reflecting back on the above you mentioned on the 9th. The voltage changes were now OK, in line with the other 3 wheels, when I tested it in circuit, the problem is that it is no longer reading all the reluctor ring segments. I also, as posted yesterday, carefully unplugged the multi-plug to the modulator and painstakingly wet and dried each pin and sprayed electrical contact cleaner on the plug and the female socket.
But thanks I appreciate the message regarding testing the sensor voltages without having to install the sensor into the hub.

One thing I need to check again this morning is the connector on the front drivers side, as this did not seem to connect as tightly as the previous two.

Larry

Heddy, It's a good point I think the new sensor I put on the front drivers side wheel was very tight and it didn't click all the way in like the old one.

I'll get on to it this afernoon.

Larry

Heddy - Checked the sensor connector and it is fully clicked home on the drivers side front.
Got a new accurate multi-meter and the voltage on the main battery is 12.2V

Measuring the sensor voltage with the ignition switched on, the feed line is 10.88 Volts and the signal voltage varies between 0.7 volts and 1.75 voltage.

This signal voltage change is precise each time, but it is not changing in a regular pattern. There are gaps when slowly rotating and marking the tyre.

On the 1st new sensor it didn't record any gaps ---- ??????

This is so annoying and inconsistent, despite being fairly diligent when taking the measurements.

What the xxxx is going on here, I've just had a new wheel bearing fitted?

I will try the 1st new sensor again tomorrow to see what the voltage readings are and if there are any gaps on the test.

Does anyone have a spare front sensor they can sell me? -- One that you know is good and has been fully functional on your car?
It seems pointless me ordering another new one, if the after market sensors are potentially duff.

I'll happily pay for it and shipping cost, or drive over and collect if close-ish to Bedford.

Regards Larry

Larry,your other sensor giving 0.5 volt was really faulty .
But with a correct GAP ....
May be you have damaged the ring while fitting the last one ?

Mike

Hi Mike, The sensors shape and sizes are identical and the hole is fixed - sliding it in and out to the butt of the hub is fixed, especially as the hole is now clean. I'm not sure how the sensor could damage the ring.

Drilling maybe yes, but sliding in a new sensor to the same depth should be quite safe, otherwise there is a serious design fault with the ABS system. Well we know there is :) otherwise everyone would have a tester that allowed them to accurately and simply identify a fault as it occurs on the system.

Regards Larry

No need of special tool (not sur it exists )
How many segments are missing out of 46 ?

The sensor is plastic encapsulated! Also, I imagine that the cast iron hub would be a very efficient heatsink. ;)
Drilling requires care for two reasons: (1) the hole isn't round, it contains a flat towards the base to ensure correct location of the sensor and (2) accurately centreing the drill bit is virtually impossible so start with a small diameter and work upwards. The sensor will break up so it's a slow painstaking process of fishing out bits of plastic. :o

Simon

Not sure why this discussion has moved to that of drilling out a sensor.

Both sensors I am fitting are brand new, and the wheel bearing which contains the reluctor ring is also brand new.
All the drilling was done on the old wheel bearing/reluctor ring, which has been removed, discarded and replaced by a new one.

Larry

MG John, agreed it's good to be open, I misinterpreted it somewhat in relation to my own thread - no worries!

On your post script message re driving a BMW -- some say an MGZT (-T) is a BMW -- or is it a sin to allude to such, just because it's fitted with a BMW common rail diesel engine?

I do wish we had a decent indigenous car industry in the UK - bring back the good old bad old, old days.

OK off to swap the sensors over again to see if the 1st one has any better readings now I have a decent meter.

Larry - PS: Thinking of getting hold of an MGB GT -- anyone any experience of having or driving one of these?

With regards to the drilling of the sensor I would always suspect the magnetic ring first if the front is involved even more so if the speedo is not working on the OS front.

Larry of cause as replaced his with new one, so therefore should not have that worry, but as I say drilling out a sensor is best done by drilling an hole in the centre and using a wood screw to help remove it. please refer to my post 28 link below.

https://www.the75andztclub.co.uk/for...5&postcount=28

PS Larry as you are going to change the front sensor again today, I would and I know it is a fair bit more work but I would remove the disc etc to double check the front bearing for debris and cleanness just a thought, good luck by the way. Arctic

I did try this suggestion of yours Steve but my sensor was so stuck that the wood screw simply tore it to pieces. :o

At the front, very small pieces fall out of the bottom of the hole. Any that lodge there can be persuaded to fall to the ground by turning the driveshaft slightly or by extracting them with tweezers.

Simon

Great photos there MGJ -- I thought it was Robert Redford for a minute there!!

Anyhow here's an update.
I changed the sensor back to the 1st new one that I thought was giving low readings.
Not the case, my meter was obviously playing up.
With the new meter I was getting close to 11 volts on the power feed and then when running the signal test I was getting switching between 0.72 volts and 1.78 volts each time and this one picked up all the segments.

So I think the 1st new sensor is a good one, the second sensor is duff.

Tested the car again just to make sure it was not fooling me, but no there is no change it still gives ABS judder at low speed.

So all the wheels have been checked, the multi-plug has been cleaned and the front discs have been changed, 4 new tyres and I'm now at a major road junction because I have absolutely no idea what is causing this ABS judder.


Regards Larry

I too am at a loss to explain it, or offer any more suggestions which might be applicable to the ABS itself :icon_rolleyes:

Which just leaves suspension and bushes etc..

Clutching at straws- I wonder if there might be a weak or damaged bush, allowing a wheel to go out of alignment enough to trigger the ABS..

Please forgive me for not trawling through 159 posts Larry, but have you carried out your multimeter test on the rear sensors? If so, maybe time for a drive to see Phil-T4 in MK?

Simon

Hi Larry.
I would suspect one of the rear hub/bearing, i say this because your speedo is working, you have changed both the sensor and the bearing on the front, so in theory the front OS should be ok.

The OS rear as you report as the newer hub/bearing with the oval slotted brass ring, therefore again we could expect this one to be ok, if you are up for a bit more graft i have a rear hub/bearing matching the brass one above which i took of a car recently i fitted it in May of this year, i removed it from the car in October as the owner no longer needed the car.

It is now in my shed and you can have it for the cost of posting, maybe change it over with the one you have fitted at the moment, and see if it makes any difference ? if not then changed or replace the one without the brass ring for the one you removed then see if that makes a difference, other than that i would suggest once more getting it on a T4, if you are interested in giving that a go please send me a PM with your full name and address and i will get the hub sent off to you Arctic.

Hi Arctic, I changed both rear hubs in 2014. When the fault started a couple months back, I took both off, cleaned up the backplates to make sure no rust and that the sensors were not clogged.

As the T4 reported a fault, the first time I paid the garage in Olney, on the rear nearside, I bit the bullet and bought a new hub and installed this, and that has subsequently passed the multimeter test.
At the time I had the front wheel bearing changed, the T4 test was done again and no faults were recorded.

Larry

Hi SD100, indeed this one is covered and both rears passed the multimeter test.

Regards Larry

"""Clutching at straws- I wonder if there might be a weak or damaged bush, allowing a wheel to go out of alignment enough to trigger the ABS."""

Harry, it's a thought, but would that not have been picked up on the MOT in October when it passed with no advisories, or is the tolerance on these pretty wide for MOT purposes?

Larry

Such should be picked up during an MOT, but they are not infallible.

You would seem to have covered all the other bases quite thoroughly, so what else is left still unchecked?

I wonder could it be a faulty wheel bearing itself?


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75 Driver - how would you know if the wheel bearing was faulty?

The rear offside was replaced in 2014, the rear nearside just changed and the front drivers side is brand new. There were no missing readings on the nearside front.

Larry

Funnily enough my car has just developed the same fault - ABS cutting in every now and again at slow speed with no alarm light coming on.

Was going to look at it today, but it was raining. Did plug my Sykes Pickavant ACR4 in though and it has read a fault code of 005: wheel speed sensor rear right (plausible), so that will be the first place to start when the weather improves.

Probably the sensor or reluctor - luckily I have a spare magnetic reluctor standing by. :Ball:

Mike if it's the rear it's a doddle, and if you need another rear hub I've got one if you need it.

Good luck with getting the old sensor out, if you decide to do that. The shallow centre drill seems like a good option, then drive a screw in.

I guess you'll be doing the chalk test on the tyre and a multimeter 1st?

Larry

Got to get round to try and sort mine out! Very similar problem despite having hubs and sensors changed! No light coming on but still getting ABS kicking in at low speed. Seems to becoming a more and more common problem, and can work out an expensive one too.
Any definitive answer would be great.:shrug::bowdown:;)
:christmas::new_year: