ABS Problem MG ZT-T

[chris75]

Originally Posted by larryr123

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.
.

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 ......

[Mike Noc]

Originally Posted by larryr123

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?

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:

Originally Posted by larryr123

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.

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.

[marinabrian]

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

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

Brian

Originally Posted by HarryM1BYT

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.

[Arctic]

.

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

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 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

[larryr123]

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

[deltic08]

Originally Posted by marinabrian

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

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

Brian

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?

[marinabrian]

Originally Posted by HarryM1BYT

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.

T4 will see it as a plausibility fault, and this will be logged in the ABS ECU Harry

Given where the OP is located, I would suggest he contact Phil-T4 in the first instance, and not bother wasting time swapping this that and the other to try and pinpoint the failure

Oh and Heddy is right, the magnetic reluctor is embedded in the bearing oil seal of the rear and front hubs, and corrosion build up can affect the magnetic flux thus interfering with the abs sensor pulse, very easily identifiable using ABS live data on T4

Brian

[larryr123]

""T4 will see it as a plausibility fault""

Not sure what a plausibility fault is, or whether that pinpoints it to the actual cause and on which wheel.
When the test was run last time no faults were shown, so I doubt there is a need for another T4 test.

Harry -- I did the chalk test three or 4 times and i honestly couldn't determine any gaps. It's not a very scientific measurement method though I assume.

There really should be a piece of test equipment that actually identifies erroneous signals and pinpoint where they are coming from. An oscilloscope perhaps, although I would have thought that would be built into the T4.

I've had the plugs in an out loads of times, so I doubt there is a connection problem, especially as I was getting a reading on the supply wire and the signal wire on all 4 wheels.

On the dirt and corrosion, I cleaned up the hubs and back plates.
Most people on here indicate that the faults usually occur on the rear wheels.
My original fault was the drivers front wheel, as the speedo failed and the ABS light came on -- but that has been rectified with a new sensor and a new front wheel bearing. Yes all the reluctor rings are embedded in the wheel bearings -- on the rears they reside in the slide on hubs and are much easier to change - the fronts a much more difficult.

Larry

[marinabrian]

Originally Posted by larryr123

""T4 will see it as a plausibility fault""

Not sure what a plausibility fault is, or whether that pinpoints it to the actual cause and on which wheel.
When the test was run last time no faults were shown, so I doubt there is a need for another T4 test.

Harry -- I did the chalk test three or 4 times and i honestly couldn't determine any gaps. It's not a very scientific measurement method though I assume.

There really should be a piece of test equipment that actually identifies erroneous signals and pinpoint where they are coming from. An oscilloscope perhaps, although I would have thought that would be built into the T4.

I've had the plugs in an out loads of times, so I doubt there is a connection problem, especially as I was getting a reading on the supply wire and the signal wire on all 4 wheels.

On the dirt and corrosion, I cleaned up the hubs and back plates.
Most people on here indicate that the faults usually occur on the rear wheels.
My original fault was the drivers front wheel, as the speedo failed and the ABS light came on -- but that has been rectified with a new sensor and a new front wheel bearing. Yes all the reluctor rings are embedded in the wheel bearings -- on the rears they reside in the slide on hubs and are much easier to change - the fronts a much more difficult.

Larry

Driving the car with the live data showing on T4 will certainly identify which wheel(s) are causing the issue.

You can't beat seeing the output speed from each wheel in real time, did Phil T4 your car, or someone else?

Brian

[larryr123]

Hi Brian, It was done with a garage at Olney. They drove the car around the car park while running the T4 test and then didn't come back with any conclusive evidence as to which wheel was faulty. This cost me £60 and was seemingly a waste of money.

I've also tried to work through this procedure, which might prove helpful to others :-

75 & MG ZT ABS
DIY Fault Finding & Fix HowTo

Introduction:

Unlike earlier MG-Rover models the Rover 75/MG ZT uses a Bosch ABS system with active (Hall Effect) wheel sensors, previous generations of cars from MG-Rover and other manufacturers used inductive sensors.
Another change from earlier models is the toothed ABS ring on the front CV joints and rear hubs has been replaced by ferromagnetic rings built into the “active” wheel bearing oil seals. The clearance between the wheel sensor and the ferromagnetic ring on the wheel bearing is critical excess clearance due wheel bearing wear can trigger the system to log an ABS fault.

Because of these changes fault finding on this ABS system without dealer level diagnostic equipment requires a more careful approach than with earlier types of ABS as both the Hall Effect sensors and active wheel bearings are more easily damaged. Hall Effect sensors on this model should not be tested by measuring resistance as a simple resistance test with an average Digital Multi Meter may burn out the sensor.

To avoid wasted time, unnecessary expense and frustration please read this HowTo through to the end before testing replacing and components.

Start-up Self Test
(1)Start-up test: at ignition switch on the ECU in the ABS Modulator performs a basic self test during which the yellow ABS illuminates. During the start-up test each sensor is tested electrically and the ABS modulator is tested both electrically and hydraulically. If the ABS light remains on it most likely points to an electrical fault or less likely an ABS modulator or hydraulic fault.

(2)In normal operation each wheel sensor is supplied with a 12v supply, if a sensor fails start-up self test the 12v supply to that sensor is automatically shut-down by the ABS ECU.

(3)If this first stage of the self-test is passed the ABS warning light extinguishes and the next stage of self-test continues when the vehicle is in motion.

(4)If the ABS fault warning light extinguishes as normal but then re-illuminates after driving a short distance (over 7.5 mph) then it usually suggest to the ABS having detected implausible signal from one of the wheel sensor.

(5)An implausible signal from a wheel sensor would usually be either a problem with an active wheel bearing or intermittent sensor or electrical connection.

(6)Another common cause of the ABS illuminating on an implausible wheel sensor error is mismatched tyre sizes, of course this will not show up on an oscilloscope test.

Important Clues on Where to Start
The ABS wheel sensors signals are passed by by the ABS Module ECU to other systems on the car this can give pointers to where start test individual sensors.



(1)Speedometer is driven by the signal from the right front wheel sensor, if the speedometer works normally the fault is less likely to be in the right front wheel sensor or associated parts.

(2)On KV6 models with cruise control the speed signal to the cruise control system is taken from the front left wheel speed sensor, If the cruise control works normally the fault is less likely to be in the right front wheel sensor or associated parts. The engine ECU also takes its' speed signal from the left front sensor.

(3)SatNav system takes data from both rear wheel sensors, if ABS and SatNav faults appear at the same time it is likely to be a problem related to a rear sensor.

Before testing individual wheel sensors it is worth removing the car's battery and battery box and unplugging the multi-pin connector from the ABS Modulator and checking for water ingress and corrosion, this is a known problem on Rover 75's particularly pre-Longbridge cars.

Premature ABS Activation
Premature ABS activation or unexpected pulsing felt through the brake pedal can be caused by problems with the the ferro magnetic rings in the wheel bearings unlike older system where a clean up and visual inspection of the toothed reluctor rings with active wheel bearings will often reveal the cause of the problem the only way diagnose this type of problem is to examine the signals from each sensor with an oscilloscope while the vehicles is driven preferably at constant speed. However with this type of fault it is worthwhile checking all the electrical connections and bleeding the brake hydraulics.

Testing an Active Wheel Sensor
The Hall Effect wheel sensors should not be tested by measuring resistance as it is likely to burn out the sensor.

Each sensor is connected to the ABS modulator by two wires, one is a nomminal 12 volt supply the other the signal return to the ABS modulator. Following a visual inspection of the wiring and connectors the first test is to measure voltage present between each of these two sensors wires and a good ground on the vehicle body.

Because the wheel sensor connectors are very difficult to back probe to get usable test connections it is more convince to break into the wiring of the vehicle either by making tap connections or making a special breakout connector using plug & socket parts salvaged from a scrap vehicle.

ABS Wiring Colour Codes
Wheel Sensor
12v Supply
Signal Output
Notes
Front Left
Red, Black stripe
Red

Front Right
Yellow
Yellow, Black stripe
Also Drives Speedometer
Rear Left
Green,
Green, Black stripe

Rear Right
White
White, Black stripe

Wheel Sensor DMM Test.
Tools required Digital Multi-Meter, Car Jack.

1.Jack up the car so that the wheel is clear of the ground and can be rotated by hand.

2.Turn ignition and allow ABS system four seconds to complete self-test.

3.Using a Digital Multi Meter (DMM) set to the 20v DC range, connect the negative lead of the meter to a good earth ground on the vehicle body then check the voltage on the 12v supply wire to the sensor. If a +12v supply is present then the sensor has passed power on self test. If the +12v supply is not present then it points to a failure in the sensor or the wiring between the sensor and the ABS modulator ECU. Connection problems due to water ingress by capillary action via the wiring from the front ABS sensor are not unknown on the 75/ZT, see notes in a later section.

4.Now with the meter on the 2v dc range check the voltage on the sensor output wire, expect this to read between approximately +0.6 volts and +1.7 volts.

5.With the wheel clear of the ground rotate the wheel by hand while checking output voltage which should switch between approximately between +0.6v and +1.7v when the wheel is rotated.

6.Repeat for each in turn wheel.

Passing this simple test will only show if the wheel sensor is energised and switching in response to movement of the active wheel bearing it will not indicate if the active wheel bearing is generating plausible signal. As the magnetic ring in the wheel bearing is relatively delicate and the clearance between the ring and the sensor is critical, failure is relatively common.

An oscilloscope is the only way to check the quality of the signal from a sensor, the output wave form should be examined using an oscilloscope when the vehicle is being driven on the road.

Oscilloscope Test on Signal from Wheel Sensor
This test is to check the quality of the signal returned to the ABS Modulators ECU by a wheel sensor. Any defect in the wheel bearing magnetic ring could generate an irregular signal which will fail the plausibility test and toggle the ABS light on as soon as the vehicle speeds exceeds 7mph.

As with the DMM test the measurements are with the ignition on and power on self-test complete, the oscilloscope connected between the the sensor signal output wire and a good earth the vehicle body.

To obtain meaningful results I would suggest the test is carried out when the vehicle is on the road driven at constant speed.

When the car is being driven the wheel sensor output viewed on an oscilloscope should be a nice regular square wave between a low of +0.6 and and peak of +1.6v. I would suggest starting with an oscilloscope timebase setting of 0.1 seconds per division.

Replacing Wheel Sensors
Take care to only purchase OEM quality sensor either MG-Rover/X-Part or Hella cheap unbranded sensors are an expensive lottery.

The sensors are held in by single 6mm diameter bolts with 8mm hex heads, so once you have identified a sensor you think is defective it should be simple to replace, well no :-(

The bolts usually come free easily enough but the sensor bodies made of a relatively fragile plastic are an extremely tight fit in the housing even when new.

The rear sensors are easiest to remove. If after soaking with WD40 they show no signs of budging then simply remove the brake disc and wheel hub to gain access to the working end of the sensor, then use a 10 or 12mm diameter drift to punch it through from brake shoe back plate side.

The front sensors are generally much more difficult to remove and attempt to remove a sensor by pulling with mole grips is likely to result in the sensor breaking up leaving the stub firmly seized in the hole leaving a choice of three options:

1. Carefully drill the sensor out with a 12mm drill, requires great care to avoid damaging the active wheel bearing also results in iron oxide, metallic dust and plastic swarf trapped in the void between the CV joint and the wheel bearing which can often cause contamination problems with the active wheel bearing.

2.Undo the outer CV joint retaining nut, bottom ball joint, track rod end and anti-roll bar link and pop the CV joint out from the hub. This allows the sensor to be drifted out.

3.Replace the complete swivel hub with a scrapyard part either with a known good sensor or fitted a new sensor. You can connect the replacement sensor parts up to the car and do a quick DMM test and self-test on the sensor and active wheel bearing before reassembly. Because of problems with active wheel bearings replacing the complete swivel hub with a tested item has become my preferred option especially on higher mileage vehicles.



Notes on Active Rear Wheel Bearings Replacement.
The rear wheel bearings come complete as part the wheel hub assembly and is very easy to change however take care to carefully examine the replacement assembly before fitting.

As the clearance between the wheel sensor and the ferromagnetic ring on the wheel bearing is critical any excess clearance or run out of the ferromagnetic ring is likely to cause problems with the signal from the sensor.

When you unpack the new part carefully check that the ferromagnetic ring as been pressed in evenly and is sitting flush with the rim of the hub.

The front wheel bearings are a fairly major job during which you can run into niggling snags, to minimise time off the road I would recommend buying a second hand swivel hub and drive flange assembly and either fitting as it comes or overhauling it with a new bearing before fitting.

hope it helps others..