ABS Problem MG ZT-T

[HarryM1BYT]

Originally Posted by larryr123

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?

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.

[larryr123]

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

[larryr123]

"""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

[HarryM1BYT]

Originally Posted by larryr123

"""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.

[larryr123]

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.

[HarryM1BYT]

Originally Posted by larryr123

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?

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.

[larryr123]

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

[HarryM1BYT]

Originally Posted by larryr123

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?

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.

[FrenchMike]

Originally Posted by larryr123

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


Larryr123

What voltage value do you get on the sensor signal ?

[larryr123]

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