ABS Problem MG ZT-T

[Greeners]

Larry

I know this is a nightmare for you, but I'm beginning to think its not ABS related on the basis that it passes the static and rolling test......Im sure if the ABS was operating the dash lamp would illuminate to show operation ???

[larryr123]

Hi Greeners, It's definitely an ABS problem, as confirmed by removing the ABS fuse. The braking returns to normal once it is removed, but the speedo is then knocked out.

Regards Larry

[Greeners]

well I guess the only other thing is the ABS ECU

[HarryM1BYT]

Originally Posted by Greeners

Larry

I know this is a nightmare for you, but I'm beginning to think its not ABS related on the basis that it passes the static and rolling test......Im sure if the ABS was operating the dash lamp would illuminate to show operation ???

I have never seen the ABS warning light come on, simply because the ABS has activated and I make a point of testing mine at every reasonable opportunity, it is a failure light after all.

I still think the problem and solution is covered by one of the suggestions made already.

[larryr123]

"I still think the problem and solution is covered by one of the suggestions made already."

Hi Harry, Correct I don't have an ABS warning light, except on initial test at ignition switch on which is entirely normal.

When you say you think it's caused by one of the suggestions earlier, could I have missed something that's not in the list below :-

1. Changed ABS sensor on Front Right Wheel, twice.
2. Changed wheel bearing on front right which incorporates new magnetic ring. (both one and two because this was suspected with speedo loss intermittently).
3. Cleaned up rear back plates to ensure no obstruction at sensor points.
4. Tested each wheel individually for consistent changes in signal between 0.7 and 1.75volts. All changes marked on the tyre in each case.
5. Brand new tyres all round the car, all identical
6. New Brake Discs and Pads fitted to front.
8. New rear left wheel bearing/hub.
9. Took out battery compartment, disconnected ABS loom plug and cleaned every terminal with wet and dry and applied contact cleaner.

Is there another test that I have missed here?

One last point -- Is there a possibility that air in the hydraulic system could be a contributing factor, as indicated in this article?
http://www.macfadyen998.plus.com/

Regards Larry

[larryr123]

As a general comment, it's time that Hall Effect sensing systems were scrapped, given the enormous difficulty and cost of resolving relatively un- testable problems related to these systems.

i.e. It does not have a method of reporting faults when they occur at the individual wheel level. At least not at a level that it accurately detects the fault and diagnoses it to the correct component. This causes huge loss of time and frustration with changing components unnecessarily.

In fact an erroneously functioning system (low speed ABS judder) goes unreported by the car's ECU system.

Regards Larry

[HarryM1BYT]

Originally Posted by larryr123

As a general comment, it's time that Hall Effect sensing systems were scrapped, given the enormous difficulty and cost of resolving relatively un- testable problems related to these systems.

i.e. It does not have a method of reporting faults when they occur at the individual wheel level. At least not at a level that it accurately detects the fault and diagnoses it to the correct component. This causes huge loss of time and frustration with changing components unnecessarily.

In fact an erroneously functioning system (low speed ABS judder) goes unreported by the car's ECU system.

Regards Larry

What you want there, is simply impossible to achieve. Software has no means of detecting the difference between that particular fault and genuine wheel slippage on braking. The same applied to the coil type sensor, as it does now to the Hall Effect sensors.

Note:- I appreciate one or two of the experts on this forum suggest that the ABS can log such faults, but I am very doubtful of its ability to log such a minor issue as you are complaining of. An earlier car of mine would log faults, but only if the fault appeared for an appreciably period - that fault I traced to a wheel sensor plug/socket which had suffered oxidisation due to moisture ingress.

In fact the Hall Effect sensors are much easier to test for the DIY mechanic, because they produce a nice easy to test pulse, no matter how slow the wheel is rotated and using nothing more complex than a voltmeter.

The coil sensors output becomes less and less, the slower the wheel is rotated. Tackling a similar fault on a coil system, I had to use an oscilloscope and it took me days to eventually find the issue.

Getting back to your car's problem, there are limited possibilities for the cause of the fault, which I have already explored with you. The issue will be one of those issues listed below....

1. Pulses are not appearing at low speed - due to rotor or sensor issue.

2. Some issue with the apparent speed of rotation of the wheel, as reported to the ABS by the sensor - due to some tyre, suspension, or poor wiring issue.

Keep in mind that the ABS does have to allow for some differences in wheel/pulse speeds, or periods, to cope with the normal braking process as the car goes around bends. Only when these limits are exceeded, does the ABS assume tyre grip has been lost and the ABS is triggered.

The engine ECU is much more complex and has many more fault codes it can generate, but even then the fault code needs some expertise to interpret the actual issue. The reported problem might be caused by something entirely unconnected to the actual report. I had a car which reported an idle speed which it could not properly control and the idle speed constantly varied. I initially thought an intake air leak and went round checking gaskets, looking for leaks etc.. and inspecting the IACV (Idle Air Control Valve). The later seemed fine, but only when I swapped it out, did the issue clear itself. Days of searching for the problem, solved in ten minutes when I got to the correct cause.

[larryr123]

"""2. Some issue with the apparent speed of rotation of the wheel, as reported to the ABS by the sensor - due to some tyre, suspension, or poor wiring issue."""

Harry w.r.t. suspension, the only detectable issue I had recently prior to the MOT in October was a broken rear spring, which was changed. This made no difference to the ABS issue. All new tyres, all balanced.

You are pretty confident this is not an issue related to contamination of the brake fluid. or air in the system?

Larry

[Mike Noc]

Originally Posted by HarryM1BYT

What you want there, is simply impossible to achieve. Software has no means of detecting the difference between that particular fault and genuine wheel slippage on braking. The same applied to the coil type sensor, as it does now to the Hall Effect sensors.

Note:- I appreciate one or two of the experts on this forum suggest that the ABS can log such faults, but I am very doubtful of its ability to log such a minor issue as you are complaining of. An earlier car of mine would log faults, but only if the fault appeared for an appreciably period - that fault I traced to a wheel sensor plug/socket which had suffered oxidisation due to moisture ingress.

There does appear to be an historic log for ABS operation Harry, which will flag up a plausibility fault if one particular wheel tends to trigger the ABS more often than any other.

Currently got this on my car - no alarms, but the ABS is activating at low speeds now and again. My ACR4 has picked up a plausibility fault for the offside rear wheel sensor.

As you say, there is no difference between a genuine fault of this type and wheel slippage, so the only way I can see it flagging up such a fault is to count the number of times it happens on each individual wheel, and if one is affected more than the others then it is shown as a plausible fault.

[HarryM1BYT]

Originally Posted by Mike Noc

There does appear to be an historic log for ABS operation Harry, which will flag up a plausibility fault if one particular wheel tends to trigger the ABS more often than any other.

Currently got this on my car - no alarms, but the ABS is activating at low speeds now and again. My ACR4 has picked up a plausibility fault for the offside rear wheel sensor.

As you say, there is no difference between a genuine fault of this type and wheel slippage, so the only way I can see it flagging up such a fault is to count the number of times it happens on each individual wheel, and if one is affected more than the others then it is shown as a plausible fault.

Thanks, that method of detecting a problem would make some sense.