ABS Problem MG ZT-T

[David Lawrence]

To get live data showing the fault would be invaluable, so it has to be worth it after such a long time trying to solve it.

My own experience of faults are that they can pass static tests, and work dynamically for a while, but then come back. Sometimes it shows the abs light, sometimes no light and a pulsing pedal, but each time i have faced it, it has always been down to sensors. I had a real struggle with aftermarket sensors. Brand new from a reputable source but gave intermittent faults, the only solution ended up a genuine MGR semsor. £60 a sensor, rather than £15 but it worked. At this stage of the game with so many parts having been changed, the T4 dynamic testing has to be the way to go.

[Mike Noc]

One interesting fault I had was a bit of rust behind the OSR sensor mounting, which had the effect of increasing the air gap slightly between the sensor and magnetic reluctor in the bearing.

Took some time to track it down as it wasn't easily visible, and the fault it caused was intermittent.

[larryr123]

Mike, How did you diagnose to that specific wheel, with the rust spot on the backplate, that was causing the problem, or was it just trial and error until you found it?
My experience of looking into the 4 wheels. is that there can not be a gap issue on the front wheels, as the Hall Effect reading is taken from the side of the sensor which fits tight into the front hub slot. There is no possibility of any lateral movement of the sensor, assuming you replace the sensor with one of the same length.

For the rear sensors, provided the hub sits flush on the back ring, the gap again can't really vary. The only possible disruption is perhaps if there is excessive dirt on the reluctor ring or around the end of the sensor protruding through the back plate.

It's a difficult problem to solve as the T4 test, as many on here have indicated, doesn't offer a difinitive indication as to where the fault originates from. I've found this from the original dynamic test from Soul's garage in Olney and from a seperate static test when replacing the front wheel bearing.

Larry

[David Lawrence]

Originally Posted by larryr123

Mike, How did you diagnose to that specific wheel, with the rust spot on the backplate, that was causing the problem, or was it just trial and error until you found it?
My experience of looking into the 4 wheels. is that there can not be a gap issue on the front wheels, as the Hall Effect reading is taken from the side of the sensor which fits tight into the front hub slot. There is no possibility of any lateral movement of the sensor, assuming you replace the sensor with one of the same length.

For the rear sensors, provided the hub sits flush on the back ring, the gap again can't really vary. The only possible disruption is perhaps if there is excessive dirt on the reluctor ring or around the end of the sensor protruding through the back plate.

It's a difficult problem to solve as the T4 test, as many on here have indicated, doesn't offer a difinitive indication as to where the fault originates from. I've found this from the original dynamic test from Soul's garage in Olney and from a seperate static test when replacing the front wheel bearing.

Larry

The sensors can move at the front, mine did that. I drove around on one for a few hundred miles then had the pulsing problem, followed by the light come on a while later. When checked i found the sensor had pushed slightly away from the hole, as it was an aftermarket one for some reason it just didnt quite fit right and after time it pushed itself slightly out.

My theory with these things is when a problem arises just after doing some work, that is the wheel to concentrate on.

When i had the pulsing on a back wheel, it was the wheel where i had just adjusted the handbrake. Sure enough, rust from the backplate had damaged the sensor on that wheel as I had disturbed it.

Front wheels are easy to determine, right one and the soeedo fails, left one and the cruise fails.

[Mike Noc]

Originally Posted by larryr123

Mike, How did you diagnose to that specific wheel, with the rust spot on the backplate, that was causing the problem, or was it just trial and error until you found it? Larry

I've got a Sykes Pickavant ACR4 with the latest Rover pod and that flagged up the OSR sensor (if I remember correctly it came up as a plausibility fault). From there I used the chalk and voltmeter test and found that, at very slow revolutions the sensor missed the odd random segment.

The sensor had recently been replaced, so I took it out and tested it by hand with another bearing and it was fine. So thinking the air gap may be a problem I checked the rear of the backplate and lo and behold a raised flake of rust was found which was preventing the sensor from seating correctly.

[HarryM1BYT]

Originally Posted by Mike Noc

I've got a Sykes Pickavant ACR4 with the latest Rover pod and that flagged up the OSR sensor (if I remember correctly it came up as a plausibility fault). From there I used the chalk and voltmeter test and found that, at very slow revolutions the sensor missed the odd random segment.

The sensor had recently been replaced, so I took it out and tested it by hand with another bearing and it was fine. So thinking the air gap may be a problem I checked the rear of the backplate and lo and behold a raised flake of rust was found which was preventing the sensor from seating correctly.

Spot on

What must be understood, is that a complete failure to generate the pulses, will trigger the ABS warning light [EDITED -] and disable the complete ABS system, but the odd few missed pulses, which usually happens at low speed, will trigger the ABS into operation with the judder because it thinks that tyre is sliding on the road surface. It is doing what it is designed to do.

Which is where the use of a volt meter (or my alternative, of a piezo sounder or earpiece) combined with rotating the wheel slowly and marking the tyre for each click, doing it repeatedly to ensure no clicks are missed, is the way to do it when all else has failed to find the issue.

[Mike Noc]

Originally Posted by HarryM1BYT

Spot on

What must be understood, is that a complete failure to generate the pulses, will trigger the ABS warning light, but the odd few missed pulses, which usually happens at low speed, will trigger the ABS into operation with the judder because it thinks that tyre is sliding on the road surface. It is doing what it is designed to do.

Yes, the only difference is if it is the same wheel every time then this will show up in an historic log and should get flagged as a plausibility fault, in as much that it isn't plausible that the same wheel will keep locking up during normal driving.

I'm only guessing mind - I don't know this to be a fact.

[marinabrian]

Originally Posted by larryr123

Hi Guys, Thanks for the response.
Given that I've tried replacing so many components, I'm really looking for a definitive method of testing and determining where the fault lies.

Clearly there is no warning light, so I'm assuming the ABS controller and the sensors are all now functional -- or is that a wrong assumption?

If I disconnect the ABS the low speed judder goes, so it's definitely ABS related.
A T4 Tester is useless as it doesn't tell you anything about where the problem lies and the chalk and meter test on each of the 4 wheels is inconclusive.

I've so far :-
1. Changed front right sensor and fitted a new wheel bearing, this extinguished the warning light and returned the speedo again -- so I'm now assuming this wheel is OK.
2. Changed both rear hubs in 2014 and changed the rear nearside again this year. Also cleaned all rust off back plates.
3. Fitted new tyres all round, two new front discs and pads, and replaced a new rear coil spring.
4. Took out the battery component and checked the controller plug was dry, and cleaned all the individual pins before reconnecting.

The car still passed the MOT with the low speed judder, because there is no ABS warning light lit.

I'm stumped and would really like to give the car to someone who knows how to fix it. !!

Regards Larry

With due respect you are throwing parts at the job and ignoring my advice, as well as dismissing as useless a diagnostic system specifically designed for the diagnosis of your car

T4 is only as good as the person operating it, and their skill in interpreting the results it is providing that operator.

Contact Phil-T4 and he will be able to correctly diagnose the fault present on your car, and repair it too

So stop messing about trying to repair it yourself, it would appear you are out of your depth and running around in circles

Brian

[larryr123]

"So stop messing about trying to repair it yourself, it would appear you are out of your depth and running around in circles "

Quite the little charmer aren't you Brian.
I wonder should you be in business at all with an attitude like that.

Both the T4 users were recognised garages, one with a good history of selling and servicing Rover and MG cars.

Larry

[HarryM1BYT]

Originally Posted by larryr123

"So stop messing about trying to repair it yourself, it would appear you are out of your depth and running around in circles "

Quite the little charmer aren't you Brian.
I wonder should you be in business at all with an attitude like that.

Both the T4 users were recognised garages, one with a good history of selling and servicing Rover and MG cars.

Larry

I rather think Brian is trying to avoid you wasting any more money wildly chucking parts at it. You have a basically simple problem to solve, yet you would seem you are going round in circles to find the cause and not even properly understanding how the ABS operates. You are not alone in this, many garages also misunderstand how it works too and struggle - as you have found.