Faulty Abs (activating at low speeds, brake + abs lights)

[Lyle]

A weird one for someone who's an expert or has had similar problems with this area of the car.

Essentially for the last couple of months my abs has been kicking in unnecessary when braking below about 25mph. I have just received the car back from my local garage after a week of attempting to find the fault, to no avail. They have told me they are completely stumped. At this stage I had no warning lights showing but when hooked up to diagnostics one of the abs sensors was showing a fault, so they changed this, problem persisted. They said one of the other sensors also looked a bit nackered although not showing faulty so they replaced that too. Issue persisted. Although the car passed its MOT recently they said they also replaced 2 of the wheel bearings as they looked a wee bit worn incase this was causing any problems but with no luck. Readings from the sensors were checked and all 4 wheels showing the same wheel speed so would not expect abs to activate. They came to the conclusion that the pump is the only thing left that could be playing up but weren't convinced so recommend since these are expensive that I live with it for a bit and see if it gets any better or worse as the car is completely safe other than the annoying feedback through the brake pedal at low speed.

For context, it failed its mot in July for uneven brake wear. When the garage got the car back from the Mot test centre they said my back left brake was sticking slightly so replaced that caliper. It was shortly after this had first been done that I noticed the feedback through the brake pedal (don't know if this is relevant or not).

For some extra complication, after having had this issue since the end of July with no warning lights whatsoever, today, about half an hour into my first drive since getting the car back, the abs, brake and airbag warning lights come on?? Don't know what to make of that. I have had airbag warnings before as the connection under my seat isn't great so it's possible that was just due to adjustment for the mechanics at the garage then back to me again but having 3 big lights flash at you in quick succession is a bit unnerving. Car was still driving perfectly though so suspect this to be some sort of electrical gremlin somewhere.

So basically, does anyone have any suggestions as to any potential solutions?
Also, out of curiosity being no expert myself, could the fact an abs warning light has now triggered actually help with fault finding because this may now show up on diagnostics?


(I should also add that this isn't a case of a garage taking me for a ride as is often mentioned with such issues because despite having replaced the 4 parts mentioned and had the car in the workshop for a week suggesting a significant amount of fault finding labour, they refused to take a penny from me because they had been unable to fix my problem).

Thanks in advance for any suggestions or comments

[Arctic]

Hi Lyle.
Did the garage say which wheel bearing they changed, now you have a light on you should try and get your car on a T4 which could help in finding the problem.

[vitesse]

Some aftermarket sensors are not up to the job. Our resident trader supplies genuine ones at the same price as the ebay ones: https://www.dmgrs.co.uk/products/rov...770e4987&_ss=r

I looked at the rear sensors as low speed activation is often a sign of problems with the rear ones, often due to contamination by rust flakes from the rear brake shields.

Arctic suggests a T4 diagnostic session if you can find a local garage, another solution is Toaf (use the search) freeware, and very easy to use. Both of these will pin point which corner is playing-up rather than the garage’s replace because it looks a bit worn approach. Might even be a Toaf user in your area who can help?

The SRS and ABS are two different systems, but T4 and Toaf can help resolve both issues.

Good luck

[Mike Noc]

Originally Posted by Lyle

Although the car passed its MOT recently they said they also replaced 2 of the wheel bearings as they looked a wee bit worn incase this was causing any problems but with no luck. Readings from the sensors were checked and all 4 wheels showing the same wheel speed so would not expect abs to activate.

So basically, does anyone have any suggestions as to any potential solutions?
Also, out of curiosity being no expert myself, could the fact an abs warning light has now triggered actually help with fault finding because this may now show up on diagnostics?

Yes that will help T4/TOAF to flag up a fault, if only for plausibility.
Whatever you do don't let them change the ABS pump unless they can prove it is faulty. The poke and hope method of ABS faultfinding where parts are replaced without proper diagnosis can easily increase the number of faults in the system.

Interestingly the wheel speeds are showing equal on the tests they have done, so maybe a very slight fault that isn't flagging up on their diagnostics.

I'd go back to basics: Courtesy of French Mike you can check all the ABS sensors and magnetic reluctors in the wheel bearings using a voltmeter.

The signal voltage changes from 1.7V to 0.7V as each magnetic segment passes the sensor, so jack each wheel in turn and slowly rotate it marking the switch points on the tyre wall with chalk. You should end up with 48 chalk marks looking something like this:



If any gaps are in the same place each revolution then likely a damaged reluctor, and if in random places suspect the sensor.


This was the rear wheel, and you can see the sensor was missing more than a few segments:



You connect the positive probe of the multimeter to the signal wire, and the negative to chassis earth:



I made up a cable from some old bell wire and a scrap ABS plug and socket to save pinning through the cable insulation. Here it is testing a hub prior to fitting:



If you are handy with the spanners you can do this yourself, or get the garage to do it for you. Also worth checking the ABS modulator plug pins to make sure they haven't corroded - not unknown on these cars.

[mileshawk56]

Not perhaps helpful from the diagnose scenario but if it reveals a faulty ABS pump and you are lucky and your car has no traction control fitted the price of a replacement unit is about £150.00 if you have traction control they are £1000.00 or £900.00. Chris S.

[Mike Noc]

I've sorted a fair few of these phantom ABS coming on at slow speeds, and none of them were due to a faulty ABS modulator.

Worn magnetic reluctors, a bit of rust behind a rear sensor lifting it away from the reluctor, cheap Ebay sensors switching at double the voltage they should and failing old sensors, but never the modulator. As long as the plug pins are in good condition I think it is rare for them to fail.

[Lyle]

Originally Posted by Arctic

Hi Lyle.
Did the garage say which wheel bearing they changed, now you have a light on you should try and get your car on a T4 which could help in finding the problem.

No, they didn't unfortunately and didn't think to ask. They just said that they had changed 2 of them.

The comment was made, that part of the problem was the car thought the system was working properly which made fault finding more difficult so I suspected a warning light may in an odd way be a good thing.

Out of curiosity, how much difference does a T4 make? They obviously have diagnostic kit that they've used on the car before, is a T4 going to provide significantly more information?

[Lyle]

Originally Posted by vitesse

Some aftermarket sensors are not up to the job. Our resident trader supplies genuine ones at the same price as the ebay ones: https://www.dmgrs.co.uk/products/rov...770e4987&_ss=r

I looked at the rear sensors as low speed activation is often a sign of problems with the rear ones, often due to contamination by rust flakes from the rear brake shields.

Arctic suggests a T4 diagnostic session if you can find a local garage, another solution is Toaf (use the search) freeware, and very easy to use. Both of these will pin point which corner is playing-up rather than the garage’s replace because it looks a bit worn approach. Might even be a Toaf user in your area who can help?

The SRS and ABS are two different systems, but T4 and Toaf can help resolve both issues.

Good luck

I recently stumbled across dmgrs myself as the other MOT failure was a shot shock absorber (which have also improved the ride massively I should add) and was really impressed with their service and prices so would not hesitate to use again.

As I'm sure you can appreciate given, they have been good enough to do all the work to date for free, I would be very reluctant to start accusing them of using inferior quality parts.

I don't know whether they looked specifically for rust flakes etc, I would guess given they replaced a sensor because it looked a bit knackered, that they would have picked up contamination at this point but might be worth me asking.

[stevestrat]

Originally Posted by Lyle

Out of curiosity, how much difference does a T4 make? They obviously have diagnostic kit that they've used on the car before, is a T4 going to provide significantly more information?

The T4 is specific to MGR vehicles and will show information that a generic diagnostic system may not. The problem would be finding a T4 system near you, the nearest I know of is Dreadnaught up in Callander.

PS Although he doesn't have a T4 I'd have a word with Dunc (Cossie Dunc), he has a garage out in Whitburn. Trader on here and a bl00dy good mechanic.

[Lyle]

Originally Posted by Mike Noc

Yes that will help T4/TOAF to flag up a fault, if only for plausibility.
Whatever you do don't let them change the ABS pump unless they can prove it is faulty. The poke and hope method of ABS faultfinding where parts are replaced without proper diagnosis can easily increase the number of faults in the system.

Interestingly the wheel speeds are showing equal on the tests they have done, so maybe a very slight fault that isn't flagging up on their diagnostics.

I'd go back to basics: Courtesy of French Mike you can check all the ABS sensors and magnetic reluctors in the wheel bearings using a voltmeter.

The signal voltage changes from 1.7V to 0.7V as each magnetic segment passes the sensor, so jack each wheel in turn and slowly rotate it marking the switch points on the tyre wall with chalk. You should end up with 48 chalk marks looking something like this:



If any gaps are in the same place each revolution then likely a damaged reluctor, and if in random places suspect the sensor.


This was the rear wheel, and you can see the sensor was missing more than a few segments:



You connect the positive probe of the multimeter to the signal wire, and the negative to chassis earth:



I made up a cable from some old bell wire and a scrap ABS plug and socket to save pinning through the cable insulation. Here it is testing a hub prior to fitting:



If you are handy with the spanners you can do this yourself, or get the garage to do it for you. Also worth checking the ABS modulator plug pins to make sure they haven't corroded - not unknown on these cars.

Thanks Mike for the very thorough step by step. Unfortunately, I'm not massively "handy with the spanners" although that gave me a smile. I'll attempt cosmetic type fixes or modifications myself but as you also say, with something like this I'd fear inadvertently increasing the number of faults in the system so any work is for someone with more expertise in this area than me. I'll pass the suggestions on incase there is anything they haven't thought to try however.

With regards to the abs pump I agree and have essentially reached this situation because they weren't convinced it was at fault and that these rarely fail but felt they had tried the other avenues.