ABS Mystery

[xpc316e]

I hope someone out there can help with my ABS problem.

My car is a 1990 S124 300E 24 valve automatic and it has a yellow ABS light that comes up on the dash when the speed rises above about 5mph. If kept below this speed, the ABS light does not illuminate. The car brakes as it should do with none of the shuddering/pedal fluctuation within the last few metres of stopping, as sometimes occurs.

I have just collected the car from an almost 2 month stay at a Mercedes Benz specialist where they were unable to diagnose the fault. Here is a list of the work done: rear ABS sensor replaced as wiring was degraded and sensor seized in differential housing, front nearside ABS sensor replaced as it was producing a low voltage (0.5v) when the wheel was spun by hand, ABS control module temporarily replaced with one from a car with no fault, all relays replaced temporarily with ones from a car showing no faults, and all wiring to sensors found to be OK with no breaks in continuity (confirmed by Ohm meter tests). The front reluctor rings have been cleaned. Basically, apart from the ABS pump, the system has been replaced with either new parts, or ones known to be okay from a working car. All brake pads are almost unworn and fluid is fresh.

I am not an ABS expert by any means, but the way in which the ABS light comes on only when a certain speed is reached to me suggests that whatever is wrong is sensor-related, so I am reluctant to replace the ABS pump.

Has anyone out there had similar issues? If so, what was the solution to the problem?

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[Bellow]

Have the tyres been checked re rolling radius - tread depth and pressures?
That the light comes on only when moving very heavily implies that what is being read is incorrect to the ECU. Electrical problems will illuminate the lamp at key-on.

 

[tonysmb]

Have the tyres been checked re rolling radius - tread depth and pressures?
That the light comes on only when moving very heavily implies that what is being read is incorrect to the ECU. Electrical problems will illuminate the lamp at key-on.

Adding to Bellow's suggestion re rolling radius, are there 2 matched tyres (make/type) on each axle (not all makes of same-size tyres are the same physical rolling radius). Years ago, the space-saver triggered the ABS light on one of my older cars (1990s)

 

[xpc316e]

Thanks for the suggestion regarding tyres: it has four new Dunlops, of identical size, at the same pressures.

 

[Bellow]

The front reluctor rings have been cleaned.

And thoroughly checked for splits?
Rears? Presumably inside the diff and unlikely to have corroded (and split)?

I am not an ABS expert by any means, but the way in which the ABS light comes on only when a certain speed is reached to me suggests that whatever is wrong is sensor-related, so I am reluctant to replace the ABS pump.

I agree. Even a sensor problem would illuminate the light at key-on. It is IMO the signals that the ECU are receiving that is at the root of this.
Going slightly leftfield... No possibility of a sensor wire picking up a stray signal? (Porsche 964 with twin distributors under acceleration and engine movement due to torque reaction brought the distributors and ABS system close enough for the HT voltages to contaminate ABS signals).

 

[Jobsworth]

OVP relay?

 

[ChrisPDuck]

Here is a list of the work done: rear ABS sensor replaced as wiring was degraded and sensor seized in differential housing, front nearside ABS sensor replaced as it was producing a low voltage (0.5v) when the wheel was spun by hand, ...

Have you looked at live data while the car is moving? You say that all parts have been replaced or substituted, but I'm only seeing two ABS sensors replaced. It's quite possible that one of the other two is giving a faulty reading that disagrees with the others, even though voltage/wiring etc are ok, and this would be enough to trigger a fault. And I could believe that the tolerance for differences might be defined in absolute mph (or wheel ticks), rather than a percentage, hence it would not trigger below a certain speed.

 

[Jobsworth]

Only 3 sensors on a w124. One on each front wheel and one on the differential. Thinking of which, I’m sure it counts on the pinion shaft. Hasn’t had a differential swap with a different ratio?

 

[ChrisPDuck]

Only 3 sensors on a w124. One on each front wheel and one on the differential. Thinking of which, I’m sure it counts on the pinion shaft. Hasn’t had a differential swap with a different ratio?

Ah, like the man in the orthopaedic shoes, I stand corrected. Ok, is it possible that the offside front sensor is giving a faulty reading?

 

[Smiley]

I believe that the ABS system is designed to disengage below 5mph.
Check the surfaces where the ABS sensors sit on the hub and make sure that they are clean - if there is a build up of rust it can hold the sensor sufficiently far away from the reluctor ring to trigger an ABS light.