The dreaded ABS/ASR light fault

[vivijiv]

Hi I have just joined this forum hoping to get some direction on my problem.I have a 1991 500sl with acceleration slip regulation(ASR).
My problem is that the ABS/ASR light comes on when driving approx.30metres and stays on.This started ocuring after I had the vehicle serviced which included a brake check.
I have cleaned the front wheel sensors,replaced the brake switch and replaced the overvoltage protection relay but the problem still persists.I have disconnected and charged the battery,I have turned the steering full lock left and then right but with no luck.
The only sensor that I have not been able to find is the rear one supposedly on the differential.
Can someone point me in the right direction as to where this sensor is ,A drawing or picture will be useful.
Also are there any other reasons for this fault to persist and is there a test that one can do to check these sensors?
Thanks,
Vivijiv

 

[Richard Moakes]

ASR on that car has no diagnostics, just a MIL light

However, the ABS can be diagnosed using a blink code reader connected to the diagnostic connector in the engine compartment.

Given the principle of look for the fault where something has changed, I would tend towards a problem with the front wheel sensors, wiring or hubs.

 

[wireman]

From the way the fault appears after the car moves its likely to be a sensor, either electrically disconnected/shorted due to a damaged cable or the toothed wheel rusted away.

The rear sensor is in the right hand side of the diff nose on my cars with ABS only, but I was under the impression that ASR had two rear sensors, one on each drive shaft or hub, there must be somebody out there who can confirm this and provide a location.

The sensors should have a resistance of around 1000 ohms and produce >100mV ac when the wheel is spun by hand.

 

[kth286]

ASR will use a sensor at each wheel.

As ASR uses a refinement of the ABS system, you need to check the ABS/ASR hydraulic control unit in engine compartment.

 

[television]

A faulty OVP relay can cause this, I do have quite a lot of info on this if you would like it or need it

 

[Richard Moakes]

ASR will use a sensor at each wheel.

As ASR uses a refinement of the ABS system, you need to check the ABS/ASR hydraulic control unit in engine compartment.

Not much to check I am afraid, they do share a hydraulic block and sensors, but that version of ASR is analogue diagnostics only, you have to measure voltages and pressures.

 

[rayhennig]

My problem is that the ABS/ASR light comes on when driving approx.30metres and stays on.This started ocuring after I had the vehicle serviced which included a brake check.

I had a similar ASR light problem on my 1991 300CE just after the warranty (from the MB dealer I bought it from) ran out. MB told me that they might have to replace the whole ABS/ASR unit at some vast price but they promised to make efforts.

Anyway, they finally told me that they'd bled the entire system several times and all seemed well. Touch wood, all is still well 15 years later.

I've just remembered, just before the ASR light would come on, there was a pulsing or throbbing noise from the front left (the ABS/ASR unit?) and that would stop as the light came on. I would always know if the light was about to come on because of that noise.

This may be irrelevant to you but good luck nonetheless.

RayH

 

[vivijiv]

I am new to this so I hope this message is seen by all that responded to my problem.
Thank you one and all for the information .My first port of call is to follow Wiremans thoughts and check the sensors with a multimeter.
Would anyone have some information on how these wheel sensors operate.
The mystery is also how does the ASR operate if there are no sensors on the rear wheels.Does anyone have any information on how the ASR operates.
Finally on the wheel sensor up at the fixing flange there is a rubber seal,does this have any significance in the operation of the sensor?
Sorry about all the questions .
I look forward to some answers.
Thanks
Vivijiv.

 

[kth286]

I am new to this so I hope this message is seen by all that responded to my problem.
Thank you one and all for the information .My first port of call is to follow Wiremans thoughts and check the sensors with a multimeter.
Would anyone have some information on how these wheel sensors operate.
The mystery is also how does the ASR operate if there are no sensors on the rear wheels.Does anyone have any information on how the ASR operates.
Finally on the wheel sensor up at the fixing flange there is a rubber seal,does this have any significance in the operation of the sensor?
Sorry about all the questions .
I look forward to some answers.
Thanks
Vivijiv.

AS I said in my previous post above, there are sensors for all four wheels.

The two rear ones are in the diff, and measure each axle shaft rotation speed.

The action of the ASR is two-fold depending on the situation and speed etc:

* torque reduction via the electronic drive by wire throttle, which throttles back the engine.

* braking of individual wheels depending on what the sensors are saying is wrong.

You say the brakes have recently been attended to.

What was the problem with the brakes ?

Was it a specialist Mercedes garage that dealt with it ?

There is a special procedure with bleeding of brakes on cars with ASR using a special loom.

Also there is a bleed point (called SP) within the ABS/SRS block in the engine bay, under the plastic cover

 

[Richard Moakes]

AS I said in my previous post above, there are sensors for all four wheels.

The two rear ones are in the diff, and measure each axle shaft rotation speed.

The action of the ASR is two-fold depending on the situation and speed etc:

* torque reduction via the electronic drive by wire throttle, which throttles back the engine.

* braking of individual wheels depending on what the sensors are saying is wrong.

You say the brakes have recently been attended to.

What was the problem with the brakes ?

Was it a specialist Mercedes garage that dealt with it ?

There is a special procedure with bleeding of brakes on cars with ASR using a special loom.

Also there is a bleed point (called SP) within the ABS/SRS block in the engine bay, under the plastic cover

On my '91 500SL I recently bled the ASR/ABS block

No special loom is required on these early cars, the system is the same as was fitted to late W126 cars. Once you release the pressure on the SP, the precharge pump kicks in and then the main ABS pump kicks in to compress the gas in the accumulator located next to the ABS/ASR block.

You are correct in that the ASR/ABS block can brake the individual wheels, however the electronic throttle is the cruise control actuator acting in the reverse direction which manipulates the linkage to the mechanical butterfly control of the KE throttle plate.

I have the full bleed and diagnostic procedures for this system and I followed them to the letter.

I believe a blink code reader to diagnose if ABS has an issue with a sensor would be a cheap way forward and is how I would proceed with such a fault.

 

[vivijiv]

On my '91 500SL I recently bled the ASR/ABS block

No special loom is required on these early cars, the system is the same as was fitted to late W126 cars. Once you release the pressure on the SP, the precharge pump kicks in and then the main ABS pump kicks in to compress the gas in the accumulator located next to the ABS/ASR block.

You are correct in that the ASR/ABS block can brake the individual wheels, however the electronic throttle is the cruise control actuator acting in the reverse direction which manipulates the linkage to the mechanical butterfly control of the KE throttle plate.

I have the full bleed and diagnostic procedures for this system and I followed them to the letter.

I believe a blink code reader to diagnose if ABS has an issue with a sensor would be a cheap way forward and is how I would proceed with such a fault.

Hi Richard,
Thank you for the feedback.I believe you and David have hit the nail on the head with regards to the bleeding procedure.The garage where I had the brakes worked on was not a MB garage and I think the bleeding procedure followed after changing the brake fluid was not correct,and is the cause of my problem.
Please can you send me the bleeding and diagnostic procedures for this system.My e-mail address is gericke@netactive.co.za
I know about the blink code reader and how to build one but not sure how to use it.
I really appreciate the input that I have enjoyed from this Forum and thank you .If I can solve this headache it will be a real victory.
Kind Regards,
Peter

 

[vivijiv]

Hi Richard ,
Sorry to bother but is it possible to send the Brake bleeding procedures that you mentioned you had.
I would like to have the brakes checked out to see if I can get rid of these burning lights.
Thanks,
Peter

 

[vivijiv]

A faulty OVP relay can cause this, I do have quite a lot of info on this if you would like it or need it

Hi Television,
I have replaced the OVP relay ,but any information is useful.I would appreciateit if you could forward it to me via e-mail.My e-mail address is gericke@netactive.co.za.
Thanks,Peter

 

[Richard Moakes]

Hi Richard ,
Sorry to bother but is it possible to send the Brake bleeding procedures that you mentioned you had.
I would like to have the brakes checked out to see if I can get rid of these burning lights.
Thanks,
Peter

Hi Peter,

I am currently away from home, will send them over on Wednesday.

Richard

 

[vivijiv]

Hooray

At last the dreaded ABS/ASR light saga has been solved.
After spending 6 hours at an Auto Electrician the fault was diagnosed as a faulty right front solenoid valve.This was eventually traced to a broken wire inside the insulation.
My thanks to Richard Moakes,Wireman.kth286 and television for their input and guidance.
The lesson learned is that it is good to try and solve problems yourself as you get a better understanding of your car but in the final analysis, find yourself a good auto electrican and you can save yourself time and money.

 

[kth286]

Forum members like nothing better than feedback on problems solved - so thanks for that.

 

[Richard Moakes]

At last the dreaded ABS/ASR light saga has been solved.
After spending 6 hours at an Auto Electrician the fault was diagnosed as a faulty right front solenoid valve.This was eventually traced to a broken wire inside the insulation.
My thanks to Richard Moakes,Wireman.kth286 and television for their input and guidance.
The lesson learned is that it is good to try and solve problems yourself as you get a better understanding of your car but in the final analysis, find yourself a good auto electrican and you can save yourself time and money.

Whereabouts was the broken wire? It would be good to know for reference.

 

[vivijiv]

There were two faults.The one was at the abs control unit a bad connection to one of the solenoids.Apparently a pin in the connector dropped out and was not making proper contact. Secondly on the Right hand front sensor between the sensor and the first connector there was an open circuit and they found that the one wire was broken inside the insulation.
I did check this visually but I did not pick up any broken wire?

 

[wireman]

The sensor wire fault is typical of the sensor cable being pulled excessively hard, the insulation stretches and the wire breaks, sometimes there is intermittent contact afterwards making it hard to pin down. Bend and flex the cable while performing a resistance check to seek out this problem.

Glad you got it sorted and thanks for the feedback.