W124 ABS fault very annoying

[LINDS HANSON]

Hi we recentley bought a 95mwy E220 estate the ABS light was on and we stupidly thought it would be a simple OVP fault or at worse a speed sensor issue. The light is on at all times even with the car staitionary, it flicks off for a second when the ignition is turned on then immediatley returns and stays on. We have swapped the OVP relay with our other car, swapped the ecu, swapped the pump unit , swapped pump and main relays, replaced rear speed sensor because resistance was at the low end of tolerence all made no difference. I have checked voltages and earth resistances at the ecu multiplug all appear to spec, if the main relay is removed the light goes out. the wiring between pump and ECU tests good for continuity. Am I right in thinking that the speed sensor's [which all test out OK anyway] do not have any effect on the warning light until the car is moving? and what needs to be done to clear a fault from the memory once repaired? What does the wire from ABS to engine ECU do? This really is bugging me now any ideas would be most welcome

 

[grober]

Since you have checked most of the " units" by substitution it tends to point to an odd voltage supply fault or intermittant wiring fault Have you checked the alternator output? Might be worth changing the regulator pack/brushes? Check all the earth points are good especially the one from the ABS hydraulic unit to chassis/engine--check the engine earth strap also. When you checked the wheel sensor wiring did you get someone to wiggle the sensor wires while taking readings ?
Just a few suggestions.

 

[LINDS HANSON]

Thanks for quick reply, the alternator puts out 14.5v and the ECU is seeing this on the blue red voltage sensing wire, I cleaned all the earths on the ns suspension tower and the pump to chassis leg. I know if all these are OK then it should work thats whats so annoying. On my other car the light comes on briefly when the ignition is turned on then goes out, then returns during cranking and then goes out once running. But this one never goes out.I have even removed the ecu cover and checked the voltages on the curcuit board just incase the multiplug was at fault. Should the fault be erased once the ignition is cycled. The other car can have the speed sensors disconnected and the light does not come on until the car is driven.

 

[Ian B Walker]

On the ABS pump there is a black cover held in with a torex screw. Remove this cover and check the connections under it and also the relay.

 

[LINDS HANSON]

Hi Ian, I have already replaced the pump and relays which made no difference, does the car have to be driven to clear fault codes?

 

[coupe deville]

Have you seen this old thread from t'other side,
w124 abs - Mercedes-Benz Owners' Forums
you seem to have been very thorough so can i assume all the fuses are in good order and not 17 years old, and the holders are clean ?
I would swap out the brake pedal switch as well [ desperation mode ]

 

[carat 3.6]

Could it be that you dont have a fault with the abs system, but a faulty light control unit? I think it is this relay that brings all the dashboard lights on as a test, when you first turn the ignition on.

I could be wrong though.

 

[LINDS HANSON]

Thanks for all your combined help on this chaps, just reassembled everything back into the car hoping for a miracle cure but still no joy. Drove the car for a mile in the hope that it may reset but alas no. Will double check the brake switch though [ the brake lights do go on and off when the pedal is pressed/released] I still think it must be a voltage issue because the light just flicks off for a second and then returns even without starting or moving the car?

 

[balge]

Only thing that spring to mind is still a duff OVP/fuse...are you sure the one you swapped in was good? Tested for 12V?
Can't find a specific diagram, is this one close?
cheers

 

[vtechnician]

I have the same problem on a w124 200.Found that the left front sensor was faulty.Replaced the sensor and the light still stays on after starting when the car is stationary.
Tested the harness for the whole ABS system,tested brake switch etc.
Exactly the same problem as you are having and all is tested before reading this thread.
From the diagram that I have, I see that the ABS light constantly gets power through a fuse.
Thus,the only solution that I could come up with is that the earth wire for the light is constantly joined to earth.

Did you road test the car to see if the ABS works or not.I did and it doesn't work.
Also make sure that the bulbs are not swapped around behind the cluster.i.e winscreen washer/abs lights.

The client never really complained about the ABS,I had some extra time and decided to try and troubleshoot it as I have repaired quite a few ABS issues on w124's.
Client picked up the car and I cannot check this further.
Will try and get a diagram up here for you soon.

 

[LINDS HANSON]

Hi sorry for delay in getting back, the light goes out if the main relay is pulled so it would appear that everything bulb wise is OK, tried the brake light switch but it only has two wires so only contacts when the pedal is pressed and the brake lights work and I get 12V at the ECU when the pedal is pressed. I roadtested the car and the ABS does not function, I mentaly ruled out the front speed sensors as when I unplugged these on our donor 124 the ABS stayed off until the car was moved, but the light stays on from start up on this car with just a brief flick off and then back on. I hope to try a used loom this week in the hope that this can be put to bed. cheers all I will update on the saga soon

 

[grober]

Have you checked for an AC component on the alternator output- might be worthwhile swapping the regulator pack/alternator as a test as you seem to have access to a donor vehicle?

 

[bolide]

I'd get the car in the air, spin the wheels in turn and measure the voltages coming off the ABS sensors. If the voltages are sensible I'd look at the waveform on a scope if possible. I'd want to measure at the ABS brain to test the cabling as well as the sensors

This will help tie down where the fault is

Nick Froome
www.************

 

[joe prosser]

A few years ago my wife reversed her estate into the drive and due to the front diff mounting bolt shearing she managed to spin her diff free of the prop end coupling. This event stretched the rear speed sensor connecting cable to the extent that the internal wiring snapped although the exterior insulation remained intact. Even when everything was reinstated the ABS light stayed on permanently until I reasoned that replacing the suspect speed sensor module would solve matters. I see that you have replaced the rear speed sensor and all I can suggest is that your next step would be to swap out the speed sensors in the front wheel hubs one at a time. As I understand it the ECU monitors the pulses from the diff and both front wheel hubs and this will cause the the ABS warning light to come on if there is a lack of broad agreement in the pulses. If just one of your front sensors is dodgy then the ABS light will light up.

 

[frodes]

Old tread, but it comes out high when googling for W124 ABS issues.

Did you find a solution to this?