ABS failure? Brake releasing and grinding 87 560SL

[Johnhazen]

Occasionally it feels like the ABS is releasing my front left brake and making it pull slightly to one side while also making a grinding noise. It happens just at the last moment when the car comes to a complete stop. It does not do this during regular braking at speeds above 2-3 mph. It usually happens most often in the mornings after the car has been sitting all night. Information: (Not always, but more often than not, the ABS light comes on after driving for a while) I have had the sensors cleaned and supposedly tested and the shop says the front ones are ok, but the rear sensor "appears" to be bad. Before I replace it (not cheap and hard to find) I wonder if there is some way to verify that it really needs it and that it really is the cause of this problem. It seems strange that a bad rear sensor would affect just the front left brake. Is this possible? More information: (Brake front pads are new and so are the rotors and calipers) Of course it could still be something wrong with that particular caliper, but the shop that did the brake job seems to think it's the ABS causing the problem. Can anyone provide the part number for the rear sensor? Thanks

[Frank Reiner]

Suggestion:

Disable the ABS by unplugging the connector from the control unit (the ABS light will stay on continuously).
Drive the car in this condition for a few days.
Any change in the symptoms?
No? Problem is not ABS related.
Yes? Continue investigating ABS functions.

Disabling the ABS does not affect brake performance; it will be as though the car were not even equipped with ABS.

[Johnhazen]

I was thinking to try that, but was not sure. Thanks for the advise, I will post back. The connector is under the black plastic cover? Does pulling the smaller relay do the same thing? Haven't removed the cover yet. Which would be easier, the relay or the connector?

[Frank Reiner]

If you are looking at a schematic, and the relay supplies only the ABS, then pulling the relay should be equivalent.

If there is a ABS sensor fault, the rear one will not likely affect only one front wheel; it would affect overall front/rear balance. The sensors are very simple devices; a magnet with a coil wrapped around it. As a tooth passes the magnet the magnetic field is altered, and that change in the magnetic field induces a current in the coil. the current is sensed by the control unit. Sensor faults are usually in the leads, rather than in the coil.
Electrical continuity of the assembly can be checked with an ohmmeter, and wiggleing/pulling the leads.

There is a used rear on Ebay for $19.99

[Johnhazen]

Looking at the picture in my CD manual it looks like there are two relays under the black cover. The smaller one is for the solenoid valve and the larger one is for the return pump. Probably either one willl disable ABS. I just noticed you said to pull the connector from the control unit not the hydraulic unit. Is that accessible by removing the fuse box cover or is it more involved than that? I did not find the Ebay listing you mentioned. The only ones for 19.99 were for a different model car. Thanks for your input on this problem.

[Frank Reiner]

The control unit is a box 7.5"x3.5"x2" Bosch P/N 0 265 101 016 In the 126 chassis it is located in the "double firewall" area just forward of the cowl. I confess I am uncertain of the location in a 107; perhaps in the right kick panel, or above the right footspace, if it is not in the engine room. The wire bundle is about 1/2" in diameter; tracing from the accuator may help.

The Ebay listing was for a SEL/SEC; the rear end center piece is the same in the SL, and the sensors will interchange. The sensor is mounted in the diff. and the trigger wheel is on the pinion.

[Johnhazen]

I found the easiest way to disable the ABS was to remove the black plastic cover on the top of the ABS pump unit. Requires a small torx tip screw driver or socket. Then I pulled the relay out. (The small silver one). This morning the ABS light came on and stayed on as expected due to the ABS unit being disabled. The symptoms in my first post went away. No grinding. No brake releasing. I am going to drive it some more to be sure, but it looks like a malfunctioning ABS system. Now to determine which wheel sensor is causing it. I too cannot believe the rear sensor would cause one front wheel to act up.

[Johnhazen]

It has been several days now with the ABS disabled and I have no more grinding, brake release symptoms. Just a yellow ABS light on all the time. So now do I fix it or put electrical tape over the idiot light. I live in AZ and am not likely to be driving in snow or even rain for that matter. Decisions, decisions... I still think it has to be the front left sensor making the ABS think the left wheel is locking up and thus releasing the pressure accordingly.

[Frank Reiner]

I agree with your analysis that a front sensor (and more likely, the flexing leads from the sensor) is the culprit. Your experience replicates mine. I'm still driving w/o ABS and do not feel imperiled.