ABS Harness

[glam99]

Chasing a rolling ABS fault, does anyone have a pinout of the ABS harness?
This is what I've done so far:
1) Tested ABS sensors for continuity, good
2) Tested ABS sensors for MV, good
3) Replaced ABS control unit, no change
4) Replaced ABS relay, no change
In particular looking for red, yellow, green and white wires from the ABS sensors to the control unit. Thanks!

[lifeiskaos]

http://wedophones.com/BMWManualsLead.htm

[jlevie]

Are you saying that the ABS initializes (light on w/ignition, off on engine start) but then the light comes back on when the car is moving? If so I'd suspect a bad speed sensor. A simple check for that is to disconnect all but one sensor. If the ABS light comes on when the car is driven that sensor is producing a speed signal and is good. Repeat the test for each of the other sensors. The bad sensor(s) won't produce a valid speed signal and thus the ABS module won't know the car is moving and thus and won't turn the light on for missing speed signals.

[glam99]

Are you saying that the ABS initializes (light on w/ignition, off on engine start) but then the light comes back on when the car is moving? If so I'd suspect a bad speed sensor. A simple check for that is to disconnect all but one sensor. If the ABS light comes on when the car is driven that sensor is producing a speed signal and is good. Repeat the test for each of the other sensors. The bad sensor(s) won't produce a valid speed signal and thus the ABS module won't know the car is moving and thus and won't turn the light on for missing speed signals.

Yes that is what was happening. Yes, I suspected bad/dirty speed sensors/dirty pulse wheels. Turns out the P.O. had put in 3 new sensors, so I put in another known good sensor and cleaned up the pulse wheels and got microvoltage at the sensor while spinning the tires. I still kept getting moving faults so I replaced the ABS control unit with 3 different units without change. Thinking that the ABS control control unit was not seeing one or more sensors, I tested each ABS sensor for continuity at the harness and didn't find anything wrong. Getting desperate, I changed the two relays on the ABS pump and bingo, light is now out. Testing further, I found that the pump relay was the problem and opening it up saw that the contacts somehow got knocked out of alignment, after realigning the contacts put this pump relay back in and it now works correctly too. Never thought that a bad relay on the ABS pump could cause a moving fault but now I know that it can, BTW, I like your method of testing the sensors, thanks!