ABS troubleshoot/help

#1

Hello.

A couple of friends and I are trying to track down and repair an ABS fault. I replaced the front sensors and received the ABS light. The car is a 2/89 build date.

This is what we did:

  • Removed the ABS computer.

  • Measured resistance of each sensor at the computer connector - all 4 sensors returned a value of 1000 ohms. Which indicated a good connection to the computer.

  • While driving,without the computer, took a voltage reading from each sensor at the computer connector, 3 sensors read 2 to 2.3 volts - the right front sensor read zero. (Pins 11/21).

  • Took another reading at the computer connector of the right front sensor and it is still reading 1000 ohms.

  • Verified that the exciter ring and sensor were correctly installed.

Any ideas are appreciated.

Tom Rust

#2

Here is a nice tutorial on how your ABS sensors work and how they could be fooled. A couple of things you might try are backprobing the connectors with the sensors connected to the control unit looking for consistent reference voltage at all of the sensors and capturing wheel sensor traces with a portable oscilloscope with the car moving and the control unit connected, assuming you can get access to a portable scope. This should help isolate the problem.

http://www.picoauto.com/tutorials/diagnosing-abs.html

#3

Thanks for the information.

#4

So this thread is not left hanging… A vendor, who shall remain nameless, sent two left side sensors. The Bosch boxes had two different Bosch part numbers NO BMW part numbers. I referenced the Bosch part numbers and installed per the vendor’s rt lft designation on the invoice not thinking to check the BMW part number on the sensor. A right side sensor is on the way and should solve the problem…

Tom Rust

#5

We’ll re-instate this thread since my ABS light is on.

From what I read - the order of diagnosing is :

  1. ABS relay - silver little box right on the ABS computer under dash and to left of steering column.

Most times it fails by the fusible link melting apparently which can result from jump starting the car. The sensor is easy to take apart - just pry open the AL box and slide it out.

Pic shows how you can jump the link w/ a replaceable fuse or something. Beware though - that fuse saves your ABS computer. Not sure what AMP fuse to run yet.

2: Not sure yet - still verifying relay but I think we’ll check a few things before we replace the ECU or ABS computer.

#6

Hey KNSbrakes1. I think a new ABS relay is pretty cheap - $40 range. I can dig up speed sensor pins diagram for the ABS computer, if needed.

Let me know if you need an ABS computer too. My 2/89 build date did not have the relay. I’m not sure if the computer part numbers are the same. I have two I picked up off ebay. From what I have heard/read, ABS computer failure is rare.

#7

Yeah -

I think we will try to jump one first.

Seems like a good thing to ‘fix’ so it can be fused.