Trying to troubleshooot Bosch ABS2
 

Not a 'benz, but it's really similar...

My 88 Audi has Bosch ABS2, just like all of our 80s Mercedes. I'm trying to troubleshoot the system, currently it always thinks a wheel is locking so whenever you apply the brakes the ABS comes on. After driving for a bit the ABS warning lamp will come on and the system turns off. I believe it is a rear wheel because it doesn't affect stopping much. Like a Mercedes both rear wheels are on the same hydraulic circuit, but each wheel has its own wheel speed sensor.

I've checked the resistance of each sensor from the computer's plug, each one is about 1 ohm. I put the car in the air and spun the wheels, each sensor makes some voltage. I checked the wiring diagram and the car does not match. Probably an error in the service manual, but changing the wiring to match the diagram didn't seem to change anything.

My next step will be to have someone else drive while I check the voltages and waveforms with an oscilloscope. The computer lives under the back seat so this shouldn't be too bad.

Any other suggestions? I'm wondering if the control unit could have gone bad somehow?

-Jason

No ideas?

-J

I finally got a helper to drive, so I was able to figure out which sensor was bad.
Here is what one of the good sensors looked like, going about 35 mph:
http://www.peachparts.com/shopforum/...1&d=1296108228

The bad sensor was easy to spot. As I suspected it was a rear, it turned out to be the driver's side. The waveform was very chaotic.

http://www.peachparts.com/shopforum/...1&d=1296108228

http://www.peachparts.com/shopforum/...1&d=1296108228

The "test rig":
http://www.peachparts.com/shopforum/...1&d=1296108228

-Jason

Nice Diagnostics !After replacing the sensor Do let us know the difference in braking performance and perhaps of the new waveforms;all wheels sensors operational.
Thanks
mak

I'll be sure to do that, I'll also post pictures of the physical problem(s) I find.

For right now I have the ABS disconnected because it errors out and turns off after about a minute of driving anyway.

There may be a problem with the front right sensor as well, its signal was a bit weaker then the front left's. The sensor may not be the correct distance from the tonewheel.

-J

The brakes on my 300se operate very well ,but i suspect there is a sensor problem. while slowing down i hear the ABS pump come on during the last couple of metres with the characteristic sound of a brake being modulated .
mak
300se 126 89

I'd suggest doing a similar test :)

-J

Nice fix! None of my E23s have properly functioning ABS if it makes you feel better. :) System is probably the same as each wheel has its own sensor.

I remember attending an MBCA Star Tech session on ABS years ago, run by Randy Durrance. The gist of it was a lot of ABS failures come from broken wires to the sensors. His recommended diagnosis was to attach an ohmmeter to the sensor terminals and flex the line up and down its length. If the resistance fluctuated as you bent the line at a certain point, you've found the break.

FWIW

An ohm check was the first thing I did :)

This trouble shooting procedure may help sort out misc ABS problems.

Regards,
bobf

Before replacing I would clean the sensor, it is magnetic and if there is metal attached to it, you will get the kind of readings you have.

Make sure it is properly seated before you put the locking screw in.

Rob

I forgot I made a video of this, figured I should add it to the thread:

ABS Sensor output on an oscilloscope - YouTube