Modern cars are very nice, the computers in them have a lot of built in diagnostics, and when there is a fault they can spit it back to you in plain english. Early cars were not so nice, the ABS in my 1988 Audi 5000 is such a system. It has fault detection, but it has no way to tell you what the fault actually is. :(

ABS wheel speed sensors are just like the tonewheels in a Hammond organ: a tonewheel attached to the wheel rotates near a magnet with a wire wrapped around it. The faster you go the higher the frequency produced. I knew something was wrong on my Audi though, the first time you would step on the brake the ABS would kick in. It didn't seem to limit stopping distances much so I suspected it was a rear wheel sensor that was giving a bad signal.

The ABS computer lives under the back seat so diagnosing it wasn't hard. I got a buddy to drive while I probed the connector with my garbage picked HP 'scope. The "test rig":

http://videokarma.org/attachment.php?attachmentid=169216&stc=1&d=1296108786

What a working speed sensor looks like at ~35 mph:
http://videokarma.org/attachment.php?attachmentid=169219&stc=1&d=1296108786

Like I suspected, the problem was a rear sensor. This was the output of the driver's rear:
http://videokarma.org/attachment.php?attachmentid=169217&stc=1&d=1296108786
http://videokarma.org/attachment.php?attachmentid=169218&stc=1&d=1296108786

What has likely happened is that junk has gotten between the teeth of the tonewheel. The axle has to come out to get at the tonewheel though, so this is a project for another day.

Thanks,

-Jason

That's a nice little scope for a trash find:thmbsp: Good thinking on how to find the problem too, glad you got 'er under control. No pun intended:D

Just keep it away from the ignition system!

I used to be a member of the Automotive Service Association and their magazine had a great article each month about diagnostics. The author was an expert with a scope and was able to do some amazing (too me) work with one. Always finding obscure problems just by knowing where to probe and how to read it. Ashame that more mechanics aren't that slick.

Ya. A lot of people are put off by automotive electronics, but they really aren't that bad. In my experience many, many problems turn out to be broken wires or bad grounds.

A friend of mine had at the time a 92 Chrysler LeBarron that was in decient shape that he picked up cheap with a "miss" to it and was told that it had a bad fuel injector. Before throwing any money at it I told him to bring it by and we'll look at it. This model had the Mitsu 3.0 in it, and the computer fires two injectors at the same time. After reading the codes I mentioned for us to hook up the o'scope. After all, an injector will appear like a solenoid as it cycles so we should be able to see a pulse with a downward ramp. The bulkhead connector for the injectors was easy to get at and sure enough, there was our "bad" injector. Ok, let's switch one with another and see if the problem follows....Ah Ha! Now our bad injector looked good so there was less of a bad feeling having to spend the money to replace the Computer! After replacement, ran like a champ!