rogman897

In my '91 200 every time I start my car, the brakes work, but not well. It feels like they grind and the pedal is stiff. They dont feel like the power assist is gone, but not quite like the abs is kicking in either. After a couple times to several times braking, the anti-lock warning light comes on, and at the same time brakes start working fine.

Ive checked and all four of the calipers are working correctly. I also made sure the lines were bled correctly and that there werent any pinches in any of the lines.

The bomb hasnt worked since I got this car 6 years ago, so I wouldnt think its that, and the steering isnt affected, so I wouldnt think its the brake/ power steering pump. Could it possibly be the abs pump? And how would I go about testing the pumps?

Any thoughts or problem experience would be a great help! Thanks!

Number68

one of your abs sensors is bad. you can try unplugging the abs relay, it'll keep you from having that problem until you get it fixed

rogman897

Ok, that makes sense... now heres the question.. anybody have a wiring diagram for the abs system? I unfortunately lost that most important of the 3 books a couple years ago. I know I can check which sensor it is with an oscilloscope (which I actually have and know how to use) but as to which points go where I do not know.

Number68

to check the sensors, you don't necessarily need the wiring diag. there's only front sensors, and the plug is right next to them, just 2 wire sensors. You probably already know this, but you hook up the scope to the 2 wires, and set it to see square waves, and then spin the wheel. You're looking for a perfectly square pulse, if it's anything but, you've found you're culprit

RussB

Are you SURE that you want to see a square wave from the ABS sensors? On my 1988 5000CSTQ, I see waveforms that approximate sine waves.

A good thread regarding ABS sensor troubleshooting: http://www.honestjohn.co.uk/forum/po...ex.htm?t=16782

A second point: I'm sure that the original poster's Audi 200 has 4 -- not 2 -- sensors: one sensor on each of the four wheels.

I usually measure the outputs of all four from one place: beneath the rear seat. Remove the large ABS computer plug and all four sensor outputs appear on it. (Unfortunately for the OP, I don't have a schematic for the 200.)

rogman897

Yes... 4 sensors as it is a quattro.

Also, square waves could work. Its not the proper way to do it, and you can miss information, but if you know how to read it, it works. Sine are much easier to figure out exactly what is causing it though.

What I was hoping to get the diagram for was basically which color goes to which wheel. Ive tested them and actually have 2 that are not reading correctly (Im hoping Im correct on this one). One isnt reading at all, and one is showing some minor disruptions that cleaning may take care of. I just dont know which those are, and dont want to pull out the two reading well and clean them if I dont have to. I did have the front wheel bearings replaced on both sides a couple months ago and am curious if those are the 2 malfuntioning.

p.s. I dont have a lift and was too lazy to jack up all four wheels on stands; so I set up my oscilloscope in the passenger seat with lead extensions from the harness to watch while I drove at a constant speed down the highway. I know it wasnt safe but thought the mental picture would give those EE/ME nerds out there a laugh!

RussB

ABS monitors the rotational speed of all four wheels regardless of whether the car has 2 or 4 driven wheels.

I have no idea what you mean in your second paragraph.

Number68

my 5000 only has 2 abs sensors in the front....

rogman897

I was referencing *Number68*'s statement about how to check function of the sensors on the oscilloscope using "square waves". (I understand that the sensors work like a generator, using the magnetic passing pulses from the teeth on the CV gears to create an alternating current in the wire between the sensor and the computer. The computer then analyzes the input and compares it to an ideal model.) It will still show whether it is functioning or not, but not much more than checking resistance with a VOM will. It wont be able to show the actual output of the sensor, just whether its outputting or not. It's all in the way to set up the scope when checking signal.