Hi thought I would share this with you as it was quite a rare one in my experience. Had an Audi A4 with ABS light on. Fault code was open/short N/S/R wheel speed sensor.so checked signal with scope no fault found. Had correct bias voltage and a good ground (compared it to the other side that was not logging a fault). Road tested and found light would come on when I actually came to a stop (intermittently) when I looked closer at the waveform I noticed the bias voltage would get pulled to ground only when the foot brake was applied. Was it a fault in the wiring loom? So I cut the wire at the computer/ pump and applied the brake and the fault was still there. Comparing this to the other side that never showed any fault. Can anyone else think of any variables other than a short inside the ABS module??? Can't be a bad ground as it gets pulled to ground? Can't be a bad power as it wouldn't just effect 1 sensor? And the brake light switch was a pull up design and had been eliminated when I cut the wire? Here is the video
I will let you know when we finally change the ABS module if it fixes the problem.
Hey sir! I think you're on the right track with an ABS unit on this Audi. I've known these modules to fail in various different ways, but this one is a first for me. The way that bias voltage trace stepped down around 57 seconds was WEIRD.
The ONLY other thing I can think of is some kind of resistive short at the ABS module connector itself. I'm thinking of green corrosion between the wheel speed sensor pin and a neighboring pin that happens to ground when the brake pedal is applied. I know, it's kinda a stretch, just thinking out loud.
Hi thanks for the advice, but I did perform a visual on the connections inside the multiplug and everything looked good. It was all so intermittent. The only real consistancy was alot of hard stomping on the brake pedal would finally reveal the fault?. Turns out I didn't even need to strip the car to find the fault. I fell in to the complacent trap, assuming it was going to be a sensor,wiring or connection fault.
That's a really cool find. Like Tyler said, with the rest of the circuit out of the loop, if it's anything but the module, it would have to be a connector issue. It sure looks like it has to be the module though.
I have seen an MG ZT with corroded connector on the Bosch ABS pump-module, I had not disimilar symptoms I didn't have any scantool at the time that would read the system but had to go the voltmeter and scope route.
In the end it was obvious when I eventually pulled the ABS module connector.
I
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Andy.MacFadyen wrote: I have seen an MG ZT with corroded connector on the Bosch ABS pump-module, I had not disimilar symptoms I didn't have any scantool at the time that would read the system but had to go the voltmeter and scope route.
In the end it was obvious when I eventually pulled the ABS module connector.
I
Andy, Welcome Back!
That's about the only scenario I could imagine apart from a faulty module.
Hi, as I said before I did pull the multiplug and everything looked good. No corrosion or water ingress of any type. Just shiny healthy looking pins and terminals
Customer finally caved in and brought back car for repair. Sent ABS pump off for test and repair. They confirmed fault and repaired. It was turned around in 3 days with a happy customer at the end of it.
Richey.brown wrote: Customer finally caved in and brought back car for repair. Sent ABS pump off for test and repair. They confirmed fault and repaired. It was turned around in 3 days with a happy customer at the end of it.
Whoa, fix outta nowhere! :woohoo: Five months later, that's gotta be some kind of record around here. Well done!
Honest question: In the UK and Europe, is it more common to send modules like this out for repair, rather than replacing them? Similar services exist in the States, of course, but the vast majority of shops in my area would opt for replacement in the same situation. Not that there's anything wrong with module repair, just curious if there's a different attitude towards it?
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