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Friday, October 30, 2020

Doug's Domain

Doug Vetter, ATP/CFI

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Wednesday, July 23, 2014

ASC/ABS Troubleshooting

I thought the ABS/ASC warning lights might be a fluke as they only illuminated on a few drive cycles after the brake job but at some point last week they turned on continuously and would not clear even when cycling the ignition. I took my technician's advice about the sensors and decided to jack up the front of the E36 last weekend to pull the speed sensors and check them for an errant metal filing or chip that might be blocking them.

If you've read my front suspension overhaul entries you know that I had a hell of a time removing the speed sensors from the kingpins. To make sure that never happened again I put some CV-2 grease in the bores of the kingpin that accept the sensors and put anti-seize on the bolts that secure them. For that reason instead of snapping another bolt off in the kingpin both sensors came out with trivial effort. That's the good news. The bad news is I did not find any flakes of metal or undue contamination of the sensors' magnetic tips – while they weren't perfectly clean, they were a hell of lot cleaner than they were when I pulled them during the suspension overhaul so I figured the sensors weren't the problem.

(Image: Closeup of old ASC and ABS relays)On Monday I took the car to see my technician. I was expecting a fast diagnosis – something like “right rear speed sensor failed”. Unfortunately that was not to be, as the SSS failed to even recognize the ASC module but interestingly it did recognize the fact that I had removed the exhaust butterfly solenoid to install the M3 exhaust earlier this year. No doubt having seen this movie before my technician asked me to open the hood, turn the key off, wait 10 seconds, and then turn the key back on to position 2 (all cluster lights on) to verify if the ASC throttle body actuator went through its expected power on self test (cycle full closed and then full open). It didn't budge.

That led him to the fuse box, which revealed no obvious signs of blown fuses. He then probed to check for power across the fuses associated with ABS and ASC. When that turned up nothing as well he pulled the ASC and ABS relays and brought them back to his bench. At this point he noted that BMW used to have an adapter for the diagnostic computer that would check the relays but BMW discontinued support for it so there's no way to check this stuff aside from pulling the cases off and perhaps checking continuity of the coil and contacts – in other words, the old school way he taught me years ago when dealing with the aux fan relays.

When the ABS relay came apart the shiny contacts indicated that this was unlikely to be the source of the problem, save for some possible issue with the coil. The ASC relay, however, was a different story, as deep concave pits were evident. Unlike the aux fan relays whose contacts usually show a lot of carbon scarring due to the high amperage arcs that form across the contacts as they deteriorate, the ASC relay contacts were comparatively clean, displaying what I'd call 16 years of normal wear and tear.

Since we appeared to have found the smoking gun the next logical step was to replace at least the ASC relay, and optionally the ABS relay. I decided to replace both. The parts desk didn't have either in stock so that meant another day of driving a vehicle with no electronic nannies or braking aids, but I have to admit I enjoyed travelling back to a time when all cars continuously threatened to teach a good lesson to those that lacked the appropriate respect for them. And there are times, especially of late it seems, where I wish I could buy such a car, brand new. It would likely be lighter, less costly, and easier to maintain, that's for sure. As we pilots say: if it ain't installed, it can't break.

The next morning I picked up the relays and quickly inserted them. Turning the key resulted in the ABS and ASC warnings momentarily illuminating as they always do upon start, but then they extinguished. Problem solved. Although I knew at this point that codes had been stored I figured I wouldn't bug my technician to clear them now as I fully expected to bug him after the interior overhaul to clear the inevitable SRS warning.

Mileage: 251700