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I think the issues is resolved. A solid week of commuting with no failures. In the past, any drive over 20 minutes had about a 100% certainty of generating a fault.
While I used all three of the testers I had, and will add details about that experience later, it was the Bosch ABS 2 tester that narrowed down the fault, mainly because it was the only tester that lent itself to driving around and monitoring.
The ABS tester showed that it was the Right Rear that was flaking out after driving for a while and hitting the bumps. As a reminder, I had replaced the RR sensor with a different used sensor as well as replacing the crumbling connector as part of my earlier troubleshooting. The sensor tested perfectly while up on liftbars, impedance perfect, signal generated when rotated and no ground faults or dropouts even when all wiring wiggled and shaken. Only failed in actual road use.
After sleeping on it, and pulling it out and looking a couple things came to mind:
First, this sensor was much tighter and harder to insert/remove than the other side or front sensors. I had checked for corrosion earlier and checked again with my pinky, but found no obvious corrosion, but both sensors I had were a very tight fit in the hole.
Secondly, the issue only happened after a period of driving, so for the first few minutes from right after ABS self test to ~10 or 20 minutes into it, I could hit bumps all day long and no issue. This made me think it was temperature related.
So my theory is this: Much like when the mounting hole is heavily corroded, the tight fit of the right rear hole ( for whatever reason) combined with a little additional expansion from warming up, compresses the sensor just enough to cause ground faults, which manifested when the sensor was subject to suspension motion shock.
I took my Dremel drum and removed a few mils on the inside of the hole. Sensor slid right in, and no faults since then, again a solid week of commuting.
Wow. Totally impressed by your jack russel approach to the problem, and I will use every diagnostic point brought up in this incredible thread this winter when I try to suss out the cause of this behaviour on my 86.5. Many thanks, and although I have no authority whatsover to grant it, I hereby award you the honorary rank of "Occam" in my book. I have been too reserved to mete out other "Occam's", but you know who you are, or be assured that at least everyone else does!
Following up on the testers. The Mitsubishi MB991131-02 was originally designed for the88-90 Sigma FWD, 89-91 Galant FWD/AWD. This looks identical to the Corvette tester, both made by OTC/Kent-Moore. Main difference is that this has an option to select between 4-channel abs and 2-channel ABS. The 4-channel ABS selection worked with the 928:
The way this tester works is that it walks through a menu of tests, with some steps telling you to take an action, like press brakes or spin certain wheels. It seems to to a capable job of testing each relay, senor, pump and component. It would be a decent tool for instant diagnosis of clearly failed relays, pumps or sensors. Instructions attached
The J-35890 Corvette ABS tester is very similar to the Mitsubishi system, except you do no chose a car model. Again, you walk through a menu of tests, with some steps telling you to take an action, like press brakes or spin certain wheels. It seems to to a capable job of testing each relay, senor, pump and component. It would be a decent tool for instant diagnosis of clearly failed relays, pumps or sensors. Instructions attached.
The Bosch ABS 2 tester was the least sophisticated. It did not have a digital display that walked you though tests, just a rotary dial and some push buttons. BUT it was the most flexible and thus useful. The Vett and Mitsubishi testers just ran a few second test of the sensors as you manually spun the wheels and then produced a pail/fail readout. The Bosch tester let me run that test continuously and I just watched the the output meter for drops, or slowed down to the expected speed range (about 4mph) to get a positive light from the sensor. When the right rear sensor failed, the meter went to zero, and the check light would not illuminate. The main thing is I could do this while driving continuously. So, in many cases, the other meters would be better, but in my specific case, the less automated testing was what I really needed.
Somewhere out there is another type of Bosch tester apparently called a K-7. The interesting thing about that sensor is that it apparently has male and female plugs, so you can put it in-between the ABS brain and the wiring harness, so you get some visibility into the ABS brain. If you will note, all the above testers only connect to the harness, so do no testing of the brain or the system in actual use. If I ever find a K-7 at a reasonable price, I may pick it up for testing.
This is the tester mentioned in the ABS Test plan attached below, from the WSM:
Just wanted to say thanks for your review of the testers! I bought myself a KDAS 0003 last night on Ebay, as I've not been able to find the fault in my '87's ABS, and driving the 928 daily for a week while BMW replace the transmission in the X5, its time to just sort it out.
Fault history:
I found a open-circuit harness issue right front with a multi-meter, and repaired the harness as well as replaced the engine-bay barrel connector - Fault still present
Swapped brain with my 89 which has working ABS - fault still present
I removed brake discs and cleaned all the sensors. While I was at it I re-insulated the left rear harness as the sheath was crumbling. - Fault went away for about 300kms - I thought "yes!".
After sitting the 87 for a couple of months - fault was back.
Driving 600kms in the last week, the fault is consistent, not intermittent.
My symptom doesn't seem to match anything I can find.
I get no light at start, or reversing out of the driveway, but as soon as I drive forwards (either from a start or after reversing), the warning light pops up, so I'm hoping the KDAS will give me some insight on where the issue is.
11-17-2018, 02:11 PM
