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Today I removed and inspected the wheel speed sensors on my 1997 C36 w/o ASR. I decided to do this after a few symptoms manifested themselves:
Intermittent engagement of "limp home mode" while driving, which restricts the transmission from shifting above 2nd gear
ABS light coming on several times (albeit intermittently)
Scanning the car resulted in several ABS-related codes concerning the wheel speed sensors
Strangely enough the limp home mode seems to occur after taking a somewhat sharp turn. However, once I put the car in park and turn the ignition on/off, the car runs/shifts normally. The car currently sits at 225,000 miles and runs great with lots of power. The intermittent engagement of the limp-home mode is quite annoying and I guess potentially dangerous depending on when it occurs. Around New Year's I ended up buying a STAR diagnostic scanner, and after resetting stored codes, driving around town with the revvy M104, and scanning the car again, the following codes resulted:
ME Codes:
P0500 "B Rear left vehicle speed signal"
P0507 "Idle speed control implausible"
P1605 "CAN acceleration info for poor road recognition from ABS speed sensor"
134 (sporadic) "fault in CAN communication with: traction system"
ABS Codes:
C1101 RPM sensor FR -007 L6/2
C1102 RPM sensor RA -016 L6
Here are pictures of the codes found on the scanner:
Both the lack of open circuits and the ABS codes being intermittent seem to indicate some sort of loose connector. After looking around online for information about these faults, I found some mentions of oil seepage into the harness connector for the transmission via the pilot bushing connected to the valve body electrical plate, which could be a possible cause of this issue. I was able to rule this out as upon inspection, the pilot bushing showed no signs of oil leakage into the harness connector (which I replaced regardless as part of a transmission service earlier this month).
After performing a diagnostic scan today I raised the car on jack stands and inspected the speed sensors. I began with the front left speed sensor. Despite the diagnostic scan not throwing any codes for this sensor, I thought I could 1) Get an idea of what the sensor should look like when functioning correctly and 2) clean off the magnet and hub-bolting components. Unfortunately I did not take any pictures of this part of the process (or near enough pictures in the first place). The magnet had a good amount of fine dust/dirt which came off easily with a microfiber cloth.
I then removed and inspected the front right speed sensor (one which actually had thrown an error code). Upon inspection, I found a similar amount of dust/dirt, and more notably, that a piece of the plastic that allows the sensor to clip into the hub was broken off. I felt something was off when after removing the two T-40 bolts that hold the connector in, the sensor came out with no resistance (as opposed to the front left sensor, which I had to carefully pry out with a screwdriver). If you look at the magnet end of the sensor right above where the large ring is, you can see the absence of part of the plastic that allows it to clip into place:
Although the sensor is very firmly fixed by the two T-40 bolts, the damage on the end that clips into the hub may be causing this intermittent limp-home mode by 1) being ever-so-slightly jostled in/out of place (would seem to explain why the fault is intermittent, and occurs so often after a sharp turn), 2) prohibiting the sensor from sitting correctly in its plug or 3) the plastic could be involved in the sensor's operation (seems unlikely but I am no expert). Whichever it might be, I am ordering a replacement and will document the installation (ideally with better pictures).
I then inspected the rear hubs, and on both ends, there were no speed sensors to be found. Not only could I not find a sensor (despite the presence of a similar plug location as the front wheels), but there were also no "teeth" (for lack of a picture or a better descriptor) on the rear axle under the magnet end of the sensor. Initially I was looking for these sensors as my scan resulted in error codes mentioning both rear wheel speed sensors. I could not find a definitive answer online. Are rear wheel speed sensors only equipped on ASR models? If so, this would seem to explain why I get only 2 ABS codes but 3 transmission/EGS codes for speed sensors. Here is a picture of one of the rear locations for the phantom speed sensor:
I will update this post (hopefully with better photography) once the new speed sensor comes in and I replace the damaged front right sensor. I bought this car in July of 2020, put 7,000 miles on it since and have performed a decent amount of the maintenance myself. The previous owner really took fantastic care of the car and was zealous (in a good way) about maintenance/repairs. I bought the car not running, but after installation of a new crankshaft position sensor I drove it ~13 hours from DC to Memphis, TN with no issue. Otherwise, the most significant work I've had done was having a new headliner made and installed for the car. I have been meaning to be more diligent and organized in documenting the maintenance, so I thought by writing it up in a forum post there's a chance it might help someone else (even if the job isn't particularly difficult or has been documented before). I hope this post (and others in the future) are not boring and at least minimally interesting, and I hope they are permitted in this forum (please let me know if it isn't).
01-28-2021, 10:59 PM
