ABS wheel sensor issues on 1993 850

[Dietmar]

Hi - my car (1993 Volvo 850 sedan, 154k miles) has had some issues with the front right wheel sensor. The error codes where
122 Right front wheel sensor, faulty signal <40 km/h
324 Right rear wheel sensor, irregular >40 km/h
I brought it in to my mechanic and he decided to replace the wheel sensor. However, I then got
312 Right front wheel sensor. open/short
and it didn't go away after some 30 miles driving. (The mechanic thought that there might be some corrosion on the wheel that first had to get scraped off by driving in order for the new sensor to make good contact.) The mechanic then did some fixing with tape or something to ensure contact and the error went away (after proper resetting of the fault codes).
However, I am now receiving
122 Right front wheel sensor, faulty signal <40 km/h
324 Right rear wheel sensor, irregular >40 km/h
again. Interestingly, they come on only when I am hitting a bump or a pothole, whereas the
312 Right front wheel sensor. open/short
came on immediately when starting the engine after resetting the codes.

I am concerned that the ABS is not going to work properly when I need it most, but it appears that my mechanic needs some guidance from this forum to fix this issue. Thank you for any advice!

[MadeInJapan]

Sounds like faulty wiring to me. The '93 ABS module hardly ever goes bad (who knows why). You'll most likely have to fault trace the wiring (and connector) for the wheel sensor on the two wheels from where you're getting these codes. Good luck and let us know what you find.
Regards,
MIJ

[Dietmar]

MadeInJapan, you were right. I switched mechanics in the past time because my old refused to do proper error elimination. Finally, I worked with the new mechanic on fixing this. One of the wires connecting the sensor was broken and interrupted the signal. However, the teeth on the axel where corroded, as well, and my former mechanic never measured whether the abs control got a strong enough signal from the sensor. Turns out that the sensor was not close enough to those teeth and we had to adjust it (the old mechanic put a washer there, which might have been the factory default, but removing the washer improved the signal).

fyi: the weak signal, after fixing the wire, was problematic at very low speeds when the amplitude generated by the sensor was so low that the abs control "believed" that the wheel was not turning at all. It then took over the breaking action, which didn't feel very gentle anymore.