My 1998 Forester suddenly began pulsing / crunching sound, from the front brakes. I had just performed full brake inspection 2 weeks before. So I removed the ABS fuse (20 amp), and the problem disappeared. Of course the ABS light is also on now, but I can drive the car. I disconnected the 4 wheel sensors from the car harness, connected an oscilloscope to each, and spun the wheels pretty fast. I get 150 MV peak-to-peak from each sensor. Must I replace the “tone rings”, or just wire-brush them? Replacing the rings are a BIG job, and the wheel bearings & seals must be replaced, since they don’t survive pulling out the axles from the steering knuckle (front), or strut assembly (rear). Anyone have this problem? Pete
Not sure what levels those sensors should be. One would imagine that 150mV is enough as that’s just a passive Hall sensor. One of those wheel speed sensors may not be getting back to the ECM, thinking that one or more of the wheels are slipping. You may want to trace the wire back, starting with the wheels that crunch.
Could it be that one of the rings is cracked?
I do know you should just be able to pull the half shaft out of the hub in the front without destroying anything - I did it just last week, as I’m replacing the automatic transmission on my son’s Impreza. Disconnect the lower control arm and the hub assembly pops out. You won’t even have to disconnect your brakes, lines, etc. Pull it out enough so you have enough clearance to pull the half shaft out, after you punch out the roll pin that attaches it to a short shaft in the transmission.
The only thing you will need to replace is the axle nut ($5 at the dealer) as that thing is staked and will require some persuading to become unstaked.
If the signals were clean sine waves and not erratic (suggesting a bad section on a ring), and the wavelengths seemed to change consistantly with the wheel speeds, than I see no reason to do anything to the rings. Signals should be similar to those on page 9 of the attached document.
Along with mountainbike, see if you can store a trace of a bunch of them, enough for one wheel rotation. With a regular non-storage scope It would be pretty hard to see one missing or malformed pulse in a train of several hundred pulses.