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Old 04-02-2022, 11:03 AM   #44
Elbert Bass
Scooby Specialist
Member#: 283429
Join Date: May 2011
Chapter/Region: South East
Location: Down yonder.
2016 Fuji Sunfire 03


Originally Posted by 95 Legacy awd wagon View Post
Thank you for very helpful Info. What a bizarrely tiny margin of error tolerance, esp. for such a slapped together design: '95 literally just two drilled holes, one for sensor, one for hold down bolt. Not the 1st time Subaru designers under-estimated R U S T and corrosion (the filler pipe plastic cover in real world acted as an insurance policy that sand/salt/pebbles would get wedged to bottom hoop of pipe and rust it worse than swiss cheese. I had dealer replace mine, but luckily just the pipe, not the gas tank or tank intermediate rubber hose.

May I clarify: backing plate means the metal part that comes with ABS sensor and has the hole for hold-down bolt to knuckle? You're saying some folks have had to REPLACE the rear knuckle to get ABS to work?!

I realized they replaced just Driver Rear sensor 'cause Pass Rear tested ok. Possible that there is enough of a mismatch between possibly worn/dirty Pass side sensor that after a mile or so now, it's throwing ABS code? Current situation: when turn car on after sitting, or after clearing obd-ii codes, the ABs light on dash is off/dark (i.e., both rear sensors are checking o.k. for Ohm resistance, yes?) But after driving less than 1/4 mile or 1/2 mile, ABS light turns on. I did get one longer "light off" drive, and took chance tk slam on brakes to get pulsing ABS, and it worked. After 2 years of inactivity, both front wheels locked and then ABS pulsed. I also several times since picking up car have felt some "whirring" sensation and vibration, sinking in brake pedal, which I guess is good news that abs solenoid servos are filling up/pressurizing (my guess, i not expert here???) So, good news is that the senoids and pulsing still works after 27 years and 2 years of no rear driver abs sensor. :-) Thoughts? Can dealer's abs reader find more detailed codes on where abs fault is? I think dealer embarrassed they had to use the under dash connector method to Morse-code read errors, as they no longer have reader for a '95. (!!!)
Speaking from experience I can tell you it takes serious late-stage rust to cause this issue. Another cause could be someone bending the backing plate while using a press to replace a rear wheel bearing, or breaking or leaving out the reluctor ring when replacing an axle hub.

The buzz noise: That is the ABS system cycling. What is happening is the ABS module looks at all 4 speed sensor for frequency and amplitude. Because one sensor has a bigger gap ( or cracked/missing reluctor) the amplitude is lower/missing. As the wheels slow down the module sees this as that one wheel's rotation slowing sooner than the other 3 and kicks in the ABS.

First - measure air gaps on both sides in rear. Also inspect for missing or cracked reluctor rings. If you have a multichannel oscilloscope and were looking at two, three, or all 4 speed sensor signals you can clearly see the difference in waveforms as the wheels slow down to a stop.

Younger techs just don't know about these issues. Subaru only trains for the latest technology. If you have no experience with older cars you just don't know this things.

Below is a picture of a backing plate (showing drum brake - disc is similar) and then the knuckle (left diagram is AWD) with reluctor ring shown on hub.

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