ABS is going wacky! on 1999 Subaru Legacy

I run a garage and I have a weird problem: When you first start this car, the abs brakes pulsate constantly while driving and braking for about 15-30 minutes of driving in traffic. After that, the ABS light will come on in the dashboard and then the brakes will stop pulsating and work great. We have changed relays, ABS pumps, and the ABS module. HELP!

by in Ellsworth, ME on December 22, 2009
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1 answer 6 comments
ANSWER by , December 22, 2009
Is the pulsation occurring only when depressing the brake pedal? Or is the pump running all the time? Have you checked the wheel speed sensors, are they clean? Are they tight and not moving around? Are the wheel bearings loose? Any movement of the sensors can cause ABS activation, so check that out.
COMMENT by , December 22, 2009
Hi Bretb; The pulsation is only when you press the brakes for the first 15-30 or so minutes of driving, then the ABS light comes on in the dash and the brakes revert back to standard braking and work great. All speed sensors are clean and have good signal, and all wheel bearings are good. The boys have been on the car for two days straight now. I am ready to pull my hair out, as I am sure they are also!
COMMENT by , December 22, 2009
Can you connect an oscilloscope at the ABS module and monitor the wheel speed sensor signals? Any loss of signal, or corruption of the wheel speed signal can cause ABS activation. I've seen this from dirty sensors, loose sensors, loose sensor ring, loose wheel bearings/axles, loose suspension etc. Any of these may be visible on an oscilloscope display. Just a thought, check that the tires aren't overinflated.
COMMENT by , December 22, 2009
Thank you, we will try that. We brought it to the local Subaru Dealer and they read the codes and charged us $38 to tell us that there was a broken ABS wire, but we haven't been able to find a thing.
COMMENT by , December 22, 2009
That makes sense, what code did they pull? That should help to identify which circuit has the broken wire.
COMMENT by , December 24, 2009
After grueling hours and hours and hours, we brought it back to the Suby Dealer and it was one of the ABS rings around the front axles.
COMMENT by , December 24, 2009
I used to call these "Learning Experiences", the next time you have a similar problem this will provide a ton of useful information. Glad to hear it is fixed. Happy Holidays!