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Help! 1986 ABS - Further Diagnosis Help Needed

chris123

Well-known member
Joined
Jul 7, 2006
Messages
75
Location
Wilkes Barre, PA
Before I ask the question, here's what I've done so far. I have a FSM, and followed most of the procedures in the diagnostic section. I've replaced the modulator assembly with a known good unit, including the wiring harness, since that had some corrosion on the terminals where it connected to the modulator. All 3 relays have been tested, as well as the diode. Fuses are good. The ABS light will come on for a second or two when the ignition is turned on, then a couple of relay clicks, and then go out. It will light up when the engine is cranking, and then go out as per FSM. One of the only tests I haven't performed yet is ohming out each of the wheel sensors. I did remove the front 2, cleaned them, and re-installed them. Haven't had a chance to do the 2 rear ones yet.


Here is the question: The ABS system will not perform the self-function test at 4-5 mph; if the system is not getting 4 correct signals from each of the sensors, will that prevent the self diagnosis test from ever happening? After I get up to 7-8 mph, the ABS light will come on and stay on until the next key/ignition cycle. Not sure if the next step is to focus on the ABS main computer/controller, or possibly one or more of the wheel sensors. I guess ohming out each of the sensors might reveal more info, however I just wanted to know first what prevents the system from not performing the self-diagnosis check at the 4-5 mph range. Not even sure I can find new aftermarket sensors if I do happen to need one. Hopefully I won't have to mess around with used ones.......

Regards, Chris :)
 
I don't envy anyone having to diagnose a problem with the early ABS because it has no on-board diagnostics and the "off-board" piece of equipment that most dealers had back in the day to diagnose those systems is exceedingly hard to find.

That said, there are some diagnostic tasks you can complete without that piece of test equipment. See an article on C4 ABS located elsewhere on this web site.
All About C4 Corvette Antilock Brakes and Traction Control
 
Things to consider......

I would pull the sensors.Inspect if there are metal shavings on the sensors, that will attenuate the pulses the " computer "( read abacus ) see's.

Also make sure there is no junk in the teeth of the pulse wheels...

What the computer needs to see are clean square waves from each wheel...

Mike
 
"vigman" is spot on with his discussion of sensor cleanliness.
 

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