Re: [No EFI] ABS Help Needed

[Shannen Durphey <shannen@grolen.com>]

Hi, Mike.  I chased a problem which was extremely similar to yours in
symptom.  I had a 92 (Vette) that would go into an antilock stop at
low speed.  At first I thought it was when I let off the pedal, but
after some driving I caught onto the real symptoms.  Anyway, on this
car a dealership tech had removed the tachometer signal filter and
spliced the lines together.  The short end of the story is that
voltage spikes from the tach line were causing interference in the LF
wheel speed sensor wires.  The brake module saw what appeared to be
one wheel moving quickly (LF), and 3 almost stopped, and determined
that 3 wheels were skidding.  It took an oscilliscope to find the
erratic signal in the LF sensor, but a scantool would show excessive
wheel speed. 

A 92 Vette is LT1 equipped, and has very different ignition module,
wiring, pcm than an 86.  I won't say you have the same problem that I
had.  But there is a tach filter, under the distributor on the LH
side.  The brake computer does not communicate with a scantool, and
you can only get trouble codes by jumpering 2 of the terminals in the
ALDL connector.  

But I have to ask, does this problem only happen once per ignition
cycle?  That would likely be the ABS self test, which the brake module
performs when the car hits 8 mph fo the first time after it's been
started.

HTH.
I'll help more, but I'm really burning the candle at both ends now. 
So my replies may not happen right away...
Shannen

Mike Turner/ENGR/HQ/KEMET/US wrote:
> 
> Where might one find any information on troubleshooting, communicating
> with, replacing, etc. an ABS unit for a late 86 (ie 87) Corvette?
> 
> Ours has developed a couple symptoms. The first occurs just after mild
> brake application. On release of the pedal, the abs kicks in and the pedal
> starts vibrating for a short period. The other symptom is the ABS light
> coming on sometimes after this event but not always. ABS action during and
> after hard braking looks and feels normal.
> 
> I have considered a complete brake line flush including the lines to the
> abs unit. Removal and replacement of the abs unit looks non-trivial to my
> untrained eye. My local vette experts indicate fluid contamination should
> not cause this symptom. A sticking pad or caliper could fool the abs (ok
> the wheel may actually be locked up and abs is just doing its job but seat
> of the pants says no wheels are locked since car is slowly rolling). Ditto
> a flaky wheel sensor.
> 
> Additional info:
> Calipers replaced a few months ago. Symptoms did not immediately appear
> after replacement.
> Brake pads are hawk autocrossers which squeal impressively.
> Rotors are stock.
> 
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