ABS light on help please

[Eric Maxon]

Hello Iain,

Thursday, October 11, 2001, 2:57:40 PM, Iain wrote:

IA> i replaced the shocks and bushes today on my Coupe Q 86 (like 4000q) and now
IA> i have the ABS light coming on, any thoughts greatfully appreciated.

First check all fuses associated w/ the ABS system-- I've been fooled
before.

Next, using the schematic (pinout of the ABS control unit plug,)
LOOOONG test leads, measure the signal produced by each wheel speed
sensor at the ECU plug while turning the wheel by hand (requires
raising a whole end of the car and unlocking the diffs)--
I believe the minimum spec is something like 10mV p-p when turning
the wheel slowly by hand.  As long as you get *something* from each
sensor, they are probably good.  If you get lousy amplitude...

Check that any low-reading speed sensor is firmly seated in its bore--
They press into the wheel bearing housing, and you may have disturbed
them whist removing/replacing the struts.  Being even .010 or .020"
farther from the teeth on the CV joing may decrease the amplitude of
the signal they deliver to the ABS control unit enough at low speed to
cause a fault condition.  (EX, when turning a wheel by hand, if near
the maximum allowable clearance, you'll only get about 5-10mV p-p!
Remeber that the ABS computer has got to get a signal all the way down to a
*stopped* wheel to do it's job, and the slower the wheel turns, the
lower the amplitude of the signal.) I have come across a few bad
sensors, but usually setting the clearance restores an acceptable
signal.

On some q's, they are press-fit into their bore, sometimes w/ a
plastic, closed end bushing designed to set the clearance between the
tip and the teeth on the CV joing.  In '86, I believe the sensors lack
the pastic bushing, and are secured in place with a small
machine screw (as an '86 5kq)-- You may want to loosen the screw, shove
the sensor in unitl it *hits* the teeth on the CV joint, then back out ever so
slightly (The closer the sensor is to the teeth, w/o touching, the
better signal you'll get.)

Happy troubleshooting,

Eric
-- 
mailto:emaxon at mediaone.net