Faulty ABS

[Mike_Buckham at ptk.pioneer.co.uk]

Thanks again, Bernard. Unfortunately, swapping in a known good component
would require me to buy one (new or used) which means that it's not really
a test - I'm just replacing the components anyway. This is why I wanted to
try and diagnose as much of the fault as possible first. I still believe
that my ABS relay is working OK because when I switch on my diff lock (yes
it has an ABS switch and manual diff lock control) the ABS relay clicks
off, and on again when the diff lock is released. The ABS switch itself
does nothing, but that could be just a faulty wire between the switch and
the relay.

According to the Bentley wiring diagrams, when the ABS relay is on, it
provides power to one coil connection of the changeover relay in the
hydraulic modulator. The other coil connection (the ground) should be
supplied by the ABS control unit itself, but only if it thinks everything
is OK. I haven't checked this connection - perhaps the control unit is
providing the right signal, but it is somehow not getting to the changeover
relay. More diagnosis required. The trouble is that I can't do much until
the weekend because it's getting dark now when I get home. Stupid Autumn
(Fall, to you guys in the U.S.A.)!

Cheers, Mike.
'89 200qta




                    "Bernard Littau"
                    <bernardl at acumenassoc       To:     <Mike_Buckham at ptk.pioneer.co.uk>
                    iates.com>                  cc:     <quattro at audifans.com>
                                                Subject:     RE: Faulty ABS
                    02/09/2003 19:21






Hi Mike,

The sensors sound like they are fine.  I think you should swap in a known
good ABS relay -- I suspect that relay does more than just power up the
hydraulic modulator.  After that I would swap in a known good ABS
Controller.  If both of these prove fruitless, then I would suspect and
chase after funky connections.

I can't remember what the '89 has as far as diff and ABS control.  Does
your
car have the ABS off switch or manual diff locks?  Activating the diff
locks
will turn on the ABS light.  Make sure a diff lock is not on.

Bernard



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