diff lock from hell

[QSHIPQ at aol.com]

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[ Picked text/plain from multipart/alternative ]
Here's how the electro pneumatic diff lox work:
FROM ALL LIGHTS OFF = OPEN REAR:
IF you press the switch it is a momentary contact 12v, which pressed once
activates the switch from vacuum open to vacuum locked (the vacuum actuator
at the rear axle).  The yellow triangle lights immediately and stays lit
until you press it again.  The LED in the panel only turns on when the actual
diff is locked, it matters not if the switch triangle is on or off.

FROM ALL LIGHTS ON = LOCKED REAR
IF you press the switch (yellow triangle is on), you send a momentary 12v to
the controller, which then turns off the yellow triangle immediately.  The
LED in the panel however stays on until the actuator has actually resulted in
an open rear differential.

Other highlights:
* 15mph disconnect is a ground signal from the ecu to the controller which
immediately shuts off the triangle, and the LED will follow when the diff is
actually unlocked.  ON ALL usa cars this is the blue/white wire at the
controller harness
*  Thermo shutdown circuit:  The controller has an overheat protection
circuit that can disable the diff lock above a certain temp.  There is
venting to these controller boxes but most are located under the rear seat so
they can get hot quickly
*  ABS enables ONLY when the LED light goes out, not the triangle
*  ANY faults in the diff lock switch or circuit, defaults the diff to open
mode

Based on your post Chris, you have a stuck, crudded or malfunctioning vacuum
actuator at the rear axle.  I suppose the actual rear electrical switch in
the diff could be bad, but I haven't ever seen that, even on 20year old
audis.  The fastest test for the rear diff actuator, is a Mity Vac on the two
lines at the controller with the rear wheels in the air.  IF the mity Vac
gives you normal operation then you have weak vacuum to the controller, which
is usually a ck valve or split line from the engine/vacuum accum routing.

HTH

Scott Justusson
QSHIPQ Performance Tuning
Chicago
'83 urq with dual electronic locking diffs