re. All wheel drive does not engage

[Ben Swann]

[My all wheel drive on my 86 Audi 5KTQ does not engage. Well the dash light
does not come on when the switch is turned to engage the rear drive.
Someone
had a thread here before saying that this is usually
a vaccum leak problem. Has anyone else experienced this problem, and did
it turn out to be a vacuum problem? Thanks, gerald]

Yes, that is likley the problem if your differential lock(s) is/are not
engaging.  Look near the vacuem actuator near the diffs.  Just go ahead and
replace the line even if it looks good.  The actuator up front is hard to
get to - on firewall behind the engine - just break your arm in a few
places.

If that doesn't fix it, then you may have a leak near the control switch.

Ben