i recently bought an 87 5ktq and am having some work done on it.
when driving it i notice that the rear differential light comes on when accelerating hard then goes out. when i engage the center differential and the light is on, it goes out with hard acceleration.
has anybody on here had any experience with this symptom before??? i'm kind of new to audis and don't have a lot of time working on them.
it sounds like a vacuum leak or problem to me, but i'm not sure!

thanks for the help guys.

When the lights are on does the car do funny things? On dry pavement whit the diffs locked, especially the rear, the car will wander like crazy and scrub the tires. I would check the electrical connections to the switches, on the diffs themselves and in the console.
HTH
Gary

the audi guy that is helping me thinks it is the check valve too. we are installing a new one tomorrow and going to test it.
so far, all the vac lines are good. i'm going to look for bad grounds or loose wires around the switch tomorrow.

thanks guys!!!