My 1990 Volkswagen Cabriolet runs fine except for when the AC is switched on. Idle drops approx. 50 RPMs and runs rough.

Shouldn't there be a kick-up of some sort to compensate? My idle stablizer valve is humming and appears to be working but whatever it is that is suppose to tell the computer the AC is on does not appear to be doing its job. I just do not know what it is that would tell the computer.