Intermittent Idle Problem, A/c Related Options

[Poonicus]

Alright so since about 3-6 months ago I've had a rough idle problem whenever the a/c is on, but whenever the a/c is off the car is perfect and this is Texas I kind of need my a/c on. Whenever the car is at running temp and even cold it idles fine at 800rpm but whenever you turn the a/c on it will go up to 1100 or so and then it will shoot down to 100-200rpm and the whole car shakes. I was wondering if this was because of incorrect belt tension, a vacuum leak, bad iac, or because that's what the celica's do when the a/c is running. Any help would be appreciated.

[Special_Edy]

AC Idle Up Control Valve according to the vacuum routing diagram under the hood. It is located on a bracket bolted to the passengerside of the intake manifold. The ST uses an interchangeable unit, but it will not properly mount to the bracket. The only source for them is the salvage yard and possibly a dealership, auto parts suppliers dont carry them.

The fact that the idle initially increases leads me to believe the IUCV is functioning correctly. Try unplugging the electrical pigtail for the AC compressor/compressor clutch located on top of the compressor and under the alternator. I THINK this should cause the idle to increase regardless, and prevent the compressor clutch from engaging, though the system may be smart enough to do neither if it senses the compressor not building pressure. If the idle does increase and the car doesnt begin idling rough, this would mean the compressor was the culprit.