[Joe37]

Did 4 spd cars use the electric idle solenoid with A/C ?

[Kenth]

Yes. From 1979 Pontiac Service Manual:

[Skip Fix]

I like an AC solenoid even with a lump cam so you can have carb idle screw down so it won't diesel when you turn it off if it idles on the solenoid.

Funny idle solenoid story for you all-the mild 400 I had on the '81 TA the kids drove to high school I adjusted the idle solenoid no. Well it must have been stuck a little when I adjusted it and when my son was driving with his friends it unstuck and started idling about 2000 rpm and with the 3.08 he had to ride the brakes to get it in a parking lot! Freaked him out.

[Joe37]

Thanks, I am rebuilding one for local fellow, he gave me three to make one good one, a '78 standard 400, a '77 TA automatic, and a rough '79 firebird 4spd. I am robbing off all three to make a '79 4spd correct numbered one for an Anniversary TA.

[nUcLeArEnVoY]

They repop them, anyway, if you want a new one. Bear in mind, though, that they don't have the strength to them to actually actuate the throttle level on their own. If you're at curb idle and you turn on the AC, you'll need to blip the throttle so that the throttle lever moves out of the way and allows the solenoid plunger to extend to its adjusted position, so that it can hold the throttle lever at the higher idle speed to compensate for the load the compressor puts on the engine. This also means that idle will be higher than normal when the compressor is cycled off and there's no load on the engine. The solenoid unfortunately isn't wired in that it's able to extend and retract in conjunction with the compressor cycling. Rather, it doesn't retract until you actually turn off the AC.