I bought a 1990 honda civic last year and it runs great. I decided to make this my primary means of transportation. To do this, I needed to have the a/c fixed. The existing after-market a/c was not working.

I took it to a shop and they replaced the major parts (condenser, compressor, evaporator,
Drier). When they gave the car back to me, I found that when I turned on the a/c, the
Idle speed would drop 300 rpms. If I'm idling or slowing/downshifting, the car would stall.
(I should note that there is no fluctuation. When the a/c is engaged the idle drops 300 rpms). If the a/c is off, no problems.

I took it back to the shop and they said they would boost the idle speed, but that is all they could do. They set the idle speed at 1100 rpms (normal idle is 750 rpms. When the engine is cold 1500 rpms). This helped some, but there were still some instances when the car would stall with the a/c on.

I did some searching online (including this forum) and found information on the idle air control (iac) valve. From what I read, it seems the iac valve engages when the a/c is turned on to increase the idle speed, compensating for the load the a/c places on the engine. Should the iac valve be engaging when the a/c is on to set the idle speed higher and prevent the car from stalling? Is there some wiring connection needed between the iac and the a/c? Or is the iac fine and the idle just needs to be set higher?

Thanks in advance for any information on this!

I have a 99 civic ex and I have exactly the same problem ,maybe you read my post
So I already try cleaning the iacv and nothing then I replace it with a working one and nothing so I don't think is the iacv and I still don't know what it is ,some people told me it culd be the computer so I'm trying to find someone with a working one to test if that's the problem.
Please if you find out first what cause the problem post mi I will do the same

When you turn on the ac, the computer tells the iac valve to allow more air to the motor. With my 96 Civic, the idle speed increases from 750 to 810 rpm (+/- 50 rpm).

If it was an iac problem, when you put load on the motor (by turning on the lights, rear defogger etc, I would expect the idle to drop. A bad IAC valve will also cause a very erratic idle.

If you AC system is overcharged, this will put excessive load on the compressor such that the idle will drop excessively. I have had this happen on my car, due to overcharging, and it was dramatic! The car was sluggish with the ac on, then lurch forward when the ac was turned off!

The compressor could also be starting to seize up (lack of lubrication, metal particles from the system not being flushed properly if the previous compressor died, etc.). This, again, could cause your idle to drop excessively.

Hope this helps.