AC and idle

I was helping a friend with his 90sc today and noticed a problem with the idle when the ac is turned on. The idle drops well below 500rpm and the engine usually dies. Sometimes it will recover and idle just under 1000rpm with the ac running. More often than not you will have to feather the gas until it recovers and then it idles normally. It also does the same thing if your driving around with the ac on and come to a stop. It will do it sometimes even with the ac off if you let off the gas very quickly,but it will recover without stalling and idle steady around 800rpm. It's a 5spd with a brand new IAC. The IAC is the ford oem. Any Ideas?

Since we can assume that your IAC is working properly, I'd say to check the engine mounts. I know it doesn't sound like it should have anything to do with the idle, but since I've had new mounts... no idle problems.

Nope, new solid mounts about a month ago. It almost seems that the IAC is not reacting quickly enough to the throttle plate closing or the ac adding the extra load. Once it recovers from the initial drop it idles as smooth as a baby's butt.

Time to break open the electrical manuals and look to see where the A/C compressor signal is fed into the EEC. The A/C has a clutch cycling switch which sits on top of the accumulator. Above a certain pressure, the switch engages the clutch, but (and I don't know this for sure) should also send a signal to the EEC that there's an impending load. At least thats what happens with the power steering. The EEC is supposed to compensate.

I don't think my Haynes manual shows that circuit.

That's the strange part though. It does compensate, just not quickly enough. Once it recovers it will idle right around 900 rpm with the a/c on and around 750 with the a/c off.

The signal is really a heads up to the EEC. If it doesn't get it, then the EEC will eventually compensate normally when the idle goes down. But its a slow compensation.

With a heads up signal, the EEC knows the load is coming and reacts quicker.

At least this is what I've been told; I don't have the diagrams to confirm it. Based on what I know about controllers though, it makes sense.

That makes sense to me and I'll check the switch. What really confuses me though is that it will do the same thing with the a/c off. If you give it some gas and release the pedal the rpm's will drop to about 500 or so and just as it's about to stall it will recover,the rpm will pick up and it will idle nice n smooth. Could it just be that the new IAC is defective or is there an adjustment I'm missing somewhere? Thank's for the help.

I've noticed the same problem on my '89 5spd 190k miles. So it's either the switch or the EEC. I'm doing my mounts soon, maybe I'll look at it then.

-Jason

Check for vacuum leaks. Our MAF sensor cars are effected by even small vacuum leaks can cause a too lean a condition at idle and a weak idle.

Jerry

I checked all vacuum lines and re-torqued and propane tested IC tubes. Every thing was good.