Car won't idle w/the AC on - IAC motor issue?

[PhoenixGTS]

Maybe once a year my car starts having a problem with idling - usually related to the battery being a bit low. This time I charged the battery and reset the PCM (Mopar HP PCM BTW - no idle hang and passes emissions = great overall) and car runs fine, until I turn the air conditioning on then it stalls/won't keep the idle up. Sounds like an IAC motor issue. What do you think?

 

[dave6666]

Does the PCM get a signal from the AC somehow that the AC is on, and change the setting to the IAC? I would think that the idle control would be independent of any signal from the AC - would rely on the tach mainly, but I wonder if there is interaction with the AC, and your HP PCM is for a car that does not have AC? Have you thought about putting the original PCM back in for a test?

Or maybe just clean or replace the IAC. Couldn't hurt.

 

[Tom F&L GoR]

The PCM does get an indication that the AC has been turned on; for example it is supposed to move the rad fan to high speed when AC is active, regardless of temp. Also, some have noticed that if you disconnect the compressor (so you can turn it to AC and simply run the fan at high speed on track days) the check engine light comes on.

I found that replacing/disconnecting the IAC means the PCM has to relearn how to use it. I swapped my good Viper IAC into a Dakota just to see what the problem with the Dakota was. Just as you described, it wouldn't idle, would stall, and did funny things with the AC on. After two trips and 15 minutes it was normal again.

If the battery is low enough to need charging, perhaps it was low enough to "forget" stuff and is relearning. Makes you wonder what else it needs to relearn.

 

[PhoenixGTS]

Building on Tom's comments, I rebooted and am currently trying to drive through the "learning period" without turning the AC on to disturb the operation. With temps of 100F this is harder than it sounds! But we got a little break in the heat today so I drove the car to work. Idling fine so far.