I just got the a/c in my 94 with a 5.0 running not too long ago. At first everything was fine but a day or two later every time I turned the ac on the rpm would drop and not come back up and the engine would stall if I stopped while backing out of a parking spot. It only got worse over the next couple days to the point of stalling when I changed gears. As soon as the clutch was disengaged that was it. It almost caused a bad accident one time. I had the alternator tested and that turned out to be bad so I replaced it and it runs better but not great. The rpm still drops when the ac comes on and it still stalls every now and then when the engine is cold but not nearly as bad as before. What could be causing the low idle and stalling? Is it something electrical like a relay or sensor? Or could it be something wrong in the ac system itself?

 

With the truck idling in park (or neutral), does the truck stall when the AC compressor is cycling? If it doesn't, I'd say it's safe to bet that the AC compressor is OK and not locking up, and it may be an issue with the IAC (idle air control).

 

check for locked up compressor, sticky IAC (happens often on these trucks, the egr generates substantial carbon and gunk buildup over time), also a bad tps (or tps going bad) can also cause such an issue.

 

rip out ISC, take apart, clean well, reinstall, see if that works, if not, replace

 

I picked up a new IAC and I'll get that put on ASAP. Even the guy at autozone said that could be the issue. The truck doesn't stall at idle but it does drop from about 650-700 RPM to 500 or less when the ac comes on. I replaced just about everything in the ac system so unless I just got a bad part from the factory, I doubt that's the issue. And the TPS was replaced not too long ago. Hopefully this will do the trick.

 

Once you have the new IAC installed, make sure you disconnect the battery for a bit, then reconnect it and idle the truck with all the accessories on, fan on high, high beams, etc. I had a little trouble with mine when replacing the IAC, and relearning the idle helped a lot.

 

Well the IAC was definitely the problem. It must have been bad for a long time but didnt really start to show itself until the ac was running. That fixed problems that I didn't even know I had. The truck drives completely different now. Thanks for the help!

 

yea a/c will highlight a bad iac. the iac has to open its furthest when the a/c is on, so it gets caked all winter (unless you use defrost a lot) and then cant go back to that point come summertime.

fwiw, disassembling the iac and cleaning it often fixes the problem with no money spent.