95 FZJ80- Rough idle only when the AC is on. Otherwise, low idle at 600 rpm. Where should I begin?

[rdcnj]

Based on the fact that your RPM's drop like the truck is going to stall, I think there is a fair amount more at play here.

It's possible that the AC compressor is starting to go bad and it's causing a massive load on the engine.

I would suggest two things:

• Check the amperage being pulled when the AC is turned on - High amps = high load - Could mean compressor is going bad and it's physically slowing the engine down when it engages.
• Check for air leaks with smoke - Air leaks will cause a serious amount of weird issues, but I would think you might notice some other drive-ability issues is it were that serious.

From the chair, my money's on the compressor. But there's plenty of testing and checking to do here.