Subject: won't idle when put in gear or the a/c is on
Question: 92 mitsubishi eclipse,1.8 auto. the engine will not idle when I put it in gear or turn on the a/c, or the cooling fans turn on.
Answer: Curtis,
You may need to have your alternator replaced. A simple test is to just leave your car turned on and in PARK, then turn your highbeams on. If your car cannot sustain idle while intense electrical items are turned on, there's a good chance your alternator is dying. If the alternator that's on your car now is still the original alternator, it's probably time for a new one. The nice thing about the alternator in the Eclipses is that they can give lots of warning before it completely dies.

If you have an AutoZone in your area, bring your car there and they will check your alternator and battery for free.
Good luck!