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Transmission problem with fan on

I have a 1999 Mitsubishi Eclipse Spyder GS with an automatic transmission. The car drives fine and has been well taken car of (I have owned the car since March 2000) Recently it has developed a problem with the transmission while the cabin fan is on.

Like I said the car drives fine, however when I turn the cabin fan on (the one for AC/Heater) the transmission will downshift, then upshift, then downshift, etc. It will hunt for the correct gear for about 15 seconds or so (as seen by the RPM speed), it will eventually settle down, but as it is I can not use the fan while driving as I don’t like what it is doing and I’m afraid it will cause further problems. I can use the fan at idle no problem. This problem happened suddenly with no warning.

I have checked everything I can think of. Battery, alternator, wire connectors on transmission, grounds, relays, fuses. I don’t know what else to do. I have also searched Google for others with the same problem but didn’t find anything.

Any help is appreciated.

Does it misbehave if the ac is off? Note defrost mode usually engages ac.

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You are amazing! I just took it down the road. It does it with the AC or defroster on. I didn’t know the defroster used the AC, and it has been cold here so I had been using the defroster.

I guess that means it’s something with the AC compressor or the AC clutch. I will have those checked.

Thank you, I was at my wits end.


On most cars the AC does come on in defrost mode to dehumidify the air going to the windshield.
That was an excellent suggestion by Barkydog, I would never thought of that!

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So, I have disconnected the AC compressor by taking the belt off, unplugging the electrical connector, removing the relay in the fuse box and disconnecting the AC fan by the radiator. Essentially the AC system is taken out of the equation and the car still acts funny when the defroster is on.
I’m guessing it is something electrical, but I have no idea what.