Another strange AC problem

The RPM jumps because the compressor clutch is dis-engaging. I'm not that familiar with a 90, so I don't know if the system uses an "amplifier" or not. If possible, I'd try to see if the compressor kicks back in when you cycle the A/C switch. There is a possibility that the pressure switch is shutting down the system, or as you said, some sort of other electrical fault. (Fan, relay, etc.)

Thanks. Going to replace the ac compressor relay after I fine it. Not sure of the location. any ideas? can't find it in my manual or online.

Nevermind, I figured it out.

Got these?

I hope the relay fixes the problem. Personally, I'd likely see if the A/C radiator fan is running after the compressor kicks out before I replaced the relay. On the other hand, the relay switches an inductive load, and after twenty years, it very well might have worn out.

We had an 89 Mazda MPV that seemed to "eat" relays. A/C relay, Rear defroster relay, relays inside the turn signal module, electric "kickdown" relay, Overdrive relay, and so forth. The typical failure mode was contacts heating up, melting plastic, and then the contacts would not close. No shorts were ever found. I believe the relay mfr over spec'd the relays in the MPV since the failing relays were all the same part number, except for those in the turn signal module, which were made by the same mfr.