Idling in (God-awful heavy) Atlanta rush hour traffic this week with A/C on full blast I notice motor temp inching past 100C.

Off goes the A/C and the temp comes right back down. (Relief) If the car is moving with the A/C on, it does not begin to run hot. On further thought, I realized that last week I could hear the auxilary fan coming on and I no longer hear it turning on.

Where to start to trouble-shoot? It is not the fuse, that is all that I know for sure - the fuses are all fine as I checked them last night. I believe there's a pressure switch on the A/C drier that is NOT to be messed with lest I discharge the A/C (I am a total novice when it comes to A/C) as well as another switch that might be my problem.

Vehicle (in signature) is '83 300CD (turbo.)