I've been doing some "searches," so let me know if I have this foundation of knowledge right on how to troubleshoot an auxillary fan not coming on (82 300SD):

If the fan is not working you could run a wire to the battery just to see if it works at all.

If it works, then the next step is to check the connections at the reciever / dryer.

Next is to look at a "relay" (also known as the "ice cube" relay).

See the pic - is that the right relay? (Originally Bosch 0 332 205 002. 5 pole terminal).

If so - Where the heck is that on a 82 126 300SD - under the cover with the rest of the fuses up by the firewall are somewhere under a cover near the fender????

From my research there looks to be two conditions when the fan is supposed to come on, and the fan has two speeds. There is a "high" speed triggered from a sensor near the thermostat when coolant temps exceeds 100* C, the low speed is triggered based on the air conditioner creating pressure in the system at the receiver / dryer (most common condition).

The auxilllary fan's main function is to pull air through the condensor to assist the A/C. So the auxillary fan helps your A/C condensor mostly, and kicks into "high" speed in severe overheating to help out your radiator.

The cooling system shouldn't need the auxillary fan to keep the car temp in the normal range - right???

There is an auxillary fan switch on the reciever / condensor, but I think if you remove it, you loose your freon. Is that correct???

So if it's not the fan, or the relay, I probably need to talk to an A/C specialist.

I'm trying to troubleshoot my friends 82 300SD... thanks all!

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__________________
~shell
As of 2/2010:
2001 CLK55 0o\=*=/o0
13.6 @ 106mph
10K mi
1984 300SD
260K mi and going and going...
97 S600
46K miles
1991 Sentra SE-R (extremely dorked with)
www.se-r.net