Had the same problem but not as severe with my 240.

First, replace fuse as has been said. Then replace thermostat - get a good one, not junk. That solved my running hot problem.

The two relays on the left front fender under teh black box control the ac clutch and the fan - they are the same and are interchangeable, so try swapping. If you still don't get the fan, when you short the pressure switch, but you can get it on with a direct wire to the fan, it's somewhere in the wiring.

I have heard of people using the clutch relay to control the fan also - that way the fan is always on when the ac clutch is activated. Just takes a bit of rewiring. Not perfect, but might get you by.

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer