Originally posted by someguyfromMaryland
AFAIK, on a 1991 300TE 4matic, the aux fan is operated on two speeds. The low speed is run through a resistor (which is open on my car) which gets its input signal from the temp switch on the receiver/dryer. There is also a pressure switch on the recvr/dryer to cut out at low head to prevent running the system without refrigerant. The high speed fan circuit receives input signal from the temp sensor on the cylinder head. If the low speed circuit is working properly, you will not see the car go above 100C unless you're in slow traffic.
I'm still looking for anyone who can confirm if the temp switch is exposed to a/c refrigerant.
I guess the Wagon is different.
My schematics and CD data for a 124.230 chassis [91-300E-4matic] show low speed fan a/c drier sw part S-32 as
a pressure switch for high side pressures of cut-in/cut out of 20/16 bar.
The other pressure sw. is , as you say, to prevent compressor running with low freon. This one has nothing to do w/fan operation.
The CD procedure calls for evacuation/recharge when changing
As an added note on the high side low fan pressure sw. systems:
If this switch is jumper and now has low fan, the switch should not be condemned until freon level has been checked. [ a slightly low freon level will not allow the system high side to get to the cut-in pressure required for the low fan and often mis-diagnosed.