The the temperature you have mentioned, the fan must come on. If not, then you have to check:

1. the fan's fuse
2. the fan's motor, you can check resistance across its terminal to see if the winding is not open. You should get a low resistance in a few ohms across the winding. If the winding is o.k., apply 12 V directly to the fan to see if it is turning. If the fan works directly, then need to check sensors that activate it.
3. There is a coolant temperature sensor somewhere on the engine, on my 87, 260E, it is the second last from the windshield side. This is actually a switch that closes when the engine temp is high. NOTE: on some models, this sensor may have 4 pins rather than 2 and you will have to find which two belongs to the aux fan. The other two belong to a signal that goes to the a/c relay. Take the connector out and short the two ends that correspond to the fan to see if the fan comes on. If it does, replace the sensor. If it does not, just connect both the pins to battery ground, if the fan still does not come on
check the aux fan relay. On my 87, 260E, it is K9 in the relay box.
4. Check pins 3 and 4 on the relay socket, they should have approx 12V. If they do and the fan still does not start, replace the relay. if you dont, then check fuse (fuse D) and wiring to the socket. If this is the problem, post again and I will look up further.

Another way besides a hot engine that triggers the aux fan is if there is high pressure in the refrigerant line.

One way to test the aux fan even when the engine is cold is by setting the a/c temp to minimum and running the blower motor at the highest speed. If the motor comes on, it tells you that the motor is fine. If it does not, you still have to do the above tests to find out if there is a problem with the fan/relay or that your a/c push button unit is not sending the right signal.

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Saumil S. Patel