Have a '92 300D with the 2.5 Turbo diesel. When my compressor is engaged the two auxillary fans don't work. They will come on when the temp gets up to 90C. The belt-driven fan is engaged when the engine temp. is 80 or higher. Question#1--do the auxillary fans turn when the compressor clutch is energized? Question#2--Where is the relay for the fans on this car? I don't have a manual for this car but I worked alot on the same system on my previous 190D. TIA. Ernie in Louisville

Dear epssax

The aux fan or fans don't have anything to see with the engine temp nor
the compressor clutch. They are controlled by the temperature of the cooling
gas in the airco.
They will be started as the temp comes to high for proper airco working.
In my car after a short stop with a warmed up engine, I can here my fan blowing ( it's rather noicy ! ) right after starting again ( only with airco on and
in warm weather ).
I hope this will help you to understand the system.

Danny

95 Y300DTA's explanation is correct except, technically, they are not turned on by refrigerant temperature, but by refrigerant pressure on the high side. This is to bring down the high side pressure to prevent excessive pressure which can cause damage to the a/c system.

Have a great day,