Originally Posted by babymog

Old thread, revived for a gas ML?

psfred was spot-on.

Anyway, on the 201 and 124, the aux fan is turned on by:

High engine coolant temperature (105C?), ... high fan speed.

Refrigerant (A/C) pressure (?? bar) during normal operation, ... low fan speed through resistor.

If it doesn't run during A/C operation, you might have low refrigerant pressure. Apply 12v directly to one end of the rectangular ceramic (speed) resistor: If it runs high speed with 12v applied to one end, but doesn't run with 12v on the other end, the resistor is bad. Common. If it runs high speed with 12v on one end, and low speed with 12v on the other end, the resistor is fine.

With the aux fan(s) not running, your A/C will not cool well when driving slow or idling, will cool down at highway speeds normally.

I've had this symptom in one 124 and one 201, in both cases it was a bad speed resistor and was cured by replacing the resistor. The engine fan is for cooling the engine, not the A/C condensor, the electric fans are for cooling the condensor, and for cooling the engine at low speeds.

There are two pressure switches on the A/C: one is the low-pressure cutout that cuts power to the compressor if there is iinadequate refrigerant pressure as tf007 mentioned, the other is the high-pressure switch that turns the fan on low speed.

And yes, the temperature switch on the engine runs the fan directly.