Hi all. I'm new here, and new to BMW-dom.

I've just inherited my father's 1990 535i, with an auto tranny and 220,000km on the clock (about 140,000mi).

I'll save the story for another time and cut right to the chase.

The aux fan never comes on. Never. The A/C also does not work. The car overheats any time the speed drops below 40mph, making the car undriveable.

Hoses, coolant and thermostat are new. Radiator is not OEM, but looks like a quality item in good shape. Probably replaced by a previous owner.

Now, my questions:

1. Could the aux fan problem be the reason why the A/C doesn't work, or is it wishful thinking that fixing the aux fan will restore the A/C?

2. Is the aux fan failure the cause of the overheating problem? The rest of the cooling system seems fine.

3. What are the steps for diagnosing the cause of the aux fan failure? I've read about aux fan switches and resistors, but can't figure out where they are or what they look like. I've ordered a Bentley manual but it hasn't arrived yet.

Any help is greatly appreciated. Thanks.