In Reply to: Auxillary Fan questions and overheating M3 at idle posted by Oliver30 on February 28, 2001 at 23:03:15:
Pre '88 cars had a one speed fan which cut in when the temp guage hit the 3/4 mark, the sensor for this is at the bottom of the radiator (which eventually silts up!), and the wires break often.
Later cars have the sensor moved to the other side of the car (the bulb like piece of rubber on the passenger side of the car where the tubing comes out of the radiator - this makes it quicker to react). This controls the two speed fan. Speed one should cut in just over the half way mark and full speed cuts in at 3/4.
An alternative (which I've had done) is to have the wiring/sensor updated but have full speed set to come in at a little over half way - in normal driving your fan should need to cut in but in a serious jam it gives you a little reassurance - just what you need in a lot of modern traffic!!