"My e28 M5 would run very hot at idle with a/c off. It did not heat up with a/c on. I replaced thermostat and fan clutch w/ no effect on problem. I then replaced the auxiliary fan switch with an 80/88 degree celcius replacement switch from bmp design and the problem went away."

So I decided to take the M5 out for a spin to test if turning on the A/C would keep it cool. It didn't help and may have actually caused it to heat up more quickly.

So what does this mean? I have a couple theories:

1) I can see that the aux fan is spinning, but maybe it's not spinning at the higher speed needed to keep the temp down at idle. Because the fan isn't working via the A/C switch or the temperature switch, I would think this may indicate that the fan itself is bad, unless of course both switches are bad. Is there a relay or something else here that could be the problem?

2) The problem is elsewhere. Such as the radiator itself or the water pump. However, I don't think these are the problem as it runs nice and cool at speed.

I replaced the fan clutch, but have not replaced the thermostat yet. I may do that tonight.

Any ideas or other suggestions would be much appreciated.

Thanks.

Sean
'88 M5
'76 2002