1990 4 valve K100RS with Motronic Box
The fan runs continuously from the moment the engine is started from cold. Have removed and verified the varying resistance of the temperature sensor in hot water then reinstalled. I then disconnected the leads to the temperature sensor which I believe provides an infinite resistance and still the fan runs. Next I removed and changed the fan relay for a new one and the fan continues to run. Looking at the Clymer schematics I see a connection to the I just betchya probably really demoralizingly expensive Motronic box and am really hoping this is not the source of difficulty. Has anyone run into this problem or can make test suggestions other than riding everything over a cliff?

I see a number of posts about installing a separate switch to turn the fan off and on manually but I would prefer to have it work as BMW intended.

Thanks
geo

what happens when you tap the sensor ?

When you say "I removed and changed the fan relay for a new one", is that the small relay for the horn and the fan or is this the temperature switching unit?
Here is a simple test to see if you have a short somewhere:
Remove the Temperature Switching Unit (TSU) from its socket and put the ignition ON. The fan should not turn. If it does, you have a short on the wire going from the TSU to the fan (or the PO has installed a switch to bypass the TSU).
If the fan is not turning remove the plug from the water temperature sensor (WTS)
Find the connection for pin 'E" on the socket of the TSU then put one probe of your Ohmmeter to this connector and the other to a good ground on the engine.
You should not get any connectivity. If you have connectivity, the wire coming from the WTS is shorted (or the PO has installed a switch to bypass the TSU).
If you don't, replug the socket on the WTS. Check the resistance between pin E on the TSU socket and the ground. It should be close to the diagram On the EFI troubleshooting page. If its not, the water temperature sensor is bad.
If the resistance is close to what it should be according to the graph temperature/resistance, your temperature switching unit may have a problem.

On the 16 Valve Motronic equipped K there is no separate temperature switching unit as there is on the 2 V that is done inside the Motronic which sends a control signal to the fan relay which is just that a simple relay with no electronics in at all.

So I would favour measuring the resistance by removing the big Motronic plug and measuring the resistance (when cool after an overnight stop) at that plug.

Now this is a guess and not tested my reading of the BMW engineers psyche is that they would build in a safety for the fan control since fan running when cold is an annoyance whereas fan not running with an open circuit or short circuit of the temperature sensor when the engine was in fact overheated would be a big problem. My guess is the fan will run if the sensor is either open circuit or short circuit.

So have a check to see if the resistance at the big plug pins is correct.
Regards,
K-BIKE

Thanks for the prompt replies. Good suggestions and I will test some more this coming week when I return from this latest work assignment some 140 miles south of home.

geo

problem eventually traced to a faulty Motronic unit. A used unit was finally located and installed solving the problem.

Hi geo,
Give the plug and socket a good clean with DeoxIT just to be sure the contacts stay in tip-top condition.
Regards,
K-BIKE