Engine Cooling

Cooling Fan Runs Continuously

--------------Problem--------
One fan runs at max at all times. The other is running controlled the way it should. I can only shut it down by pulling the connector out on the fan.

I've tried disconnecting:
>1. the water temp sensors (both on the front of the engine)
>2. the coolant temp sensor (rightside down on the radiator)
>3. freon pressure and freon temp (at the unit just in front of radiatoir)
>4. manifold air temp sensor (on top of the manifold)
>5. is there still one more sensor for fluid temp of the AT ? (don't know)

Checking means: pulled out the sensor connectors and left open all at once.

--------------Solution---------
This is the procedure :
The control unit is located on the front-left side of the engine compartment.
It is the one with the finned heat sink on top and the round connector.
1. unplug the connector and the 3x10mm screws
2. pry off the bottom plate with a screwdriver. (carefull, you'll need it later)
3. take out the insulation plate (brown, loosely placed inside)
4. take off the rubber seal ring
5. take out the 4 philllips screws that hold the PCB
6. carefully push out the round connector (it has a rubber ring that seals it)
7. there are two assembly's that carry the power drivers for the fans
both sets are independant for one fan. They are equipped with 2x BUZ11a
N-channel MOS FET's (like transistor type). Specs: 50V, 26A max load
8. if in doubt of correct functioning, just replace the components (2$ each)
9. drill out the part that holds the FET in place on the heat-sink
10. solder out the old FET (s). Carefully; use a TIN suction pump if available.
11. drill a hole through the heatsink and make screw thread in it to hold the new FET
12. attach the new FET (s) using a standard screw and use silicone paste to make a good temp-conducting mount on the heat-sink
13. solder in the new FET (s)
14. re-attach everything in reverse order. Try to keep the bottom plate fixed.

Guess what.... it runs OK again.
If you experience problems with the control unit itself... don't worry.
Just replace all non resistor and capitors. All are standard components.
The PCB doen't look half as complicated as the fuel computer.

Again, thanks to Bob for his help in the matter!!!!
Theo Jenniskens
'88 928s4 Cherry Red
The Netherlands