92 300D fan problem
 

The fan in my 92 300D only runs on low speed, no matter what speed it is switched to. I am talking about the fan for the ac and the heat. Bad switch?

Either the pushbutton unit or the fan speed regulator module, which is mounted on a heat sink below the blower motor, could cause the problem that you describe.

You need to measure the control voltage coming from the pushbutton unit. There is a 3 terminal connector behind the brake booster that has black (ground), red (+12 volts) and yellow (control) wires.

The connector is quite difficult to get to. I made an adapter to measure the voltages but you could remove the plastic connector housing, re-install the terminals and then probe them to check voltages.

The voltage on the yellow wire should vary from 1.0 (low speed) to 6.0-10 volts (high speed) depending on your pushbutton unit settings. If your voltage does change but your fan speed does not then your speed regulator module is defective. If the voltage stays low, then it is your pushbutton unit.

Plan on it being the regulator. In either case I have both parts if you need them

Ok, I changed the regulator and no change, I also changed the push button unit and no change. I also hooked up the fan directly to the battery and the fan is fine. What do I do now?

Did you replace the regulator and heat sink as one assembly or did you replace just the regulator and bolt it onto the old heat sink?

If you did the latter, then you need to be sure to use heat sink paste between the regulator and the heat sink.

I had this problem on my car until I replaced the regulator and put some fresh heat sink paste between there.

Heat sink paste? No i did not put any heat sink paste between them, that will cause it not to work? Where do I get the heat sink paste? Thanks for all the heip.

I know that if the heat sink compound is not present it can cause the resistor to overheat. The paste helps transfer the heat between the two surfaces. Also, on my car, the previous owner had replaced that part without the heat sink paste and the resistor burned out. When I replaced it, I put the paste between the heat sink and the resistor and the blower now blows very strongly and has been doing so for over 2 years.

You can find it at most electronics stores, but you can also buy it online.

Also, did you test the voltage going to the resistor to make sure that the CCU is working properly?

get a tube of the heatsink paste from frys or best buy - its in the custom computers aisle.