91 300TE 4Matic I'm trying to troubleshoot an intermittent electrical problem with the heater fan motor (without a schematic). I replaced the fan motor a few months ago due to bad bearings. The new one worked fine for a few months but now only works intermittently. It usally works briefly in the mornings for maybe five minutes. One morning the fan started when the driver's door was slammed shut.

The motor checks out OK.. works fine connected directly to the battery. Following the wires back to the firewall (behind the master cylinder) there are 3 coming through a connector, 12V (red), GND (blk), and control (yellow). When the fan isn't working there is no voltage on the red (12V) wire. The control (yellow) wire seems to function fine. When max fan is selected the voltage is 7.5V. When the climate control is off the voltage is zero. When auto fan is selected the voltage varies from 3V to 7.5V.

I have tried to trace the 12V wire back to a connector on the climate control panel with an ohmmeter but couldn't get continuity.

I have the circuit board out and have looked for bad traces or corrosion, but haven't found anything obvious. I've done a search and read a good DIY page for the circuit board. It addressed bad solder joints on the circuit board.

Are there any relays controlling power to the fan? Any common areas with loose connections? Does anyone know the orgin of the twelve volts going through the firewall connector and to the fan?

Thanks Barn

91 300TE Climate Control Circuit Board

Attached Thumbnails