More CCU woes - PeachParts Mercedes-Benz Forum

LUVMBDiesels · #1 03-30-2007, 02:11 PM

I have been going through the posts on climate control units but I need some more help. The car is my 1981 300SD

Here are the symptoms:

Monovalve circuit is not working. The "hot" side is always reading 12v, but the "cold" side never connects. This forces the valve into full heat mode no matter what the temp setting is.

I have had to make a jumper from the monovalve to ground which activates the valve and causes me to go to no heat.

I checked the infamous fuse #10 and replaced it but the problem did not go away. I know fuse #10 is working because I have backup lights and the kick-down on the tranny is working.

Has this happened to anyone out there? Can somebody tell me if this could be an external relay or if I need to pull apart the ccu? If I need to pull apart the ccu, can somebody point me in the right direction for troubleshooting ie; is it a transistor issue or maybe a relay.

Thanks in advance!

Rick76 · #2 03-30-2007, 04:31 PM

The pushbutton assembly is a common source of problems causing a wide variety of symptoms. There are two circuit boards (that the connectors plug into) that mount vertically on the main board. The solder connections joining the two crack from age. Also solder connections on the relays will need to be re-soldered as well.
Inspect the copper tracks for any signs of overheating (portions may even be missing) due to shorted components such as the aux. water pump or monovalve.

LUVMBDiesels · #3 03-30-2007, 04:49 PM

Thanks, I will disassemble and check with weekend.

Regards,
Joe