My CCU for my '83 300SD had some mechanical issues (off button would not disengage any of the other buttons). Turns out that there's a slide rail that shifts as you press a button to release all the other ones. I ended up desoldering the entire unit, took off the daughterboard, then took off the switches from the mainboard.

I THOUGHT I soldered it back on properly, and when I plug it in, I hear the vents opening/closing, but when I put it on defrost, the fan does NOT turn on. Nothing I can do (not even turning the fan speed to HI) will turn the blower on.

I suspect I failed to solder a very specific spot, but I've been trying to look over the electrical diagram to see how the blower is turned on and I can't see how the CCU does it. I've looked at the bottom and don't see any cracks in the side connectors' solder.

How can I test the CCU with a multimeter to verify that it tells the blower to turn on? I have the CCU plastic housing off so I can always follow the circuits to see what joints I may have done poorly.

I take the blower control module's plug and wire #12 (power) to #6 (blower motor), and the blower turns on like a charm.

And if by some chance it's not the CCU (the blower did work this morning before I did any of this work), where do I check next?

I've already checked the fuse box and #10 fuse (powering the CCU) is OK. I'm gonna resolder the whole thing again and see if that helps.