Car is a 1992 300D; a 124.

First one flow of air went intermit, then the max vents went intermit, then the blower went intermit on defrost. Now nothing works except heat. Blower is not working; heat blows through vents when car is moving.

Unplugged the mono-valve; this stopped the constant heat.

Removed the control panel. Tested for power to unit (plug on left side of control panel) and power is there; tested for power out of unit (plug on right side of unit) and was getting 12v from every pole no matter which buttons were pushed and no matter what fan speed was selected.

Also: When testing for power out of unit I had to have the output side unplugged. When I switched on the key to give power to the unit the blower came on at high speed. When I plug the output side of the unit back in the blower motor will not run.

Think this is a bad control head? Is there a way to test the output of the unit by seeing which pins are hot when a button is pushed in?

Any answers will be helpful. Anyone have a working head they wish to sell?

Sam

Sounds like your push button control unit is going south. The hot air while driving is a classic symptom. There is a 3 pole connector near the brake booster that you can test for output voltage coming from the PBC. Do a search for the spec values coming from the yellow wire and test. You should not be seeing continuous 12v output at different fan speed settings.

The PBC unit is known to develop bad solder joints as it ages. If you're proficient in soldering you can take apart the unit and look for any suspect solder joints to re-solder.

Another possibility is the blower regulator but as far as I can tell these rarely go bad.

Good Luck

__________________
-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi