'87 300TD Climate Control/Mono Valve problem.
 

Hi. I was hoping to get some insight on an issue with my climate control. I just got an 87 300TD that only blows hot air out of the vents. I checked the mono valve and of course it is not closing. It has 12v on one pin, however, the other pin is never grounded, even when the climate control is set at its lowest setting. I jumped the ground pin of the mono valve to a ground, and the valve vibrates a little bit, but doesn't shut off coolant flow. I am guessing that the mono valve is shot. I will check the current draw of the valve when I have my multimeter handy. My questions are...

Can I fix the mono valve with one of those kits?
If the coil of the valve shorted, would it take out the climate control unit?
Is this a common problem that can be fixed by changing a component/ or fixing a burnt trace?

I have decent mechanical and soldering skills, so if these things are fixable, I would like to give it a shot. I like to not have to buy a $300 CCU.

Thank you for the advice.

HVAC CCU AND Mono-Valve
 

As you'll notice,one wire from the Mono-valve goes to the CCU
and the other goes to the Auxiliary Water Pump.
(AND the Aux Pump feed back into the CCU.)

Aux Pump may be BAD

OR [And, I hope not]
The CCU is NOT grounding the Aux.Pump/Mono-Valve Circuit.

http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/ETM/ETM.pdf

I didn't notice that. I will check it out. I did notice that the Ground wire on the monovalve is the same color as the L10 wire on the CCU harness. Maybe the wire runs through the AUX pump. Thanks for the response.

the schematic says that the ground goes directly to the controller. I guess I will check for continuity and cross my fingers.

Does anyone know where I could possibly get a schematic for the control unit itself? possibly from a rebuilder?

If you open the controller you can resolder the joints between the boards. That usually helps. Also look for burned traces. I had to run a wire to replace a burned out trace on my 300SD.

There was a recall on the original Bosch unit, bad solder joints ('87).

I think I got everything all straight. The AC even works after I charged it. The previous owner said there might be a leak. Next I need to figure out why the electric fan isn't cutting on. Thanks for the advice and the helpful schematic.

What was the solution?