My MB 250D-92 had the same probs,the air from the vents was hotter then hell...
Monovalve should always have 12-14 V on the middle pole of contact, and the electronic circuit grounds the valve when it closes....intermittent.
The temperature on the air depends of the time ratio the valve is open/ closed.....so the walves should "klick" intermittent when itīs functioning...
If you not have 12v on the middle pole with ignition "on", check the fuse....

Excuse for my bad English, im one of the Swedes reading this exxelent forum =)