How far should the monovalve move? I put voltage to it manually and it just made a slight click with no discernible movement. Should I be able to grab the exposed top of the plunger and move it up and down? If so, how far?

If your car is set up like my 300D, then the monovalve is basically a solenoid that is opened and closed incrementally by a variable resistor -- the temp wheel in the CCU cluster. Should open partially or totally, depending on the temp setting. As for power to the compressor, I think there are a couple of switches in series with the relay that controls it. I'm not sharp enough on AC work to know just what they are and have had a difficult time finding exactly the correct wiring diagram. On my car, one is beside the glove box. Can't recall where the other is. Probably not a good idea, but I wired around them and my AC worked.

__________________
John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln

The valve moves about 1/2" down. You should hear a click as the solenoid force the plunger to go down. The plunger blocks off the flow of hot coolant. It is 0V ( open ) when calling for heat and 12V ( close ) when calling for ambient or A/C. If you get hot air when it is not called for, try measuring the voltage across the connector. It should be 12V.

Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.