The boot tearing is the most common cause of no heat. A 5 cent rubber part. The boot can be taken off via a clip on top and you just replace that if you have a boot that is not torn. Used monovalve with a good rubber boot will do the same thing. The cylinder and pin in the cylinder is not defective, just the boot for 99% of the time. Head to your local junkyard and pull monvalves until you find one with a good rubber part and you're good to go. Takes all of two minutes to remove and replace. Gas engines have the same part.

I wonder, is there a way to source just the rubber part and replace that?

Not that I know of besides the junkyard.

I believe unplugging the monovalve gives full heat. It closes when it gets power I think.
However, if the diaphragm is torn, unplugging it does not always assure full heat.

DAMN you guys are GOOD! That was going to be my next question! I thought of that as a possible temprorary solution last night after I went to bed! If I did this, wouldn't I still be able to control the heat by turning the fan switch to high/low/off? No way to change the temperature of the air output, but if I can turn the heat to high/low/off, that will do for our trip tomorrow.

That's an easy one to test - I can just unplug it before I head home from work tonight and see what it does. If that doesn't do the trick (temporarily) then I'll pull the innards out tomorrow morning.

BTW, I did an eBay search on "Mercedes monovalve" and only got one hit for a rebuild kit - for 56 bucks!

My problem is no heat on my feet while moving - W123 '85
 

Heat works fine while stationary, defrost and footwell are toasty. When I get up above maybe 20 mph the heat is no longer coming out on my feet. Does this sound like it would be fixed by the monovalve repair also?

Does the air from the defrost vents go cold above 20 mph too? If so then that pretty well matches my symptoms as well as those of the member who started this thread, so I'd be inclined to say, yes, you have the same problem.

If the air from the defrost stays warm, but just the airflow to your feet stops, I'd be more inclined to think it is a problem with the contoller. The solder joints on the board break and then airflow direction and control goes all whonky from what I've read - but I'm certainly no expert on these systems - still learning myself.

You might want to carry a clamp so that you can make a primitive adjustment (partially or completely clamping a heater hose) to the flow through the heater core. Full heat can be unbearable, even with the fan off.

Good news and bad news.

The bad - unplugging the power from the monovalve did nothing. The one on my car apparently needs power to OPEN, not to close. The rest of the bad news is that the diaphragm in it is totally shot - ripped all the way around where it attaches to the plunger.

The good news is that removing the "guts" worked! All I had to do is remove the e-clip from the end of the shaft and slide the plastic spacer ring, rubber stopper, and filter screen off the shaft and slide the soleniod and the rest of it back into the body of the monovalve. I left the outer portion of the diaphragm in place because it didn't look like it would seal up water-tight without it.

Took it for a test drive and VIOLA' I have all the heat I need! With the blower off it barely puts out just a tiny bit of warm air, but with the blower on high it warms the interior up real nice and real quick, and with the blower on low it still blows enough warm air to keep it nice and comfortable inside.

For anyone else in this situation, this is just a temporary fix, but one that will work quite well until you can get the parts to fix it permanently...

Don't bet on it. With a torn diaphragm, electrical control is fairly irrelevant.

Well, I'm just guessing by looking at the way it is made, but since it is solenoid operated, and the at-rest position position for most solenoids is with the center bar sticking out of the coil, it seems like the at-rest (de-energized) position seems to be closed - with the rubber stopper on the end of the plunger pushed down into the hole where the bottom hose attachment tube is. It at least LOOKS like it would be closed with no power applied to the coil. But I certainly could be wrong - wouldn't be the first time.

When I replace it I'll mess around with the old one I take off to see what it does with and without power.