Like pawoSD, I suspect it is drawing in outside air when it shouldn't. Read the Owner's Manual since the temp controls are non-intuitive, though the problem is likely not user-error.

As mentioned, in the engine bay, feel the tubes going into and out of the heater core. If both are warm/hot, the problem is in the cabin in the air box (flappers doors, controller). If they stay cold, and the mono-valve is unplugged, and you rev the engine a bit (water pump doesn't cirulate much at idle), the problem may be a clog, either in the heater core or in the electric auxiliary pump blades (near alternator). I removed mine (leaking shaft seal) and replaced with heater hose. I get plenty of heat (when the controller cooperates), just not much at idle.

Its still available.
Pelican Parts - Automotive Parts and Accessories - Porsche & BMW

Higher engine rpm results in higher coolant pressure at the monovalve. Either the diaphragm is allowing coolant flow to the back side (which will prevent the valve from opening) or the spring in the valve is too weak to overcome the higher coolant pressure at high rpm. Or some combination thereof.

Be mindful that the monovalve is closed electromagnetically and spring-loaded to the open position. If you don't believe that the monovalve is the problem, replace it with a piece of copper tubing and you will have more heat than you can stand. I guarantee it.

Your problem isn't electrical. It isn't airflow related. It's the monovalve.

That's a little hard to do at highway speeds.

Aftermarket parts are often worth less than the box they came in. A poor quality monovalve insert will cause precisely the symptoms that you report.

A friend has this exact issue in his '84 300D. He's replaced the diaphragm on his monovalve using the aftermarket rebuild kit from Pelican. His ACC is working flawlessly in every other way. All dash pods are working, including recir. He gets perfect heat on surface streets, luke warm air on the freeway. If he unplugs the monovalve, he gets full on blazing heat all the time. There is something not working correctly in his monovalve that is not related to a torn diaphragm. Hopefully this thread will lead to something that can help both of these guys.

Only the $128 genuine Mercedes part is called a lifting magnet. Kinda expensive heh? Must be a translation issue. It can't be a magnet. An electro magnet yes. Plain old magnet no.

Thats what mercedes calls it.

Is there a source for just the monovalve diaphragm rubber?

Seems like the Bosch units have a C or E clip that can be removed, and the diaphragm (which is almost always torn on bad monovalve diagnoses) can be replaced. I just found out mine is torn (again), even on a Bosch unit I replaced a few years ago.

MTC seems like the only unit available but I don't want to get the whole thing if there is a source for the diaphragms only. Has anybody sourced these?

Thanks

100% true.


/

there is one thing which happened to me with monovalve
it melted completely and the plunger inside of the coil stayed
in closed position in a way that spring was weak to re-position
it on lack of voltage.

it was not obvious from the outside,..it melted inside the housing.


.

The electromagnet is the coil where the monovalve, AKA lifting magnet, slots into. Monovalve "rebuild kit" itself IS the magnet.

I got new one from MB, $108 through my parts guy. Same rubber labeling as my old Bosch unit, which also ruptured years later. MB has 1 year warranty on item.

Mercedes calls it a magnet but that is technically incorrect, as I pointed out earlier. Just think, if it really is a magnet, it would stick to anything that is iron and stay in one position and not able to be moved into the open/ closed positions.

Wikis on electromagnets and solenoids:

Electromagnet - Wikipedia, the free encyclopedia

Solenoid - Wikipedia, the free encyclopedia

An electromagnet is basically a coil of wire wound around an iron core (what you are incorrectly calling a magnet, the part that has to move, to do the actuation).

This monovalve assembly is an electromagnet, or solenoid:
http://www.peachparts.com/shopforum/...y-mvc-855f.jpg


Monovalve iron core actuator (what Mercedes calls a magnet? which it is not):
http://www.peachparts.com/shopforum/...y-mvc-863f.jpg

Someone should be able to sell you the mono valve as shown in the last pic. They are plentiful and cheap at junkyards. If you can't find one pm me, I can get them anytime.

Just to be completely OCD on definitions:

An electromagnet is a device that has a core and coil. The core is fixed. Common examples are the big pickup device on the end of the crane in a scrap yard or the device that holds doors open and lets them close in case of fire.

A solenoid is a coil with a movable core. Common examples are valve actuators or door chimes or the device that pulls the starter shaft in contact with the ring gear.

Agreed. Just wanted to clarify that it the monovalve element itself cannot be an electromagnet as you suggested - that's all. :) Sure it's not a magnet either!