I have a 1985 300td and the heater has begun to work intermittently. When driving on the highway it will blow hot air for a while and then randomly start blowing frigid air.
I took out the monovalve and looked at it yesterday. The black material at the tip appeared to be torn. I'd love to replace it, but it's Christmas day (merry Christmas) and I'm leaving on a car trip in a few days. I'd really like to have heat before I go!
I was wondering, since the monovalve is a control valve, is there a way to rig the thing so that it constantly sends the coolant to the blower? Would that be idiotic?
I won't need AC for a while and if that could get me until I return from my trip, I would be so happy.
Anyway, let me know your opinions.

I have nearly the same problem with mine. For full heat just unplug the wire at the mono valve. Then just switch on the heat and turn it off when your face turns red

Now as to why you have heat for a while and then is stops because it want's to let me know. It seams that if the mono valve works it works. Especially when you remove the power source and get full heat. That's telling me that the controller thinks the care is too warm inside. It seems all the climate controllers are brain dead, or at least confused. I have replaced the sensor tube already without success. Can anyone punch in here and us where to locate the temp sensor?

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)

There are two temp sensors, one in the water jacket on the engine and another in the air box under the passenger side of the dash. The sensor in the water jacket tells the ACC that the coolant is hot enough to warm the car. The one in the air box helps ACC regulate temperature in the passenger compartment.

The mono valve defaults to open, meaning, if it has no power it will allow coolant to flow through. So, to turn it on all the time, just unplug the wires. If it is clogged somehow, you can simply remove it and make a jumper from pipe or hose. Some people have even inserted a mechanical valve at this point.

When you have time, there's a repair kit that you can buy for the mono valve. Dealer price is about $75, less via FastLane (supports this forum) and other online Mercedes parts suppliers.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

That typically won't work when the monovalve diaphragm is torn. The heat still won't work at higher engine rpm's.

So what your saying is that if the mono valve is torn the heat will work at low rpm but when you get to highway speeds the heat will stop?

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)

My 80 300SD is having similar symptoms.
While running high engine RPM's heat is outrageous . With lower RPM's blows ICE cold (outside temp is -5F ).
I already have the automatic valve replaced with a manual valve, so that water always flows to the heater core.
I have heard about an auxilary water pump, but not sure if mine has it.

I also notice that my temp gauge reads lower at low RPM's, so I'm thinking it's got to be water pump / thermostat related.

Does anybody else have any great ?

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1980 300SD W116 - Daily driver. 362,800 miles and counting. (about 100 miles / day) -- on hold, have a ticking vacuum pump. New one on the way.

That's correct.