So it is my understanding, that the monovalve shuts off when energized, and is open when not energized. When I switch off the engine (at operating temp) for a while, and then restart and drive, the AC first blows hot air out off the vents for a short while an then goes back to cold.

So my thinking was: if the engine is off, there is no voltage on the monovalve and it will stay open, letting the heater box fill with hot water. After restarting the engine and driving with AC setting cold, the monovalve will close immediately, but the remaining heat from the filled heaterbox will be blown into the cabin first.

Ok, but in the shop manual I found a word about a checkvalve within the monovalve, which apparently avoids exactly the above mentioned problem.

Ok, then I bought a brand-new Bosch monovalve (not the repair-kit insert, the whole thing) for over 100 bucks, installed it --- and: problem persists!!!!!!

So is there really a checkvalve? If so, is it part of the monovalve? Or somewhere separate in the heating system? Or did my new monovalve come with a broken checkvalve?

Aah, forgot to say: it is a 1985 300D (Turbo) California car.

Thanks for any hint.

Michael

I don't know what the checkvalve does but from what I've read a faulty checkvalve will allow hot coolant into the cabin when the monovalve is closed.

It surprises me that the heater core can get so hot with the engine off. The water pump and auxiliary pump aren't moving coolant so how can so much heat get to the heater core?

I really wish someone would come up with a tropical monovalve that is closed by default.

Sixto
... stay tuned