On my '93 300D I have consistently had some issues with running too hot and some strange issues with the HVAC.

I have replaced both electric auxiliary fans(they are working fine), the radiator, fan clutch, water pump and thermostat.

I have also switched from the green stuff to Zerex G-05.

Off and on I have had some strange activity from the A/C.

Today, like a few other days in the past, when the A/C stops cooling once the desired cabin temp is reached it doesn't just shut down the cooling it actually starts sending HEAT into the cabin.

It does this from the two outside vents while the center vents remain cool.

I also noticed that today it was 85-87F outside and my coolant temp was almost to 110C at highway speeds with the A/C on.

When the A/C cycled off I would get heat in the cabin immediately and my coolant temp would then drop almost immediately.

My question is whether or not those coolant temps are normal and whether there is something linked between the heating system and the engine cooling system that could be causing this.

I will say that this usually happens for a few days after I go through the car wash.

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1993 W124 300D
-297K on the clock as I type this.

Monovalve been checked? Perhaps the monovalve has failed therebye providing constant heat to the heater core, AC is working trying to cool the evaporator at the same time and is constantly engaged. As soon as the AC cut off you get only the heat? With the AC engaged the condensor is dumping its heat in front of the radiator also.

No, it hasn't been checked.

From what I can tell by looking at it and from the parts available there are two components connected.

There is the auxiliary water pump that seems to work fine.

It really moves the coolant to the heater core pretty quickly.

Does the monovalve separate from that platform they share or is it all one part?

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1993 W124 300D
-297K on the clock as I type this.