Heater control valve question

[Duke2.6]

'88 190E 2.6... I need to do a coolant change.

With the engine off I assume there is no vacuum to the heater control valve, and if this is the case, is the valve open to allow coolant to drain from the heater core, or do I need to remove the cowl screen and apply a vacuum to open the valve.

I had to replace it about 15 years ago, but, unfortunately, threw the old one away.

Duke

[Frank Reiner]

Most of us here are probably thinking in terms of an electrically operated monovalve, rather than a vacuum operated valve. Is the 201 unique?
If it is electric, no power = open.

[Duke2.6]

My '88 190E 2.6 has a vacuum operated heater coolant flow control valve that allows or blocks coolant flow to the heater core, but I would expect there is an electric solenoid buried somewhere in the HVAC system that allows/disallows vacuum to the valve. I've always thought that other Mercs of similar vintage have the same vacuum operated valve.

I know I've heard the term "monovalve" to describe what I've always thought of as a heater core flow control valve, but I didn't know that some models are electric. Is the S-class heater control valve electric?

Maybe the 201/124 is different because they were designed a few years later.

Does anyone with a 201 or 124 have an answer to my original question.

Duke

[Frank Reiner]

It would appear that, in fact, the 201 is unique among MBs of that era in using a vacuum operated heater water flow control valve. Other models employ a solenoid operated water valve.
The 124 uses the electric "monovalve".

[Frank Reiner]

The 201 vacuum heater valve is normally open, i.e., no vacuum = valve open.

[Duke2.6]

Thank-you!

Duke

[lsmalley]

I've considered upgrading the vacuum heater valve on the 201 to the w124 electric monovalve, I just never got around to it. Seems easy enough to just wire it in using a relay so that whenever the aux water pump is running (heater on) then the valve opens up.