my question is how does 93 mercedes 300se duo valve works do water only enter the valve are water enter and exit the duo valve. im trying to figure if its my heater core or duo valve thanks.

It's my understanding that the valves are closed (in other words, keep hot water out of the heater core) until a heat setting is chosen on the pushbutton climate control unit. Then the valves open, allowing hot water to circulate through the heater core. What problem are you experiencing?

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

If you pull the electric control plug off the duo valve then water should flow through to both sides of the internal heater. The control works in " reverse" there is power applied to the valves at all times when the ignition is on This closes off heated water, "turning up" the heater controls in the cabin opens up the valves depending on the heat required

Been through all this with him already. I don't see a good way to find out if it's the duovalve or blocked heater core without actually "doing" something. He's going to have to check for a blocked core but trying to flush it out, or try a new duovalve. Maybe he can find a used one to save some money.
Gilly

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