If you still have no heat with the monovalve unplugged (no power) then there is something mechanically wrong with the monovalve or something is plugged up. It doesn't matter in this case but the monovalve is controlled by the control panel. I'm not sure where the heater hoses hook on the 300SE (mine is a V8) but both should be hot. If one is cooler than the other, the system is plugged up, either the monovalve is stuck off, or the heater core or one of the heater hoses is plugged. The only thing I can think of now is to unhook both heater hoses from the engine, rig up an adapter so you can connect a water hose to one of the heater hoses, turn on the water and see if it comes out the other heater hose. In other words, flush out the heater core.

Paul

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84 500 SEL (307,xxx miles)