What scenario involving hot air would not involve the monovalve?

There should be system voltage at the monovalve regardless of whether a/c is "called for" or not, provided that the key switch is in the "run" or "start" position.

There are other scenarios. Burnt fuse is one. CCU not putting out 12V is another. The monovalve controls the water flow but the problem leading to the monovalve not in the right position can be caused by other factors. I suppose one can start from the monovalve and work backwards.

The monovalve voltage is 12V for A/C, 0V and anything in between is for heat when the key switch is in the "run" or "start" position. I do not know what 'System Voltage' you meant. The 300SDL service manual does not have that term.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

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