The monovalve always has system voltage (12 Volts) at one of its two pins regardless of the temp wheel setting. It's the ground pin that the CCU modulates to regulate the flow of hot coolant through the heater core. Heat at the defrost vent is the default setting. It means that the CCU is effectively "off."

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970