When running the A/C I can hear gurgling from the heater core indicating trapped air in the coolant system. Shouldn't coolant flow be stopped when the A/C is working? How does the monovalve work and would this be a sign that it is malfunctioning?

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1989 300CE

The monovalve is controlled by the CCU, bat voltage = off. I think the CCU pulses the signal line so if the CCU is turning it on you will have something less than 12 volts. If you have 12volts (battery voltage) on the valve then the diaphram may be torn and leaking hot water. A "rebuild kit" cost me about $50 last year.

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K