OK let me try to get this straight. The imput side of the heater core is attached directly to the engine block or water pump. The coolant comes from the engine into the heater core unmolested. Exiting the heater core on the drivers side is the AUX water pump. Basically sucking water from the heater core. It pushes into the monovalve and if open circulates hot water through the system.

Please correct me if I am wrong.

Two questions:

1. Is the Aux water pump always working or does it cycle?
2. What circuit tells it to come on or off?

MY monovalve lines are cool when a/c is on, but my aux lines out the fire wall are hot.

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1990 300d 2.5 new to me.