Originally Posted by jay_bob

Yes it's an electromagnetic valve. It is a two-state valve similar to your washing machine or dishwasher water control. They cycle it on and off rapidly to modulate the temperature.

To Rocky Racoon's comment above about bad connectors, the nice thing about these connectors is that they are repairable. The housings and plug pins are available from the dealer, or a junkyard plug can be removed, the existing wires unsoldered, and the car wires soldered to the pins, so no inline splices are needed. Everything on the older cars (older than W210) has maintainable connectors. It's also possible to disassemble the connector, dip the pins in tarn-x (use no abrasives, or you'll strip the silver plating), rinse and dry, then carefully tweak with needle nose pliers and you'll be good as new.