They do pulse, I mean, it's like this:

The valves' natural, relaxed state is "open", they must be energized in order to close.

You want full cold/no heat? Then pins 1 and 3 are grounded. The valves are energized, which means they are closed. No water flow, no heat.

You want full heat? Then pins 1 and 3 are open/disconnected, that is, at full voltage. The valves are not energized, which means they are open. Full water flow, maximum heat.

You want medium heat? Then pins 1 and 3 are pulsed with a ground - the valves open and close at a rate designed to give enough water flow to make the right air temperature in the ducts. There are two thermistors, heat sensors, in the heater core - one for driver, one for passenger - they tell the climate control what the temperature is, and it then sends pulses to the valves to 'modulate' the temperature in compliance with what you have dialed on the two wheels.


Merry Christmas, Bob. Wish I'd see this thread earlier, but I'm in Michigan (where it is, now, right now, 3F outside) visiting my kids.