Confirming failing Heater Valve prior to dropping the $500 on the part.

Symptoms

Monday, exterior temp is 80 degrees, engine at operating temp. Max heat from any and all vents at all pushbutton settings.

Tuesday, exterior temp is 38 degrees, engine at operating temp. Cold air from any and all vents at all pushbutton settings.

the 95 has the monovalve/auxpump combo unit.

In my searching I found that the monovalve should default to open allowing coolant into the heater core under monovalve failure.

Tuesday continued: I pulled over and unplugged both plugs at the valve without turning off the car. Zero change in vent temps (cold air all pushbutton settings). Plugged the valve back in. Very confused at this point.

Went for another drive, cold air.
turned car completely off, unplugged both plugs at the valve.
Turned car on and HEAT!
Turned car off and plugged in just the tall cylinder on the unit - HEAT!
Turned car off and unplugged tall unit and plugged in short unit - NO HEAT!
Turned car off and reversed it Short unplugged and tall plugged for the winter.

So, in conclusion, I am thinking that the short cylinder is the mono valve and the tall cylinder is the aux water pump. (this also makes sense as a pump would need a larger housing due to a pump being more complex than a valve)

The remaining question I have is that if the monovalve is failing, why would I need to unplug it to get heat if its natural state of failure is open. When I plug it in it closes and when I unplug it it opens during cold exterior temps but in warm exterior temps its open all the time.

Fan speed, flaps, and temp wheel all function beautifully.

What controls the voltage sent to the monovalve?

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