W126 sudden heat loss

[LandYaghtLover]

Not sure why, but my 91 560 SEL suddenly has no heat. Upper radiator tube is hot, washer tank is heating just fine. But the hoses behind the tank, which I think are the windshield heater, and HVAC heat are cool.

If I adjust the temp, the fan controls speed. Seemingly matching actual air temp so I dont think the cabin temp sensor is out. All fuses look good. If I go full defrost, I just get major air movement that is cool.

I am not familiar with the system. Heat was HOT before. I know there is a modulator thing that can go bad, but typically its more gradual. And there is the aux heat circulation pump. If that goes, wouldn't the main pump still pump coolant?

Any ideas or things I can rule out?