The saga thus far:

Oil was changed to 10W 40 from 15W 40 in early November in preparation for the winter. Anti freeze was also partly changed (didn't drain the block, just the radiator) then.

Noticed engine temp was a little bit higher. Consistent, though.

Planned on replacing the thermostat but haven't gotten around to it yet.

Recently, the heater stopped working. I had noticed that the vents in the middle of the dash weren't functioning and I figured that this was related to the fact that I could look into the vents and see some sort of white plastic that had seemed to peel away from the top of the "vent foyer" (for lack of a more accurate term) behind the vent-venetian-blinds and fall down. I was putting off dealing with this until it was warm enough to make it more likely that I could take off the dash without cracking it. (I might crack it anyhow, but don't want to fiddle with it in this cold snap.)

BUT, now the fan blew nothing but cold air at any setting.

So, looked for mono valve and unplugged electrical connector.

Voila, HEAT!

But, nothing but. And, I was idling while this was going on and saw the engine temp drop right down to 80 after unplugging that.

If I rotate the temp. adjusting wheel until it is all the way to the coldest setting, the heat goes off. But not otherwise. No modulation of heat.

Checked fuses. All OK.

Any ideas?

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1984 300SD: Hilda the veggie vundercar