My question is just that. On my 84 300sd, the aux electric coolant pump is always on. Is that correct? Right now it is summer and I have the ac selected. I see no reason the coolant pump is required. It stays on even if I select to turn the climate control off.

As far as I know that's the way it works. I unplug mine in summer but I'm sure the coolent just passes through the pump based on the way it's designed.

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Mad_Matt's Current Stable:
Oil Burners:
09 Jetta TDI The highway queen @ 105k
83 300D. Sold
Gassers:
16 A4 sport line
10 Passat
83 Recaro Trans AM WS6 Bone Stock Survivor
11 Harley Ultra

The aux pump is controlled through a relay in the push button unit. That relay is energized to the open position. There are several possible failure modes which would result in constant power to the aux pump. Do your footwell flaps operate properly? Or are they open all the time?

The ambient temp sensor must be bad as it turns the pump on, i believe anything lower than 65-70degrees it runs the pump.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

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