The '83 turbo swap in to my '77 300D is on pause while I wait for a new thermostat gasket to arrive, so I figured I would use the down time to sort out a puzzle.

The turbo motor, like the NA motor it replaced, has a temp switch mounted in a vertical orientation above the thermostat with a single pin. When I connect the wire from the '77 chassis to that switch on the '83 motor, the aux fan runs any time the ignition is in the run position regardless of engine temp. What the heck is that about? (The aux fan never turned on once in the years that I ran around with the original motor, and I was actually surprised to find that it still works.) I was under the impression that the aux fan was only controlled by the temp switch in the AC system.

There's a lot of confusing information regarding that temp switch, the blower fan, the AC system, and the like on this forum. Maybe the switches are different between the two engines, but they sure look the same to me. To make matters worse, the wiring diagrams from the FSM disc are pretty illegible for the '77, and always make me feel like I need glasses. (Okay, I do need glasses, but I have them, and that's not the problem!)

I've left the switch disconnected for now, but that of course means I have no blower other than in the defrost setting. (Also weird, because I was under the impression that the blower motor temp switch was controlled by the goofy little switch built in to the bottom of the evil servo, but maybe I'm remembering the purpose of that switch incorrectly.) Before I pull out the multimeter and start reinventing the wheel, I thought I'd see if anyone has any idea what is going on.

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- K.C.Adams

'77 300D Euro Delivery
OM617 turbo / 4-speed swap
404 Milanbraun Metallic / 134 Dattel MB-Tex

Current status:
* Undergoing body work

My '77 300D progress thread