300D temp switch has me puzzled
 

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.

On an '83 model, that switch is associated with the cabin blower fan. It prevents it from running in the heat mode when the engine is cold.

Can anyone tell me the normal functioning of that switch? This one conducts between the pin and body at room temp. I have not tried heating it to see if it stops conducting when it warms up. I might just need to swap switches with the old motor.

It should be open cold, closed when above approx 110*F.

ll In that case, it's obviously stuck closed. Well, I guess I'll swap in the switch from the old motor. That will give me the opportunity to test it with a pan of water before I install it, too. I suspect the old switch must be failed open, as the one overheat incident I had with that car resulted in no aux fan action, and in the 77, it is obviously intended to kick on not just tor ac heat as the later models do, but also for engine coolant overheat incidents as well.

An aux fan switch would have a much higher trip temp than the blower fan cut-out.

That makes sense. I only suspect the 77 switch of being nonfunctioning because the car hit 200 degrees one day and that fan never turned on. (my temp gauge is Fahrenheit, not Celsius.)