Okay, confession time.

I did some more checking and the reason the LED was on was because the relay was wired wrong. The relay was always activated with the power to the fan being controlled directly by the thermo switch. It worked that way but the thermo switch was providing the ground side of the fan power rather than controlling the relay. I suspect that is why my original thermo switch failed (not rated for that type of power).

Now that I have rewired things correctly the relay is controlled by the thermo switch and the fan turns on when the thermo switch triggers the relay to power the fan. The LED is still connected to the power feed to the fan (after the relay) but does not light unless the fan has power (or is moving). When the fan power is removed the LED dims as the fan is slowing and goes out when the fan stops. I suspect that if the fan does windmill due to airflow from the truck moving it may light the LED with the brightness bing an indication of how fast the fan is moving.

I'm glad I decided to add the LED because the confusion it caused resulting in my finding my previous error before I damaged another thermo switch.

I'm sorry that my wiring error resulted in you all needing to try and help me out but once again I appreciate your help confirming that my original intention should have worked if not for my error.

1949/50 3600 Project
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