Actually test thermoswitch to make sure it turns ON when hot. The sensor that drive gauge doesn't have anything to do with fan.
1. measure resistance/continuity across switch at room temperature
2. put thermoswitch into pot of water, and bring to boil
3. very 20-degrees increase or so, check resistance/continuity of switch
4. at what temperature does switch close?
5. or does it never close?
There's your answer.
Another way I've monitored this is to attach idiot-light on dash connected to fan. When switch closes to turn on fan, it also lights up bulb.
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