The fan cuts in at a pre-determined temperature (like how the thermostat will open at a given 89c of instance) which is usually around the same as a stat.
It is entirely possible there is a short between the switch and the fan, but in my experience, the fan never cuts in while traveling at speeds above 10mph as the through flow of air keeps the coolant at ambient.
If the engine overheats seriously enough for it to cut out, you would feel the heat in the interior cabin go up beyond the normal full heat. Also, there would normally be bubbling sounds and/or steam escaping from the radiator cap or worse, coolant boiling out the overflow!
I would imagine a short circuit from a chafed wire or a blown switch.
I remember several years ago rescuing two babies from a Renault. I was heading towards it and heard peeping & shouting from irate drivers. But I could see what they couldn't, the front lights were glowing and smoke was pouring out from under the bonnet. The car wasn't moving as the engine had died. The mother was frantic (and the selfish peeping clowns weren't helping
) so I told her to unhook the belts & get the babies into my car while I dealt with the fire.
Turned out the wire from the fan to the loom went direct to the battery + and some twit, upon replacing the battery with a more powerful new one, had placed the battery on top of the wire. Subsequent chafing made it wear through and start melting the plastic, which caught fire. Worse still, the fuel line ran alongside the wire
Ha ha ha, she called the local radio to say thank you to me as I was a hero :worship:
I wan't, I was just in the right place at the right time. But I was pleased at the thanks from the fire chief