I have two SPI minis, both with the cooling system you describe. The engine cooling is controlled primarily by the mechanical belt driven fan. The cooling system can and will keep the engine at the correct temperature without the electric fan connected providing the system has no other issues. I have a mini workshop and often get SPI cars in with broken or dead electric fans which don't overheat because of this.

The electric fan is purely auxiliary and only seems to activate under extreme conditions. In my case the fan only comes on during summer driving conditions when caught in traffic and even then only activates briefly. It should not be coming on at idle under normal driving conditions.

If you are having overheating issues I would suggest looking more closely at the cooling system. Check or better still replace the thermostat, check the fan blade is fitted the correct way around and that the blades are not starting to curl. This happens as plastic fans age. Check the fan belt and replace or adjust if required. Also it's not uncommon for radiators to silt up, particularly if an engine has been run for extended periods without adequate anti-freeze.

this is a long shot

I do not know your model.
do you have AC?

There are some cars (not mini that I know of)  whose auxiliary fan circuit is activated whenever the AC is turned on.
Could it be that there is an either /or set-up ? (aux fan runs if either AC is on and/or coolant hot?)
Is your temp sensor getting its 12v feed from the AC circuit? 

Using a heat gun may not be a valid test anyway. A coolant temperature sensor would trigger somewhere around 100C or 212F. Your heat gun would put out 375C (700F) @ 1000W to 575C (1000F). To test a sensor (or a car thermostat), you need to put it into a fluid with a thermometer and heat the fluid until it reached the set point of the sensor and see if it triggers. Your sensor may be functioning at a temperature too high for the cooling system.

You are talking about the switch that is at the front bottom of the radiator?

Hello,

I have a classic mini with a SPi engine setup.  I've recently run into some overheating problems at idle, which i'm almost 100% positive is the Auxiliary fan not turning on. https://ometv.onl https://chatroulette.top https://omegle.wtf https://bazoocam.cam  My dad and I trouble shot the wiring harness and traced it back to what we thought was the fan switch.  We unplugged the wires from the switch and jumped them and it ran the auxiliary fan just fine.  We also checked the relays and fuses and haven't found any faults there either.  However, when we let the car idle and get up to temp, the auxiliary fan never kicked on.  We took the fan switch out and put it under a heat gun to check for continuity, and sure enough, the switch works fine.  Does anyone have any idea what might be going on here?

thanks
alexsunny