Interesting observation, a/c on lowers coolant temp.

It should be pointed out that a/c is a heat pump--it is taking heat from the interior and pumping it outside. Thus the fans have to kick it on to dissipate that heat.

it's kinda cool if you think about how it works. How do you take "heat" from inside a car and put it outside--when the inside of the car might be 70 and outside 90? Answer: the temperature of the refrigerant goes way above and below either.

I took a class or two in thermo in college; lots of fun stuff there.
 
I've never had to actually do this, but if my engine were overheating and I wanted to cool it down, I'd probably turn the heat on full blast and set the vents to defrost. Turning the heat on cools the engine down (you're taking heat that WAS in the engine and dumping it into the cabin). Most cars will turn on the A/C compressor on defrost mode, even if you're using the heat instead of A/C. That's because the A/C is excellent at removing moisture and helps the defogger work much better. So you'd also have the benefit of the fans running constantly to help cool the engine further.
 
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