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My car has been running hot and I'm going to be putting in a Thermostat to fix this.
However, I have noticed that when I turn on my Air Conditioning, the temperature goes down.
I would expect turning the Heat on to cool the engine as it uses the Engines Coolant to heat the air (which would cool the air somewhat) but not the A/C.
Why would turning on the Air Conditioning cause the temperature to drop?
Your car's coolant is cooled via a radiator mounted at the front of the engine bay. Typically, this radiator has one or more fans on it, which the car turns on and off as appropriate to control the amount of air moving through the radiator. This lets the car's control systems manage the temperature of the coolant even when in different operating conditions (i.e sitting still in traffic vs moving down the highway).
Your car's air conditioning system uses a condenser - which is, basically, a specialized radiator - to cool the hot AC refrigerant. This condenser looks like a very thin radiator, and it typically sits right in front of the main radiator, behind the grille. When you turn on your AC, hot refrigerant is pumped to the condenser, where it dumps all of it's heat as cool air flows over it. This adds a significant heat load to the front of the engine bay. Hence, in most cars, there is a mechanism which forces one (or more) of the radiator fans to automatically tun on, no matter what, whenever the AC is on. Otherwise, the additional heat load from the condenser could cause problems.
So, in your situation, it's likely the case that the AC being on is triggering fan(s) to run, which is dropping the coolant temperature.
