Why does my mustang overheat when the a/c is turned on?
First of all I would send the car into a shop to have the cooling system flushed/cleaned, it's not very expensive and it could alleviate your problem entirely. Whatever happens, cleaning your cooling system always helps, but in the event that it does not help as much there are a few things to check. If you have a electric fan you will see three wires going into it, one is the ground, one is for 'low' speed, and the other is for 'high' speed. The 'low' speed wire is used when the A/C is not operating, and the 'high' speed wire is for when it is working. I guess I should of told you to check to see if the fan IS even working, but I assume that it is seeing as it doesn't overheat when the A/C is off.
Two things that would cause your fan to malfunction are the thermostat (tells the computer the temp. of the motor and adjusts the fan speed accordingly), and the Constant Control Relay Module (controls the 'high' speed setting wire on the fan when A/C is on). The relay is located behind the coolant overflow canister, but I don't know where the thermostat is located on the 4.6L, just the 3.8L.
Hope this helps.