Originally Posted by psfred

Your fan or relay is dead -- aux fan should run at slow speed with AC on. You also have a defective electric fan clutch or relay as the engine temp should never get above 105C this time of year, even it traffic. You should also check the resistor for the slow speed on the fan, a broken or corroded wire will prevent it running at slow speed. Should still kick on at 110C , though.

The AC will kick off when the head pressure gets too high, as it is when sitting it traffic with no fans running. This is to prevent the high pressure hoses from blowing up.

Last, check the radiator cap. If it's old, replace it, else it will allow the engine to boil and greatly reduce cooling.

I suspect a bad fan clutch, myself.

Peter