OK as the title states my engine only overheats when the AC is on. If I drive with it off the temp is at normal position. Things I have done is a new water pump, new fan clutch, replaced the coolant and a new serpentine belt. What on earth can be the problem? Do you think the radiator is so old it can't handle the extra load of the AC being on? And to note the AC still blows ice cold air.