5-90 hit on it a little but I will hit on it in more generic terms. The A/C system produces heat, just like the engine does while its running. The A/C(tetrafluoromethane) pulls the heat out of the passenger compartment, where it pushes it away out at the condensor. With the condensor being in front of a radiator, and as you drive the cooling air(well while you are moving), pushes the heated air from the condensor into the radiator, which actually adds heat to the water that is trying to release said heat itself. The engine driven fan(for those that still have one), moves some air, but while sitting in traffic, not enough keep the temps low(especially in like AZ where it can hit 115+). All the added heat required the engineers to come up with a neat way to remove heat while driving or idling. The electric fan on the later models(97+) supposedly moves quite a bit of air, however the earlier model one(pre 97) does a pretty good job as well. All I can say is the additional heat from running the A/C caused the engieers to rethink their initial design, and add the electric fan to come on when the A/C is switched on. Does that help a little??
Jeff