The blower has two electrical connector. One four prong and one two prong. If I am understanding this the two prong resistor limits the voltage across the four prong causing a slower fan speed. When shorted the fan will blow faster. Not sure why this is needed . I think it is for preventing icing.

I just want to know what is happening before I do something rash

 

I haven't look at the diagram for a while but I thought the four prong was just the resistor (there's three of them for the different fan speeds, although you probably already knew that) and the two prong was the actual fan supply. I'm not quite sure what order everything runs in, but from what I recall the **** either switches ground or power (not sure which) through the four prong resistor of which the fourth wire either goes to ground or the blower motor power (two prong). If there is a resistor on the two prong as you said, it's probably just there to knock down the overall speed while the four prong is the actual speed control.

Oh wait a second, now I know what you're talking about. Yeah, I never really understood why the AC button shorts (to ground I believe). For me - and I don't have AC - all it ever did was cause electrical issues so I just unplugged it. If what you said is the case I don't really see why it's needed. Yeah, it could reduce icing, although then what about the lower fan speed settings? Perhaps it's there to just bump everything up so that the lowest speed won't ice? Sorry, this probably hasn't been much help. It is a curious thing though, and I'll be interested to hear other's responses.

 

I’m thinking you are correct about the icing. At least that’s where I find the logic.

 

Its merely to speed up the blower to help overcome the additional "blockage" of the evaporator core so you don't have diminished airflow.