The aux. fan has a low and a high speed. It sounds as though you are not receiving the low fan speed(for starters). Check out Arthur Dalton's comments in this thread:

high speed aux fan operation

Accordingly, the low fan speed(the first fan speed that would kick in) is controlled by a pressure switch on the a/c systems receiver drier. I'm assuming that your a/c is supplying cold air.

There have been posts here about bridging connections off of the receiver-dryer in order to do problem analysis. In addition, there are countless posts on the fan issue.

I'd be suspicious of a pressure switch on the receiver-drier; then again, it could be many other things.

More ideas will follow.

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