I had this problem once, cost me my A/C dryer and a refrigerant refill.
Turned out that the reason was a broken fan speed resistor (it's a
dual resistor for low and high). The low speed part was broken so the
ventilator came on at high speed only, and this happened when it was
really hot already, causing high pressures in the A/C system that
eventually resulted in the monitoring window in my A/C dryer to burst
and spill all refrigerant.
Go check the resistor (behind the front-left headlamp) and tell us if
that was the culprit...

On 9/11/07, Tom Salicos <Tom@salicos.com> wrote:
> I have the A/C running after a couple of summers w/o.  Subie yes, but the
> van has always done this.  The high fan speed comes on when it shouldn't be
> required (as far as I understand).
>
> Sunday it was parked for about two hours on an 85 deg. Day.  When I started
> it the high rad fan came on immediately  and stayed on for at least ten
> minutes as I drove home.  Came on and off a few times.  The temp guage
> didn't show the H20 temp even to half way to the LED.
>
> Turn the A/C off and the hi fan stops.
>
> Other times it has just come on at high speed, even though it was no way
> near hot.  Stays on for a minute then shuts off.
>
> W/O the A/C the high speed fan never has to operate due to high temps.
>
> So, what controls the high speed fan when the A/C is on?
>
> Thanks for any tips.
>
> Tom Salicos
> '87 Syncro Westy EG-33
>