Stu,
On my '85 van the low speed fan seldom comes on.  It does work, I checked it on the weekend.  The high speed fan always comes on when the A/C is on.  I think there is a pressure switch in the A/C system that turns on the high speed fan when the coolant reaches a certain pressure.
Hope this helps.
George 
'85 Westfalia

Date:    Mon, 22 Jul 2002 18:52:06 -0400
From:    Stu Schwartz <sschwartz@OS2BBS.COM>
Subject: Fans again..I think its in the archives....

My '91 westy usually runs fine...when it gets hot the fan (it is the lower speed fan) comes on...

All of a sudden yesterday (very hot and I was using A/C)...an extremely high speed fan came on...the lower speed fan no longer comes on I don't think..

Is there a discussion of exactly how these fans work.

Shouldn't the low one come on and then the high one?

--peace--
    stu