David,

I have the 98 S70 GLT so my system may be slightly different. My fan switch goes from AUT (automatic based on temp) then 0 for off, then no numbers just continuously variable.

Had similar symptoms this summer - intermittent fan. The AC light would be green & I could tell it had power but fan would be slow or die. Mine was the fan motor itself. Can be replaced but a bit of a pain. The other possible thing is a circuit attached to a large heat sink that is installed into the duct. (this may be the power stage noted in other posting). Both this circuit & the fan are behind the glove box. You have to remove the box and the metal panel behind/below it for access.

I know I followed some posts on brickboard that showed how to bypass this circuit (has the fan resistor for variable speed) and test the fan directly. The circuit can be repaired by replacing a resistor which is cheap but easy only if you are electrically inclined.

Search on Heater fan or AC fan and you should find the posts. Replacing either is not too bad, much easier than on a 240 which fortunately I never had to do!

Mike
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1988 245, 245K mi, 5 speed