Sounds like - "welcome to the club of blower resistor failure" - When your fan will only run on high, I would suspect the Blower Resistor assembly. Reason: My blower fan recently died and would only run on high, turns out that when the resistor assembly failed, the HIGH speed was the only one that would work as it was DIRECTLY connected to battery power and did NOT go thru any resistors to "absorb" voltage and make it run slower.
I suspect that when you drive at highway speeds, the blower doesn't actually blow, but you still get cooler air just from forced air flow from outside. My blower resistor assembly was located under the dash, passenger side, costs about $11.00 and is a simple fix. To all reading this post with a "runs on Hi only" symptom, this will be a good candidate for your problem!
I just jumped back and checked a previous post about the same problem, pls read "AC BLOWER" dated Wed 01Aug07, currently on page 3 of this forum - page will change as more folks log in and add.
Good luck and Please post results of check - we all learn from each other!
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Saturday, September 15th, 2007 AT 11:21 AM