There is a resistor pack on most HVAC fans to control the speed, most of the time if there is a problem it will only run on high speed. If you can find the motor on the firewall trace the wires and they should lead you to the resistors.
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Jim J
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi
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