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Re: 2002 pont. grand prix-Heater/AC Only works on High
You need a new speed control resistor... this is something you will have to get from the dealer, but you should be able to replace it yourself - it is under the dash on the passenger side mounted near the bottom of the fan motor.. you will remove a couple of screws, and it will come out of the black casing it is mounted inside.
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Follow the 2 wires back from the motor under the dash. The speed controller is the fault. It's default when it fails will allow for high speed operation only. The unit that the 2 wires come from is the controller. Replace that unit.
i have been having that same exact problem with my 2002 grand prix for over a year now, and when my ac is on the lower setting the it doesnt work plus the headlights and lights on my dash go out. On Max everything works. i changed the blower resistor(about $35), still doesnt work. i changed all relays and fuses related to the ac, still doesnt work. i changed out the actual ac control switch in the dash, still doesnt work. i have changed everything but the blower motor and have asked every mechanic and auto parts retailer i come across and no one knows the answer. The best answer i got is that it is probably a wiring problem like a short somewhere.
I don't claim to know why the lights are affected but when a heater blower only works on the fastest setting, it is normally due to the failure of a stepped series resistor which has taps to set the different speeds.
As emissionwiz stated, that is the most common cause.
This resistor may be mounted on the firewall under the hood or near the blower motor.
Without an electrical diagram at hand, I can't say for sure but there may be a relay failed that bypasses the resistor completely when set on the highest speed or diverts the current through the resistor.
chk. to see if you got a heater resister, it should be somewhere in the visinty of the blower motor 2 screws hold it in the heater box and it will have 4 or 5 wires in a plug going to it. samething happend with my truck and thats what it was. i also found out sometimes that plug with the wires to the resister burn, if thats the case you will have to chg. that too.