Question about 2004 Chevrolet Silverado 1500 Crew Cab

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Relay/resistor heater blower

Thanks for your help. I'm not too experience with electronics. I don't understand how 12 volts can continue to power blower with fuse out. How is power not disconnected. If there continues to be power circulating, why does it not stay on with engine off as it does with fuse in? It's like it's working backwards. Is it a hazard to run without fuse?

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There are two fuses for the fan. One is probably a 20 Amp in the fuse panel in the cab and marked "Heater". or "Heater/AC" The second is under the hood may be marked Heater, Blower, or Fan and should be 30 Amp. The one in the cab supplies switched 12 volts to the fan speed switch. The one under the hood supplies the 12 volts to the relay. The relay is normally energized by the 12 volts through the 20 Amp fuse in the cab and through the switch. I think you find that the fan has the normal speeds except for missing HI with the underhood fuse removed, and will turn off with the ignition that way. There is no hazard in operating it that way but you may want to replace the resistor/relay assembly before it gets too cold. The relay is a pretty trouble free device in most cases, maybe you can find a deal on a good one at a junkyard.

The whole idea with the relay under the hood started back in the Sixties believe it or not. There is considerable voltage drop in the wiring between the fan switch and the blower fan motor. By adding the relay under the hood right next to the motor, the voltage drop was reduced, both by the shorter wire, and by using heavier wire to boot. The motor gets really close to the full system voltage when the relay is closed. In your case where the relay keeps the fan running, it can drain a battery rather quickly too!

Posted on Dec 18, 2008

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Thanks for using FixYa. The possible causes due to which the blower is not working normally are fuse, blower resistor or a bad blower motor. Test the voltage at the blower with a test light. Please check the motor relay switch fuse under the dashboard. Use a multi meter to measure ohms for continuity. If these are fine, bypass the blower motor relay switch to make sure blower motor is running fine. Motor relay switch is right next to radiator fan relay switch under the hood. Pull out the blower motor relay switch to see 4 female pin holes. Test the relay switch using a 12 volts power source and a multi meter to measure continuity. If fan works after jumping the pin holes, then blower motor is fine but relay switch may be faulty. You may also need to replace the blower motor resistor. If still the problem persists, then you need to replace the blower motor.

Please do rate the solution and revert for further assistance.


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