Question about 2004 Chevrolet Silverado 1500 Crew Cab

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2004 Chevrolet Silverado Heater/AC Blower on without Ignition ON

2004 Silverado Heater/AC Blower on WITHOUT the Ignition Switch ON. Is there a Relay in this circuit? If so, where is it located? None of the under hood relays are identified as controlling the Heater/AC Blower. Have previously replaced both the Blower Motor and twice the Resistor module. Where can I find a schematic for this circuit? What specific color wire is associated with High Speed only? From reading the other difficulties on this site, I find you state that the Resistor is only used for Lower Fan Speeds. Last time the Resistor failed I had no Blower speeds and a Dead Blower Motor. Vehicle was purchased new and this circuit and components seem to be a group of recurring problems. Is the Fan Speed Switch the culprit? The Blower was definitely OFF last evening when I parked it for the night. When I went out to use the Truck today, the Blower was running as I walked up to the truck.

Sign me: Rapid Robert

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  • Robert Lemon Oct 04, 2008


    I fully appreciate your insight and thank you for any help you can provide.

    Apparently you are unfamiliar with a 2004 Chevrolet Silverado. First it has 5 Blower speeds and Dual Temperature Controls. A/C is Standard Equipment. I am very much aware of the location of the Fuse in the Left side of the Dashboard. There are also many other Electronic modules due to the "Maintained Accessory Power" circuitry. I would think that a Short Circuit in the Harness and/or Connectors may be a remote possibility but, I suspect that the problem is more than likely another defective Resistor Module. 12 Volts is always present on one lead to the Resistor module as it feeds back to the Speed Control Switch. The Switch positions 1-5 then determines how many resistors are placed in Series to select any fan speed. Speed 0 is open circuit when everything is functioning normally. This is why I suspect the Resistor Module is now defective.

    Rapid Robert

  • Robert Lemon Oct 04, 2008

    Chevrolet Silverado, GMC Sierra, etc. Heater A/C (Manual A/C) Blower running on High Speed with Ignition OFF.

    For whatever it is worth, the following link will lead you to an insightful article for trouble shooting this problem.

    This article is very clear and concise and describes how to test for the problem. In this case the Resistor Module also contains a Relay which is obviously Welded Closed and the Blower runs at High Speed regardless of the Ignition Switch being On or OFF.

    Rapid Robert

  • Robert Lemon Oct 06, 2008

    Dear keemo68,

    You state "that there is no relay for the heater fan". That is incorrect! The relay is located within the Resistor Module and is utilized for the Highest Blower speed only. The problem has now been diagnosed to be a sticky relay. This condition was apparent only if High Speed was selected and the Ignition switch was then turned off.

    Three possibilities exist as follows: 1). Broken Spring to return the relay to Normally Open upon de-energization, 2).Residual Magnetism holding the relay closed, 3). or a deteriorated contact which effectively is welded closed. One thing is certain, the engineers that designed this module should have inverted the Relay so that if it fails the contact would be held OPEN by gravity.

    Upon carefully examing another defective Resistor module one will find the Relay is mounted with the contact on top when the module is mounted to the heater housing. If the return spring is broken or has come off, then gravity will close the contact completing the circuit. When I do replace the Resistor Module, I will disect it and forward the exact cause to "FIXYA".

    Rapid Robert

  • Anonymous Mar 12, 2014

    replaced fan resistor now all speeds are high blower

  • Anonymous Mar 24, 2014

    Blower motor does not run

  • Anonymous Mar 28, 2014

    where is heater resistor located excaty


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There is no relay for the heater fan it's a 15 amp fuse and is on the accessories circuit. the reostat is responsible for all 3 speeds .with the key off there should be no voltage to the blower motor whether the fans speed fan switch is on or off. the fuse will be located in the fuse paneleither on the drivers left side in the dash or underneath the dash in the fuse block. you have a short in the harness that is under the dash and most likely it's a bad plug on the harness. your going to have to do some searching but that's where the problem short is... please rate

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Where is the heater blower resistor located in a 2004 Silverado 1500?

Posted on Nov 22, 2008

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The blower motor control module is shorted under right side of instrument panel next to blower motor replace and it will fix

Posted on Nov 29, 2009

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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!

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