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Truck Trend Garage: 2009 GMC 4x4 Bring The Heat

Alex Steele
Jun 4, 2010
Photographers: Steve Austin
Question: I have a 2009 GMC 4x4 extended cab with a 5.3-liter engine. I really like the vehicle, but have one problem the dealer can't or doesn't want to address. At times, the heater doesn't send any air into the cab of the truck. I can hear the fan running, it changes speeds, but won't blow any air. It is possible to direct the air to defrost, heat, or vent, but there's no volume. It seems as if no air is going to the fan. The service department thinks it's the fan resistor module, but I can hear the fan changing speeds. It certainly feels like a louver supplying air to the fan. This problem seems to arise when we have wet, foggy, damp weather, just when I really need the defroster.
Photo 2/2   |   07z Truck Trend Garage BRING ON THE HEAT
Answer: Interesting condition -- I'm assuming your truck is equipped with manual HVAC as opposed to automatic. According to the Alldata service manual information, only the manual system incorporates a blower motor resistor assembly. The "louver" is referred to as a mode door, which is controlled by a mode actuator -- an electric motor that positions the door. Typically when a mode actuator fails (or a mode door gets stuck), air will be directed in a fixed position -- for example, between heat and defrost ducts -- or a fixed temperature. Temperature position should not affect the volume of air coming out of the ducts. Air direction could affect it, if the fan was stuck between two positions, splitting the volume of air, but you would no longer be able to change positions. If the recirculation door were in a fixed position, air would be coming from the outside, passenger compartment, or both, still with minimal effect on volume. The only way I can think of that you're losing volume due to air routing is if a foreign object were partially blocking air entry. That would require a good visual inspection and possibly a partial disassembly of the HVAC assembly to know for sure. (I once actually found a rat stuck in a blower motor.) As far as the blower motor resistor assembly being the cause of the problem, that would be atypical, but possible. The blower motor could be receiving insufficient power or ground causing it to run at lower speeds than normal. I guess the blower motor itself could be shorted and running slow, also. If the problem can be duplicated in the shop, I would direct 12 volts and a good ground directly to the blower motor to see if it runs at full speed (same as in the high position). If not, the insufficient voltage or ground that controls blower speed needs to be traced back to the resistor assembly, HVAC Control Module, and so on. If running direct power and ground runs the blower motor, but it still sounds like it's not running at full blast, I would swap it with a known good motor and evaluate the difference.

If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.
Can't wait for help with a problem you're having with your Truck or SUV? Ask the expert we trust here at Truck Trend Garage — visit Alex Steele at

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