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Well, with these guys it is one of two things.
1. The a/c control head is the cause 80% of the time.
2. The blower motor resistor or wiring to the resistor is the cause 15% of the time.
3. The last 5% divided between the blower motor & wiring issues & both are rare.
Bulletin No.: 05-01-38-012C
Date: November 02, 2006
EI05082 - Loss of Blower Motor Speeds Except for Low Speed or Inoperative (Replace Blower Motor Resistor and Resistor Module Connector)
2004-2006 Chevrolet Avalanche, Silverado, Suburban, Tahoe, TrailBlazer, TrailBlazer EXT
2007 Chevrolet Silverado (Classic) Built Prior to September 1, 2006
2004-2006 GMC Envoy, Envoy XL, Sierra, Yukon, Yukon Denali, Yukon XL
2007 GMC Sierra (Classic) Built Prior to September 1, 2006
with HVAC System AC Front, Manual Temp Control (RPO CJ3) or Heater Only (RPO C42)
This bulletin is being revised to clarify the Warranty Information. Please discard Corporate Bulletin Number 05-01-38-012B (Section 01 - HVAC).
Some customers may comment that the blower will only work on low speeds or has become inoperative.
REPLACE THE BLOWER MOTOR RESISTOR AND RESISTOR MODULE CONNECTOR.
Replace the blower motor resistor and module connector using the following procedure:
Remove the blower fuse.
Fullsize Trucks and Utility Vehicles - Htr A/C & HVAC 1 fuses
Midsize Utility Vehicles - Blower Fuse 35 located in the Underhood Fuse Block
Lower the hush panel or close-out panel enough to gain access to the resistor module connector.
Disconnect the connector from the resistor module.
Remove the tape from the wiring harness to expose the wiring.
Cut the wires back far enough from the connector to eliminate any melted insulation on the wire.
Use the old connector as a map for splicing the wires for the new connector. Be sure to use the correct crimping tool from the terminal repair kit J 38125. Use only Duraseal splice sleeves. Other splice sleeves may not protect the splice from moisture or provide a good electrical connection.
Install the new connector. Use the yellow splice sleeves provided with the connector.
Install new blower motor resistor.
Reinstall the fuse or connector block. Test the blower motor to make sure all speeds are functional.
Reinstall the hush panel/close-out panel.
Bulletin No.: 06-01-39-002C
Date: January 18, 2008
Blower Motor Inoperative or May Not Shut Off in Extremely Low Ambient Temperature Conditions (Replace Blower Motor Control Module)
2004-2007 Buick Rainier
2003-2006 Cadillac Escalade Models
2003-2006 Chevrolet Avalanche, Silverado, Suburban, Tahoe
2003-2007 Chevrolet TrailBlazer Models
2007 Chevrolet Silverado (Classic)
2003-2006 GMC Sierra Models Yukon Models
2003-2007 GMC Envoy Models
2007 GMC Sierra (Classic)
2003-2004 Oldsmobile Bravada
2003-2006 HUMMER H2 Models
with Automatic Temperature Control HVAC System (RPO CJ2)
This bulletin is being revised to update the model years that are affected and add the Oldsmobile Bravada. Please discard Corporate Bulletin Number 06-01-39-002B (Section 01 - HVAC).
Some customers may comment that the blower motor is inoperative or continues to run after the vehicle has been shut off.
These conditions may be caused by a blower motor control module failure. This failure may occur when the vehicle is operated in ambient temperature conditions of-23°C (-10°F) or below.
Replace the blower motor control module if the vehicle experienced a blower motor control module failure in extremely cold ambient temperature conditions.
Technicians are to replace the blower motor control module also called the Linear Power Module (LPM) with the P/N listed below. Refer to either the Blower Motor Control Processor Replacement or Blower Motor Processor Replacement-Auxiliary procedures in SI. After replacement of the blower motor control module the customer may notice a slightly higher than expected blower motor speed at any of the blower speed settings.