The four-wheel drive system in a 2005 Chevrolet pickup truck can be either a manual, lever-type control system or controlled by buttons on the dash. If the system uses buttons on the dash, it is operated with electronic controls. If the vehicle experiences a failure of part of the electronic system, the "service four wheel drive" indicator will be shown in the driver's information center at the bottom of the instrument panel.
The transfer case is shifted into different modes and ranges using an electric motor mounted to the transfer case, called an encoder motor. This motor provides a clockwise or counterclockwise motion that shifts the transfer case. It also provides an encoded feedback to the transmission control module to indicate a shift has been completed as requested. If the encoder motor indicates a shift has not been completed as intended, it may turn on the "service four wheel drive" indicator.
The transfer case shift switch is a three- or four-button switch on the dash, which the driver uses to indicate the desired transfer case range or mode. The switch sends a signal to the transfer case control module, which then operates the encoder motor to complete the shift. If the switch is faulty, it can provide an incorrect signal to the TCCM, which may turn on the "service four wheel drive" indicator.
The TCCM is the main microprocessor that operates the electronic four-wheel drive system. It is responsible for interpreting commands from the driver through the transfer case mode switch and operating the vehicle components to deliver the desired drive range or mode. If this module experiences a problem with its programming or internal circuits, it could turn on the "service four wheel drive" indicator.
The front axle actuator is a motor with a gear drive that shifts the front axle from its freewheeling mode to engaged so that it may be driven by the transfer case front output shaft. When the motor fully engages the front axle, it sends a signal to the TCCM to indicate that engagement is complete. If the TCCM does not receive this signal, it will turn on the "service four wheel drive" indicator.
The "service four wheel drive" indicator may also be turned on because of defective wiring. If a wire is corroded or broken, the control circuit is not complete, and different components may not receive the required signals for correct operation. The broken wire may prevent the TCCM from receiving a signal confirming correct operation as well. If this is the case, the TCCM will indicate this fault with the "service four wheel drive" indicator.
