I have an 05 GMC Sierra and for the last few years ive had 4wd issues it all started when i was driving out at a farm and hit a hard bump the service 4wd light came on for a minut then went back off a

DTC B2725 ATC Mode Switch Circuit Malfunction
Circuit Description
The range/mode switch circuit consists of 5 modes. The transfer case shift control module supplies 5 volts to the switch through the 5-volt reference circuit. The current travel through the resistor of the currently active mode. The current is then returned to the transfer case shift control module through the switch signal circuit.

The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit.

When each of the modes are selected they complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the Neutral, AWD, 2HI, 4HI, or 4LO mode was selected by the driver. DTC B2725 ATC Mode Switch Circuit Malfunction Circuit Description The range/mode switch circuit consists of 5 modes. The transfer case shift control module supplies 5 volts to the switch through the 5-volt reference circuit. The current travel through the resistor of the currently active mode. The current is then returned to the transfer case shift control module through the switch signal circuit. The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit. When each of the modes are selected they complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the Neutral, AWD, 2HI, 4HI, or 4LO mode was selected by the driver.

DTC C0306 Motor A or B Circuit Malfunction
The transfer case motor is a bi-directional, permanent magnet, DC motor. When energized, through motor control A or motor control B, the ground is provided by the opposing motor control circuit and then grounded through the transfer case shift control module ground circuit, the motor, through a series of gears, rotates a shaft which moves the mode and range forks to shift the transfer case between the following ranges:

• 2HI

• 4HI

• 4LO

• AUTO 4WD

• Neutral

This DTC detects an open, short to voltage, or short to ground in the motor control A or motor control B circuits, or an open, or short to ground inside the motor.

DTC C0321 Transfer Case Lock Circuit Malfunction
The transfer case shift control module controls the transfer case lock solenoid.

The transfer case lock solenoid is released by being energized. This is accomplished by grounding the lock solenoid control circuit during gears shifts, and in the AUTO 4WD mode. Locking action is applied when power or ground are taken away from the lock solenoid, the transfer case motor is prevented from moving. The system is capable of providing a 2HI, 4HI and 4LO lock-up without the need of additional vehicle power to hold the transfer case in these positions.

This DTC detects an open lock solenoid coil, open motor lock control circuit, or an open motor lock feed circuit.
DTC C0327 Encoder Circuit Malfunction
Circuit Description
The transfer case encoder is an analog sensor that returns a signal that represents the physical position of the transfer case sector shaft. This signal represents the mode or position that the transfer case is currently in. This sensor is made up of the following 3 circuits:

• 5-volt reference circuit

• Encoder signal circuit

• Encoder low reference circuit

This DTC detects an out of range reading from the encoder signal, a short to voltage on the 5-volt reference circuit.
Conditions for Setting the DTC
The transfer case shift control module has a predefined range, 0.5-4.5 volts, in which the encoder feed back should stay within. The DTC sets if the encoder signal feed back falls out of this range. DTC C0306 Motor A or B Circuit Malfunction The transfer case motor is a bi-directional, permanent magnet, DC motor. When energized, through motor control A or motor control B, the ground is provided by the opposing motor control circuit and then grounded through the transfer case shift control module ground circuit, the motor, through a series of gears, rotates a shaft which moves the mode and range forks to shift the transfer case between the following ranges: • 2HI • 4HI • 4LO • AUTO 4WD • Neutral This DTC detects an open, short to voltage, or short to ground in the motor control A or motor control B circuits, or an open, or short to ground inside the motor. DTC C0321 Transfer Case Lock Circuit Malfunction The transfer case shift control module controls the transfer case lock solenoid. The transfer case lock solenoid is released by being energized. This is accomplished by grounding the lock solenoid control circuit during gears shifts, and in the AUTO 4WD mode. Locking action is applied when power or ground are taken away from the lock solenoid, the transfer case motor is prevented from moving. The system is capable of providing a 2HI, 4HI and 4LO lock-up without the need of additional vehicle power to hold the transfer case in these positions. This DTC detects an open lock solenoid coil, open motor lock control circuit, or an open motor lock feed circuit. DTC C0327 Encoder Circuit Malfunction Circuit Description The transfer case encoder is an analog sensor that returns a signal that represents the physical position of the transfer case sector shaft. This signal represents the mode or position that the transfer case is currently in. This sensor is made up of the following 3 circuits: • 5-volt reference circuit • Encoder signal circuit • Encoder low reference circuit This DTC detects an out of range reading from the encoder signal, a short to voltage on the 5-volt reference circuit. Conditions for Setting the DTC The transfer case shift control module has a predefined range, 0.5-4.5 volts, in which the encoder feed back should stay within. The DTC sets if the encoder signal feed back falls out of this range.