Circuit/System Testing GMC Sierra 2500 HD

Circuit/System Testing

Check the interior dimming circuit pin 2 at the transfer case shift control switch for voltage for the 2 seconds after turning the ignition on during the bulb check.

1. If less than battery voltage check the interior lights dimming circuit for an open or high resistance. If the voltage is low or non existent and the circuit tests good replace the BCM. If the interior dimming circuit and BCM test good go to the next step.

1. If the 4 high indicator is on at all times check the 4 high indicator control circuit for a short to ground. If the 4 high indicator is inoperative during bulb check, check the 4 high indicator control circuit for an open or high resistance or short to voltage. If the circuit checks good continue to the next step.

1. With the ignition on shift the transmission to neutral and using transfer case output controls attempt to command the 4 high indicator off and on. If the 4 high indicator is still inoperative short to 4 high indicator control to ground, if the AWD indicator lights replace the module, program, and complete the motor learn procedure. If the 4 high indicator is still inoperative replace the switch.

1. Perform the «Diagnostic System Check - Vehicle»(ref-278018-S19353494792008012100000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-278018-S03493196652008012100000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-278018-S38264170562008012100000) provides an overview of each diagnostic category.

DTC B0780

1. The ignition is ON.
2. System voltage is 10-18 volts.

1. The system monitors the voltage on the 4 low indicator control circuit.
2. If the system detects a voltage of 2 volts or less for 1.5 seconds when the 4 low indicator is not commanded, the DTC is logged.
3. If the system detects a voltage of 4.5 volts or more when the 4 low indicator is commanded, the DTC is logged.

The SERVICE 4WD indicator is not commanded on for this DTC.

1. The transfer case shift control module clears the DTC if the condition for setting the DTC is not currently present.
2. A history DTC clears after 40 consecutive warm up cycles without a fault present.
3. History DTCs can be cleared using a scan tool.

A short to ground on the 4 low indicator control circuit will cause the 4 low indicator to remain on at all times. An open or high resistance will cause the 4 low indicator to be inoperative during the 2 second bulb check.

Check the interior dimming circuit pin 2 at the transfer case shift control switch for voltage for the 2 seconds after turning the ignition on during the bulb check.

1. If less than battery voltage check the interior lights dimming circuit for an open or high resistance. If the voltage is low or non existent and the circuit tests good replace the BCM. If the interior dimming circuit and BCM test good go to the next step.

1. If the 4 low indicator is on at all times check the 4 low indicator control circuit for a short to ground. If the 4 low indicator is inoperative during bulb check, check the 4 high indicator control circuit for an open or high resistance or short to voltage. If the circuit checks good continue to the next step.

1. With the ignition on shift the transmission to neutral and using transfer case output controls attempt to command the 4 low indicator off and on. If the 4 low indicator is still inoperative short to 4 low indicator control to ground, if the 4 low indicator lights replace the module, program, and complete the motor learn procedure. If the 4 low indicator is still inoperative replace the switch.

1. Perform the «Diagnostic System Check - Vehicle»(ref-278018-S19353494792008012100000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-278018-S03493196652008012100000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-278018-S38264170562008012100000) provides an overview of each diagnostic category.

DTC B0785

1. The ignition is ON.
2. System voltage is 10-18 volts.

1. The system monitors the voltage on the 2 high indicator control circuit.
2. If the system detects a voltage of 2 volts or less for 1.5 seconds when the 2 high indicator is not commanded, the DTC is logged.
3. If the system detects a voltage of 4.5 volts or more when the 2 high indicator is commanded, the DTC is logged.

The SERVICE 4WD indicator is not commanded on for this DTC.

1. The transfer case shift control module clears the DTC if the condition for setting the DTC is not currently present.
2. A history DTC clears after 40 consecutive warm up cycles without a fault present.
3. History DTCs can be cleared using a scan tool.

A short to ground on the 2 high indicator control circuit will cause the 2 high indicator to remain on at all times. An open or high resistance will cause the 2 high indicator to be inoperative during the 2 second bulb check.

Check the interior dimming circuit pin 2 at the transfer case shift control switch for voltage for the 2 seconds after turning the ignition on during the bulb check.

1. If less than battery voltage check the interior lights dimming circuit for an open or high resistance. If the voltage is low or non existent and the circuit tests good replace the BCM. If the interior dimming circuit and BCM test good go to the next step.

1. If the 2 high indicator is on at all times check the 2 high indicator control circuit for a short to ground. If the 2 high indicator is inoperative during bulb check, check the 2 high indicator control circuit for an open or high resistance or short to voltage. If the circuit checks good continue to the next step.

1. With the ignition on shift the transmission to neutral and using transfer case output controls attempt to command the 2 high indicator off and on. If the 2 high indicator is still inoperative short to 2 high indicator control to ground, if the 2 high indicator lights replace the module, program, and complete the motor learn procedure. If the 2 high indicator is still inoperative replace the switch.

1. Perform the «Diagnostic System Check - Vehicle»(ref-278018-S19353494792008012100000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-278018-S03493196652008012100000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-278018-S38264170562008012100000) provides an overview of each diagnostic category.

DTC B0790

1. The ignition is ON.
2. System voltage is 10-18 volts.

1. The system monitors the voltage on the neutral indicator control circuit.
2. If the system detects a voltage of 2 volts or less for 1.5 seconds when the 2 high indicator is not commanded, the DTC is logged.
3. If the system detects a voltage of 4.5 volts or more when the 2 high indicator is commanded, the DTC is logged.

The SERVICE 4WD indicator is commanded on for this DTC for the remainder of the ignition cycle.

1. The transfer case shift control module clears the DTC if the condition for setting the DTC is not currently present.
2. A history DTC clears after 40 consecutive warm up cycles without a fault present.
3. History DTCs can be cleared using a scan tool.

A short to ground on the neutral indicator control circuit will cause the neutral indicator to remain on at all times. An open or high resistance will cause the neutral indicator to be inoperative during the 2 second bulb check.

Check the ignition 1 voltage circuit pin 4 at the transfer case shift control switch for voltage with the ignition on.

1. If less than battery voltage check the ignition 1 voltage circuit for an open or high resistance. If the voltage is low or non existent and the circuit tests good replace the BCM. If the interior dimming circuit and BCM test good go to the next step.

1. If the neutral indicator is on at all times check the neutral indicator control circuit for a short to ground. If the neutral indicator is inoperative during bulb check, check the neutral indicator control circuit for an open or high resistance or short to voltage. If the circuit checks good continue to the next step.

1. With the ignition on shift the transmission to neutral and using transfer case output controls attempt to command the neutral indicator off and on. If the neutral indicator is still inoperative short to neutral indicator control to ground, if the neutral indicator lights replace the module, program, and complete the motor learn procedure. If the neutral indicator is still inoperative replace the switch.

1. Perform the «Diagnostic System Check - Vehicle»(ref-278018-S19353494792008012100000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-278018-S03493196652008012100000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-278018-S38264170562008012100000) provides an overview of each diagnostic category.

DTC B2725

1. The ignition is ON.
2. System voltage is 10.5-18 volts.

1. The system constantly monitors the voltage on switch signal circuit.
2. If the system detects a voltage level under 0.38 volts or greater than 4.86 volts for 60 seconds, the DTC is logged.

1. All shifting is disabled.
2. The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.

1. The transfer case shift control module clears the DTC if the condition for setting the DTC is not currently present.
2. A history DTC clears after 40 consecutive warm up cycles without a fault present.
3. History DTCs can be cleared using a scan tool.

A short to ground on the neutral indicator control circuit will cause the neutral indicator to remain on at all times. An open or high resistance will cause the neutral indicator to be inoperative during the 2 second bulb check.

The 5-volt reference circuit and the switch signal will have to be checked for shorts to ground, opens, high resistance, and shorts to voltage by following the below listed steps.

1. Reference the mode switch supply voltage with a scan tool. The scan tool should indicate 4.8-5.2 volts. If the scan tool indicates the voltage is within this specification then with a jumper wire short the switch signal circuit pin 12 on the switch to the 5-volt reference circuit pin 11 on the switch. If the mode switch return voltage on the scan tool measures 4.8-5.2 volts then replace the switch.
2. If the mode switch return does not read 4.8-5.2 volts check the switch signal circuit and the 5-volt reference circuits for a short to ground, short to voltage, open, or high resistance. If no shorts or high resistance are found on the switch signal or 5-volt reference circuits then replace the transfer case shift control module, reprogram, and complete the motor learn procedure.

1. Perform the «Diagnostic System Check - Vehicle»(ref-278018-S19353494792008012100000) prior to using this diagnostic procedure.
2. Review «Strategy Based Diagnosis»(ref-278018-S03493196652008012100000) for an overview of the diagnostic approach.
3. «Diagnostic Procedure Instructions»(ref-278018-S38264170562008012100000) provides an overview of each diagnostic category.

DTC C0306

1. The ignition is ON.
2. System voltage is 10.5-18 volts.

The system tests the motor circuits by checking for unwanted voltage.

1. Then, the system supplies voltage on each of the motor circuits and reads the voltage back on the other circuit.
2. If the system detects a condition with the circuits, the DTC is logged. The transfer case shift control module senses a low voltage return in the motor control A or motor control B circuits when a high voltage is expected.
3. The fault must remain current for 30 seconds to set the DTC.

1. All shifting is disabled.
2. The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.

1. The transfer case shift control module clears the DTC if the condition for setting the DTC is not currently present.
2. A history DTC clears after 40 consecutive warm up cycles without a fault present.
3. History DTCs can be cleared using a scan tool.

It is possible to have windings inside the motor shorted to the case resulting in a short to ground causing this DTC to set.

The motor A control and motor B circuits need to be checked for shorts to ground, opens, high resistance, and shorts to voltage by following the below listed steps.

1. With the ignition off disconnect connector X3 from the transfer case control module, and also disconnect the connector at the transfer case actuator. With a DMM check the motor A control and motor B control circuits for an open, high resistance, or short to ground and repair as necessary. If all check good go to the next step.
2. Connect X3 at the transfer case control module and turn on the ignition.
3. Check to motor A control and motor B control circuits for a short to voltage. If the circuits shows to indication of a short to voltage go to component testing.