Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category |
DTC Descriptor
DTC B1440 00: Power Mode Master Input Circuits MalfunctionCircuit/System Description
This vehicles power mode master (PMM) is the body control module (BCM). The ignition switch is a low current switch with multiple discrete ignition switch signals to the PMM for determination of the power mode. The PMM receives the ignition switch signals and additionally a reference voltage signal from the BCM, through the ignition switch and back to the BCM. There are redundancies within the PMM system that can allow operation of certain subsystems when there are faults in the ignition switch circuits. Other information such as serial data input from other modules and DTCs is also used to determine what power mode signals and information is transmitted by the PMM. The BCM logic uses all this information to identify the operators desired power mode and activate specific discrete signals and serial data messages for the operation of many different subsystems as necessary. Other modules which have switched voltage inputs may operate in a default mode if the PMM serial data message does not match what the individual module can see from its own connections. The PMM will activate relays and other direct outputs of the PMM as needed according to the calculated power mode. Several relays controlled by the BCM are switched ignition voltage outputs they pass through circuits within the BCM directly from the ignition switch. If these circuits short to ground, the fuse to the ignition switch will open. Some system functions may operate one time and not the next with a specific fault in the ignition switch circuits depending on the vehicle operating conditions when the fault occurred.
Conditions for Running the DTC
Ignition switch is in the ACCESSORY/RUN/CRANK position.
Conditions for Setting the DTC
| • | BCM senses an open or short to voltage on the off/run/crank voltage circuit when both ignition accessory/run and ignition run/crank parameters are active. |
| • | BCM senses an open on the battery positive voltage circuit (to the ignition switch) while engine is running. An open on the battery voltage circuit can be caused by a short to ground on any of the following circuits. |
| ⇒ | Battery positive voltage |
| ⇒ | Accessory voltage |
| ⇒ | Run/crank ignition 1 voltage |
| ⇒ | Off/run/crank voltage |
Action Taken When the DTC Sets
| • | The BCM operates in a fail-safe power mode, dependent on the last valid power mode detected and the state of the engine run flag data on the GMLAN serial data communications circuits |
| • | The other modules on the vehicle operate in a fail-safe power mode dependent on the last valid power mode transmitted by the BCM and the state of the engine run flag on the GMLAN serial data communications circuits. |
| • | The vehicle may not start. |
Conditions for Clearing the DTC
| • | A current DTC clears when the malfunction is no longer present. |
| • | The history DTC will clear after 50 fault-free ignition cycles. |
| • | The DTC can be cleared using a scan tool. |
Reference Information
Schematic Reference
Body Control System Schematics
Connector End View Reference
Electrical Information Reference
Scan Tool Reference
Circuit/System Verification
With a scan tool, access Body Control Module, Power Mode data display list and compare all parameters listed with the corresponding ignition key position. The PMM Power Mode Parameters Should correspond to the ignition key position being selected.
Circuit/System Testing
- Ignition ON, observe the DTC information with a scan tool.
- Perform the ignition switch component test
- Ignition in the ACCESSORY position, test for 12 volts at the BCM accessory circuit.
- Ignition in the RUN position, test for 12 volts at the BCM RUN circuit.
- Ignition in the CRANK position, test for 12 volts at the BCM CRANK circuit.
| ⇒ | Verify that no other DTCs are set. |
| ⇒ | If the ignition switch fails, replace the ignition switch. |
| ⇒ | If no voltage at the BCM, test the ACCESSORY circuit for an open/high resistance. |
| ⇒ | If no voltage at the BCM, test the RUN circuit for an open/high resistance. |
Important: Ensure vehicle is in park or neutral with park brake applied.
| ⇒ | If no voltage at the BCM test the CRANK circuit for an open/high resistance. |
| ⇒ | If all circuits test normal replace the BCM. |
Component Testing
Ignition Switch
Using the Resistance Test below, check the resistance between the pins of the ignition switch listed in each switch position listed.
| ⇒ | If any reading is other than specified, replace the ignition switch. |
Ignition Switch Position | Pins 2 and 1 | Pins 2 and 3 | Pins 2 and 5 | Pins 2 and 6 | Pins 5 and 6 |
|---|---|---|---|---|---|
Off Key Out | Infinite | Infinite | Infinite | Infinite | Infinite |
Off Key IN | Infinite | Infinite | < 5.0 ohms | Infinite | Infinite |
Accessory | Infinite | < 5.0 ohms | Infinite | Infinite | Infinite |
Run | < 5.0 ohms | < 5.0 ohms | Infinite | Infinite | < 5.0 ohms |
Start | < 5.0 ohms | Infinite | Infinite | Infinite | 1275-1325 ohms |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Control Module References for BCM replacement, setup, and programming
