* Diagnostic Procedure Instructions (See: Testing and Inspection/Initial Inspection and Diagnostic Overview/Diagnostic Procedure
Instructions) provides an overview of each diagnostic category.
DTC Descriptor
DTC B1440 00
- Power Mode Master Input Circuits Mismatch
Diagnostic Fault Information
Circuit/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 the are 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.
Conditions for Running the DTC
Ignition switch is in the RUN 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.
* A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.
Diagnostic Aids
* A short to ground in the accessory wakeup serial data circuit will cause an open fuse to the ignition switch. DTC U1814 should be current.
* A short to ground in the run/crank ignition 1 voltage circuit of the run crank relay will cause an open fuse to the ignition switch. DTC B1428
should be current.
* The power modes and controls are determined using BCM logic. Comparing the Body Control Module Power Mode data display parameters
listed in Circuit/System Verification will determine if there is a power mode fault.
