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Circuit Description

WARNING: This page is about a different car, the 2005 GMC Canyon and 2005 Chevrolet Colorado. However, it is still accessible from the selected car via links, so may be relevant.

The body control module (BCM) monitors the ignition inputs Ignition Off/Run/Crank, Ignition 1 Run/Crank and the Ignition Accessory signals supplied from the ignition switch. The BCM uses the sequence that the signals appear, their voltage levels and the engine run flag (ERF) status to determine the power mode. Battery voltage is supplied to the ignition switch by the IGN TRANSD fuse. The ignition key resistor signal from the BCM provides a 5-volt supply voltage to the ignition switch. The table below shows the correct ignition switch inputs and power modes.

Correct Ignition Switch Inputs and Power Modes

Ignition Switch Position BCM Input and Scan Tool Display
Ignition Accessory Ignition 1 Run/Crank Ignition Off/Run/Crank Crank Voltage (Volts) Power Mode
OFF Inactive Inactive Active
  • 5, w/key in1
  • 0, w/key out
 Off-Awake or RAP
ACC Active Inactive Inactive 0 Accessory
ON Active Active Active 4.4 Run
START Inactive Active Active 1.7-4.2 Crank
1 The correct value should be B+, but the parameter was scaled to maximum 5 volts, to increase reading accuracy in the lower range.
Power Mode Mismatch

Step Action Yes No
Schematic Reference: Body Control System Schematics  orPower Distribution Schematics in Wiring Systems
Connector End View Reference:  Computer/Integrating Systems Connector End Views  
DEFINITION: This table is used if the observed vehicle power mode does not match the ignition switch position.
1 Did you perform the Diagnostic System Check - Vehicle? Go to Step 2  Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2
IMPORTANT: The engine may start during this procedure. Turn the ignition OFF after verifying this power mode.
  1. With a scan tool, under the Diagnostic Circuit Check menu observe the Class 2 Power Mode parameter.
  2. Turn the ignition to all positions and compare the parameter to the correct ignition switch inputs table above.
Does the scan tool display the correct power mode?		 Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems Go to Step 3 
3
IMPORTANT: The engine may start during this procedure. Turn the ignition OFF after verifying this power mode.
  1. Observe the Crank Voltage parameter with the scan tool.
  2. Turn the ignition to all positions and compare the parameter to the values from the table above.
  3. Observe the Ignition Off/Run/Crank parameter with the scan tool.
  4. Turn the ignition to all positions and compare the parameter to the correct ignition switch inputs table above.
Do the voltage displayed for Crank Voltage and the Ignition Off/Run/Crank signal match the values from the table above?		 Go to Step 4  Go to DTC B1441, B1442, or B1443  and use that procedure to diagnose the concern
4
IMPORTANT: The engine may start during this procedure. Turn the ignition OFF after verifying this power mode.
  1. Observe the Ignition 1 Run/Crank parameter with the scan tool.
  2. Turn the ignition to all positions and compare the parameter to the correct ignition switch inputs table above.
Does the Ignition 1 Run/Crank signal display correctly?		 Go to Step 5  Go to DTC B1372, B1373, or B1374  and use that procedure to diagnose the concern
5
IMPORTANT: The engine may start during this procedure. Turn the ignition OFF after verifying this power mode.
  1. Observe the Ignition Accessory parameter with the scan tool.
  2. Turn the ignition to all positions and compare the parameter to the correct ignition switch inputs table above.
Does the Ignition Accessory signal display correctly?		 Go to Step 6  Go to DTC B1382, B1383, or B1384  and use that procedure to diagnose the concern
6
  1. Disconnect the body control module (BCM).
  2. Inspect for poor connections and terminal tension at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 8  Go to Step 7 
7 Replace the BCM. Refer to Computer/Integrating Systems Connector End Views  for replacement, setup, and programming.
Did you complete the replacement?		 Go to Step 8  -
8
IMPORTANT: The engine may start during this procedure. Turn the ignition OFF after verifying all power modes.
  1. Reconnect all disconnected components.
  2. With a scan tool, under the Diagnostic Circuit Check menu observe the Class 2 Power Mode parameter.
  3. Cycle the ignition switch through all possible positions one at a time.
Does the displayed power mode parameter match the actual ignition switch position?		 System OK Go to Step 1 
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