Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Serial Data Power Mode Master: Notes
Power to many of this vehicles circuits is controlled by the module that is designated the power mode master (PMM). This vehicles 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 that will be sent over the serial data circuits to the other modules that need this information, and so the PMM will activate relays and other direct outputs of the PMM as needed. The PMM determines which power mode (Run, Accessory, Crank, Retained Accessory Power, or Off) is required, and reports this information to other modules via serial data. 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 receives 3 ignition switch signals to identify the operators desired power mode. The 3-wire Ignition Switch table below illustrates the state of these inputs in correspondence to the ignition switch position:
| Ignition Switch Position | Accessory | Ignition 1 | Off/Run/Crank | Power Mode Transmitted |
|---|---|---|---|---|
|
IMPORTANT:
States marked with the * indicate a derived positive voltage level of 3 volts. States marked with the ** indicate a derived positive voltage level of 4 volts.
|
||||
| Off | 0 | 0 | 0 Key out 1 Key in | OFF/Awake or RAP |
| Start | 0 | 1 | 1 * | Crank |
| Accessory | 1 | 0 | 0 | Accessory |
| Run | 1 | 1 | 1 ** | Run |
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.