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
On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle the PMM is the body control module (BCM). The BCM receives three signals/circuits from the ignition switch: Acc/Run/Crank (Ignition 0), Run/Crank (Ignition 1) and Acc/Run (Accessory).
To determine the correct power mode the BCM uses:
- The state of these signals/circuits, either switch closed (B+ = 1) or switch open (open = 0)
- The sequence of switch closures received by the BCM
- The status of the engine run flag
The chart below indicates the modes detected and transmitted by the BCM:
| Power Mode | Engine Run Flag | Sampled Ignition Signal State | ||
|---|---|---|---|---|
| Selected/ Expected | Serial Data | Accessory | Ignition 1 | Ignition 0 |
| OFF | 0 | 0 | 0 | 0 |
| RAP | 0 | 0 | 0 | 1 to 0 |
| ACCESSORY | 0 | 1 | 0 | 1 |
| RUN | 0 | 1 | 1 | 1 |
| CRANK | 0 | 0 | 1 | 1 |
| RUN | 1 | 1 | 1 | 1 |
NOTE: Any ignition switch states not covered in this table indicates a DTC setting situation.
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.