BCM Functions
The body control module (BCM) performs multiple body control functions. The BCM can control devices directly connected to its outputs based on input information. The BCM evaluates this information and controls certain body control systems by commanding outputs on or off. The BCM control inputs can be:
| • | Sensors and switches that are directly connected to the BCM. |
| • | Class 2 serial data received from other control modules connected to the class 2 serial data link. |
The BCM is also capable of controlling other vehicle systems that are not directly wired to the BCM. The BCM does this by sending specific messages on the class 2 serial data link. The control module capable of performing the required function will respond to the BCM message.
The BCM controls these functions:
| • | Audible warnings--Refer to Audible Warnings Description and Operation in Instrument Panel, Gages and Console. |
| • | Automatic door locks--Refer to Power Door Locks Description and Operation in Doors. |
| • | Automatic headlamp control--Refer to Exterior Lighting Systems Description and Operation in Lighting Systems. |
| • | Interior lighting--Refer to Interior Lighting Systems Description and Operation in Lighting Systems. |
| • | Keyless entry (AUO option)--Refer to Keyless Entry System Description and Operation in Keyless Entry. |
| • | Passlock theft deterrent--Refer to Vehicle Theft Deterrent (VTD) Description and Operation in Theft Deterrent. |
| • | Retained accessory power (RAP)--Refer to Retained Accessory Power (RAP) Description and Operation in Retained Accessory Power. |
Serial Data Power Modes
On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle the body control module (BCM) is the PMM. The BCM receives 2 signals/circuits from the ignition switch. These are the ignition 1 and ignition 3 signals/circuits.
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 status of the engine run flag |
The chart indicates the modes detected and transmitted by the BCM:
Ignition Key Position | Power Mode Parameter | Engine Run Flag Serial Data | Sampled Ignition Signal State | |
|---|---|---|---|---|
Ignition 1 | Ignition 3 | |||
Off | Off-Awake | 0 | 0 | 0 |
Unlock | Off-Awake | 0 | 0 | 0 |
Accessory | Off-Awake | 0 | 0 | 0 |
Run | Run | 0 | 1 | 1 |
Crank | Crank | 0 | 1 | 0 |
Run | Run | 1 | 1 | 1 |
Power Mode Fail-safe Operation
Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place should the BCM fail to send a power mode message. The fail-safe plan covers modules with discrete ignition signal inputs as well as those modules using exclusively serial data control of power mode.
