This vehicle is equipped with a computer system capable
of performing multiple engine and body control functions. Vehicle components
(windows, locks, mirrors, etc.) are controlled directly by their system module.
Each system communicates with other systems on an ``interconnecting bus''
or serial data line. These systems are all part of the vehicle's computer
system providing two-way communication between various engine and body control
systems.
Some electrical components are directly connected and controlled by
their system module. Others systems accomplish this by sharing data information
with other systems on the serial data line to execute various vehicle functions.
All system data information (sensors, switches, etc.) can be monitored by
any system module that completes the computer system.
All systems on the same serial data line use a communication process
similar to a telephone system, where enormous amounts of information can
be exchanged on a single data wire.
Communications between each system is accomplished by sending a digitally
coded message, consisting of specific information a system module must follow.
Each system module is assigned its own recognition code (address). So when
a message is sent out on the serial data line, only the particular system
module that was assigned that recognition code will responded to the message.
The BCM, which is the power mode master, is responsible for sending
a power mode message on the serial data line to other systems. When the power
mode message is sent, other systems respond to this message by entering
a power-up state and sending a State of Health Code (SOH) code message. This
SOH message indicates they are able to communicate information between other
systems.
By communicating data information with other systems on the same serial
data line, either input information or specific program information within
the systems memory, can provide more accurate and reliable computer system
control functions. The BCM uses information from other systems in order to
execute some functions. Therefore, before further diagnosis, review the general
information on how the BCM computer system operates and how each system interacts
with the BCM. This will save diagnostic time, future misdiagnosis, and unnecessary
parts replacement.
BCM Functions
The BCM is capable of performing multiple body control functions. The
BCM outputs control the systems that are directly connected to the BCM. The
BCM controls the outputs using this input information received from the sensors
and the switches directly connected to the BCM. The BCM also borrows information
from other systems on the serial data line in order to control the outputs.
The BCM evaluates this information and commands an output ON or OFF in order
to control certain body systems. The BCM also commands other systems to control
systems that are not directly wired to the BCM. The BCM accomplishes this
by sending specific messages on the serial data line. The system capable
of performing this function will respond to the message.
The BCM is capable of performing the following functions:
| • | Bulb over voltage protection |
| - | Daytime Running Lights (DRL) |
| - | Headlights flash-to-pass feature |
| - | Front turn signal monitoring |
| - | Rear fog lamps (export only) |
| - | Illuminated entry feature |
| • | Monitored loads control |
| • | Pass-Key® (Personalized Automotive Security System) |
| • | Steering column lock/unlock control |
| • | Universal Theft Deterrent (UTD) |
| • | Vehicle configuration (RPO codes) |
BCM Wake-up/Sleep States
The BCM is able to control or perform all of the BCM functions in the
wake-up state. The BCM enters the sleep state when active control or monitoring
of system functions has stopped, and the BCM has become idle again. The BCM
must detect certain wake-up inputs before entering the wake-up state. The
BCM monitors for these inputs during the sleep state, where the BCM is able
to detect switch transitions that cause the two wake-up states, activated
or deactivated. Multiple switch wake-ups are need in order to sense both
the insertion and the removal of the ignition key. This would allow the BCM
to enter a sleep state when the key is in the ignition.
The BCM will enter a wake-up state if any of the following wake-up inputs
are detected:
| • | Any activity on the serial data line |
| • | Park/fog/headlights are on |
| • | The BCM experiences a battery disconnect and reconnect condition. |
| • | The ignition is turned ON. |
The BCM will enter a sleep state when all of the following conditions
exist:
| • | No activity exists on the serial data line. |
| • | The ignition switch is OFF. |
| • | No outputs are commanded. |
| • | No delay timers are actively counting. |
| • | No wake-up inputs are present. |
If all these conditions are met the BCM will enter a low power or sleep
condition. This condition would indicate the BCM, which is the power mode
master of the vehicle, must have sent a message to the other systems on the
serial data line to stop broadcasting their state of health message.
Power Requirements
The BCM has two main power feeds (high and low current), and two main
grounds. The low power feed (battery 1) is used to provide power
for the BCM's logic and internal driver operation. The high power feed (battery 2)
is used to provide power for systems that draw higher amounts of current
(motors, lights, etc.). Operating current at the BCM's battery should not
exceed 3.0 amperes at battery 1, and 7.5 amperes (for
2 seconds) at battery 2 while the BCM is awake, and 2.5 milliamps
when the BCM is asleep. For most BCM functions, the BCM will operate properly
with a system voltage of 9.0-16.0 volts. If system voltages exceed
16.0 volts the BCM will provide protection by disabling certain functions
that may damage due to higher than normal system voltages.
BCM Input/Output Information
The following components provide direct input to the BCM. Other systems
can use this input information in order to carry out certain functions:
BCM Inputs
| • | Ambient Light Signal Processing -- provides the BCM with
the ambient light input information for interior lamp illumination. |
| • | Column Feedback Switch -- provides steering column feedback
information to the BCM in order to determine if the steering column was successfully
locked or unlocked. |
| • | Door Ajar Switches -- provides door ajar status to the BCM. |
| • | Electronic Traction/Suspension Control Switch -- provides
traction control switch activation input to the BCM in order to turn the TCS
status ON or OFF. |
| • | Fog Lamp Switch, Front -- provides fog lamp switch activation
input to the BCM in order to turn the front fog lamps ON or OFF. |
| • | Fog Lamp Switch, Rear -- provides fog lamp switch activation
input to the BCM in order to turn the rear fog lamps ON or OFF. |
| • | Hatch Ajar Switch -- provides hatch ajar status to the BCM. |
| • | Hatch Release Switch -- provides hatch release switch information
to the BCM. |
| • | Hood Ajar Switch -- provides hood ajar status to the BCM. |
| • | Ignition Switch -- provides the BCM with ignition position
status. |
| • | Key In/Out Ignition Switch -- allows the BCM to detect when
a key is in or out of the ignition. |
| • | Monitored Loads -- monitors electrical loads left ON after
the ignition switch is turned OFF. |
| • | Pass-Key® Resistance -- determines the resistance of
the key for the PASS-Key® system. |
| • | Rear Defogger Switch -- provides rear defogger switch activation
input to the BCM in order to turn the rear defogger ON or OFF. |
| • | Turn Signal Monitor, LH/RH -- monitors the turn signal status
for the DRL. |
The BCM directly controls the following outputs. Other systems may request
the BCM to control these outputs for certain functions:
BCM Outputs
| • | Backup Lamp Relay -- provides output control in order to
energize the backup lamp relay for UTD and Approach Lighting. |
| • | Column Lock/Unlock Control -- controls the steering column
lock/unlock motor position. |
| • | Console Lamp Dimming Control -- controls the console lamp
illumination when the headlights or the parklights are ON. |
| • | Courtesy Lamp Relay -- provides output control in order to
energize the courtesy lamp relay when an ON request is received. |
| • | DRL Relay, LH -- the energized relay allows the BCM to use
the LH front turn signals for DRL, Approach Lighting, and UTD functions. |
| • | DRL Relay, RH -- the energized relay allows the BCM to use
the RH front turn signals for DRL, Approach Lighting, and UTD functions. |
| • | Fog Lamp Relay, Front -- provides output control in order
to energize the front fog lamp relay when an ON request is received. |
| • | Fog Lamp Relay, Rear -- provides output control in order
to energize the rear fog lamp relay with a fog lamp switch ON request, or
for Approach Lighting and UTD functions. |
| • | Horn Relay -- allows the BCM to energize the horn relay for
UTD and RFA system functions. |
| • | HVAC Lamp Dimming Control -- controls the HVAC lamp illumination
when the headlights or the parklights are ON. |
| • | Monitored Load Relay -- provides output control for the load
monitor relay, allowing the BCM to shut off certain electrical loads in order
to prevent battery rundown. |
| • | PRNDL Lamp Control -- controls the PRNDL lamp illumination
when the headlights or the parklights are ON (A/T only). |
| • | Rear Defogger Relay -- provides output control in order to
energize the defogger relay with a switch ON request. |
| • | Theft Deterrent Relay -- allows the BCM to energize the theft
deterrent relay for UTD. |
Monitored Loads
In order to minimize any battery rundown, the BCM can detect if certain
electrical loads have been left on. When the BCM detects that the ignition
has been cycled to the OFF position, the BCM immediately checks the status
of the load monitor input. If the BCM detects that a load is present (ground
on CKT 640), the BCM turns ON the load monitor relay for 15 minutes.
If after 15 minutes the BCM still recognizes that a load is present,
the BCM will turn OFF the relay, removing the battery voltage from the loads.
The BCM continues to monitor this circuit for a switch transition. The BCM
will again turn ON the relay for 15 minutes if a switch transition
exists.
Load Shed Control
The BCM can turn off the rear window defogger and heated outside mirror
electrical loads when the vehicle is in a condition where these loads may
discharge the battery. The BCM will also remove these loads when engine demands
are higher.
Interior Lamp Over Voltage Protection
The BCM disables the interior lamp bulbs when the system voltage is
above 18.0 volts in order to extend the bulb life.
Driver in Vehicle Detection
Using the ignition switch/door ajar inputs, the BCM can detect whether
or not the driver has left the vehicle. If the ignition is turned to OFF with
no door ajar status detected, the BCM assumes that the driver is in the vehicle.
As soon as the BCM detects a door ajar, the BCM will assume the driver has
left the vehicle. The BCM uses this information for the RAP functions, then
sends the information on the serial data line to the other systems also responsible
for the RAP functions.
BCM Fail-Soft Condition
If a particular BCM malfunction would result in unacceptable system
operation, the BCM takes a fail-soft action in order to minimize the condition.
A typical fail-soft action would be the substitution of a fixed input or
output value when the BCM is unable to interpret data correctly.
