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
the power mode messages on the serial data line to other systems. Power
mode messages consist of ignition switch position status information,
and allows other systems (that are not directly wired to ignition power)
away to determine which position the ignition switch is in. 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 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
BCM to wake-up when activated or deactivated. Multiple switch wake-ups
are needed 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 or OUT of 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/headlamps 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
messages.
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 BCMs 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
BCMs 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 various functions:
BCM Inputs
| • | Ambient Light Signal Processing -- provides the BCM with
the ambient light input information for interior lamp illumination and automatic
headlamp operation. |
| • | 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. |
| • | 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 (export)-- provides fog lamp switch
activation input to the BCM in order to turn the rear fog lamps ON or OFF. |
| • | Folding Top Ajar Switch--provides folding top ajar status
to the BCM. |
| • | Folding Top Release Switch -- provides folding top release
switch information to the BCM. |
| • | Hatch Ajar Switch -- provides hatch ajar status to the BCM. |
| • | Hatch Release Switch -- provides hatch release switch information
to the BCM. |
| • | Headlamps OFF -- provides the BCM with headlamps ON or OFF
status for automatic headlamp operation. |
| • | Hood Ajar Switch (export)-- 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 pallet 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. |
| • | Traction Control System (TCS) Switch -- provides traction
control switch activation input to the BCM in order to turn the TCS ON or
OFF. |
The BCM directly controls the following outputs. Other systems may request
the BCM to control these outputs for various functions:
BCM Outputs
| • | Automatic Headlamp/Parklamp Control -- provides output control
to turn ON the headlamps and parklamps when it's dark outside. |
| • | Backup Lamp Relay -- provides output control in order to
energize the backup lamp relay for UTD and Approach Lighting functions. |
| • | Console Lamp Dimming Control -- controls the console lamp
illumination when the headlamps or the parklamps are ON. |
| • | Courtesy Lamp Relay -- provides output control in order to
energize the courtesy lamp relay when the courtesy lamp switch is ON. |
| • | DRL Relay, LH -- when energized, the relay allows the BCM
to use the LH front turn signals for DRL, Approach Lighting, and UTD functions. |
| • | DRL Relay, RH -- when energized, the 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 the fog lamp switch is ON. |
| • | Fog Lamp Relay, Rear (export)-- provides output control in
order to energize the rear fog lamp relay when the fog lamp switch is ON,
or for Approach Lighting and UTD functions. |
| • | Folding Top Release Relay--provides output control in order
to energize the folding top relay when the folding top switch is pressed. |
| • | Hatch Release Relay--provides output control in order to
energize the hatch release relay when the hatch release switch
is pressed. |
| • | 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 headlamps or the parklamps are ON. |
| • | Monitored Load Relay -- provides output control for the monitored
load 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 headlamps or the parklamps are ON (A/T only). |
| • | Rear Defogger Relay -- provides output control in order to
energize the rear defogger relay when the rear defogger switch is pressed. |
| • | Steering Column Lock/Unlock Control -- provides output to
control drive A and drive B for steering column lock/unlock operation. |
| • | Steering Column Lock Relay -- allows the BCM to disable steering
column lock functions. |
| • | Theft Deterrent Relay -- allows the BCM to energize the theft
deterrent relay for UTD functions. |
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 greater.
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.
