Circuit/System Description
The internal fault detection is handled inside the control module. No external circuits are involved.
Some control modules must be configured with serial numbers, vehicle options, or other information. If a control module was not properly configured after installation, that module may set DTC B1001.
The internal fault detection is handled inside the control module. No external circuits are involved.
When the ignition is turned ON, the microprocessor runs a diagnostic program to detect an internal fault when power up is commanded. If an internal malfunction is detected, the control module will store a DTC C0550 in its memory.
Modules connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have programmed information about what messages are needed to be exchanged on the serial data circuits. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module.
DTC U0020 will set when fewer modules are detected on the bus before the modules can learn the other modules IDs. If the communication bus is opened, or if the modules power supply is lost when the vehicle is in the OFF power mode, and then placed in the RUN power mode, U0020 will set in specific modules.
The serial data circuits are serial data buses are used to communicate information between the control modules. The serial data circuits also connect directly to the data link connector (DLC).
Modules connected to the high and low speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules.
The body control module (BCM) activates the serial data communication enable and the accessory wakeup serial data circuits by applying voltage when the ignition key is in ACC, ON or START. Only the modules using high speed GMLAN serial data are connected to the enable or wakeup circuits. To determine which modules are activated using the enable or wakeup circuits refer to Data Communication Schematics .
The accessory wake up serial data circuit is a voltage wake up signal from the body control module (BCM) to the engine control module (ECM) and the transmission control module (TCM). The signal is activated in 2 ways; any time the ignition switch is in the ACC or RUN position, or anytime the BCM determines that communication is needed from the powertrain. When the ignition switch is in the ACC or RUN position, the accessory voltage circuit input to the BCM is internally connected to the accessory wake up serial data circuit output, and voltage is supplied to the modules. The BCM also supplies voltage to the accessory wake up serial data circuit when it determines that communication is necessary with the powertrain and the ignition is not in the ACC or RUN position such as during remote start.
The body control module (BCM) applies voltage to the data communication enable circuit when the ignition key is in ACC, ON, or START to enable several modules for bus communication.
Depending on option content, modules that may receive the signal are as follows
- Transfer case control module (TCCM)
- Electronic brake control module (EBCM)
- Headlamp module
The electronic brake control module (EBCM) activates the other modules connected to the CAN serial data circuits by applying voltage when the ignition key is in ACC, ON or START. The EBCM, Yaw Rate Sensor, and the Steering Angle Sensor are the only modules connected to the CAN bus serial data circuits. For more information on CAN bus serial data refer to Data Link Communications Description and Operation .
The data link connector (DLC) is a standardized 16 cavity connector. Connector design and location is dictated by an industry wide standard, and is required to provide the following
- Scan tool B+ voltage at terminal 16
- Scan tool ground at terminal 4
- Common ground at terminal 5
Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other than OFF. The high speed GMLAN serial data circuits must be operational for the vehicle to start due to body control module (BCM) and engine control module (ECM) communications.
Modules connected to the low speed GMLAN serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other than OFF. The low speed GMLAN serial data circuit uses JX 205 and JX 405 as common connection points between some modules and the data link connector (DLC).
Body Control System Description and Operation
The body control system consists of the body control module (BCM), communications, and various input and outputs. Some inputs, outputs and messages require other modules to interact with the BCM. The BCM also has discrete input and output terminals to control the vehicle body functions. The BCM is wired to the GMLAN High speed serial data buss and the GMLAN Low speed serial data buss and acts as a gateway between them. If the BCM does not communicate the vehicle will not start due to the inability of the engine/powertrain control module (ECM/PCM) and vehicle theft deterrent (VTD) control module to communicate without the BCM providing the gateway function.
Controller Area Network (CAN) Description
The EBCM communicates with the yaw rate sensor and the steering wheel angle sensor exclusively, when equipped. This serial data system does not communicate with the scan tool however; there are loss of communication DTCs for this system which set in the EBCM. The yaw rate sensor and the steering wheel angle sensors are in effect modules, exchanging essential information with the EBCM. For more information about these sensors refer to ABS Description and Operation .