Module Communications Network
COMMUNICATIONS NETWORKAnti-Lock Brake System (ABS) Module
The ABS module uses the controller area network (CAN). The ABS module controls the brake pressure to the 4 wheels to keep the vehicle under control while braking.
Driver Seat Module (DSM)
The DSM uses the universal asynchronous receiver transmitter (UART)-based protocol (UBP) communications network. The DSM controls the positioning and programming of the driver seat and pedals.
Electronic Automatic Temperature Control (EATC) Module
The EATC module uses the UBP communications network. The EATC module controls automatic climate functions that maintain the vehicle interior temperature at a constant setting.
Instrument Cluster
The instrument cluster uses both the CAN and the UBP communications networks. It is an analog face instrument cluster with a digital odometer. The instrument cluster displays a variety of information.
Parking Aid Module
The parking aid module uses the international standards organization (ISO) 9141 communications network. The module controls sensors in the rear bumper that detect close objects when the vehicle is in reverse.
Powertrain Control Module (PCM)
The PCM uses the CAN and uses this communications network to communicate with the diagnostic tool. The PCM utilizes an internal termination resistor for normal interface with the CAN. Damage or loss of this termination resistor results in intermittent operation and or failure of the entire CAN. The PCM controls the engine for better fuel economy, emissions control, failure mode detection and storage as well as all 4 wheel drive and speed control functions. Refer to Computers and Control Systems for diagnosis and testing of the PCM.
Restraint Control Module (RCM)
The RCM uses the ISO 9141 communications network. The RCM controls the deployment of the air bags based on sensor input.
Vehicle Security Module
The vehicle security module uses the UBP communications network. The vehicle security module controls the remote keyless entry (RKE) functions for the vehicle.
PRINCIPLES OF OPERATION
The vehicle has 3 module communications networks:
- controller area network (CAN)
- universal asynchronous receiver transmitter (UART)-based protocol (UBP) communications network
- international standards organization (ISO) 9141 communications network
The CAN has an unshielded twisted pair cable, data plus, circuit 1827 (WH/LG) and data minus, circuit 1828 (PK/LG). The UBP communications network uses a single circuit communications network circuit 1906 (VT), as does the ISO 9141 communications network, which is a single circuit communications network, circuit 70 (LB/WH). All 3 communications networks are connected to the data link connector (DLC). This makes diagnosis and testing of these systems easier by allowing one diagnostic tool to be able to diagnose and control any module on the 3 communications networks from one connector. The DLC can be found under the instrument panel between the steering column and the audio unit.
The ISO 9141 communications network does not permit inter-module communication. When the diagnostic tool communicates to modules on the ISO 9141 communications network, the diagnostic tool must ask for all information; the modules cannot initiate communications.
The ISO 9141 communications network does not function if the circuit is shorted to ground or battery voltage. Also, if one of the modules on the ISO 9141 communications network loses power or shorts internally, communications to that module fails.
The UBP communications network does not function if the circuit is shorted to ground or voltage. The UBP communications network allows intermodule communication. If one of the modules on the UBP communications network loses power or shorts internally, communications to that module fails.
The CAN permits inter-module communication between the DLC, the anti-lock brake system (ABS) module, the instrument cluster and the powertrain control module (PCM). When the diagnostic tool communicates to modules on the CAN, the diagnostic tool must request all information or initiate module commands.
The CAN does not function if the circuit is shorted to ground or battery voltage. Also, if one of the modules on the CAN loses power or shorts internally, communications to that module fail. Both the PCM and the instrument cluster have internal termination resistors required for correct communications network operation.
The PCM communicates with the diagnostic tool through the CAN circuits 1827 (WH/LG) and 1828 (PK/LG). If one of the communications network circuits become shorted to ground or voltage, or if some but not all termination resistors are lost, communications can continue. Check circuits 1827 (WH/LG) and 1828 (PK/LG) between the PCM C175b and the DLC C251. The total resistance values must not be more than 5 ohms. If the resistance is more than 5 ohms there is an open circuit in the CAN, damage to the DLC C251, damage to the PCM C175b, or a problem in the in-line connector.