Principles of Operation

WARNING: This page is about a different car, the 2005 Ford Pickup. However, it is still accessible from the selected car via links, so may be relevant.

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 inter-module 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.