Principles of Operation

WARNING: This page is about a different car, the 2004 Mercury Mountaineer and 2004 Ford Explorer. However, it is still accessible from the selected car via links, so may be relevant.

The vehicle has three module communication networks:

controller area network (CAN)
universal asynchronous receiver transmitter (UART) based protocol (UBP)
international standards organization (ISO) 9141

The CAN has an unshielded twisted pair cable, data bus plus, circuit 3126 (TN/YE) and data bus minus, circuit 3125 (RD/YE). The UBP communications network uses a single wire network circuit 693 (OG), as does the ISO 9141 communications network, which is a single wire network, circuit 70 (LB/WH). All three 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 three networks from one connector. The DLC can be found just above the parking brake release handle.

The CAN permits inter-module communication between the DLC, PCM, instrument cluster EATC module, tire pressure monitoring system module, and the ABS modules. When the diagnostic tool communicates to modules on the CAN, the diagnostic tool must request all information or initiate module commands.

The CAN will not function if the wire 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 will fail. Both the PCM and the instrument cluster have internal termination resistors required for correct network communications operation.

The UBP network allows inter-module communication. The UBP communications network will not function if the wire is shorted to ground or voltage. If one of the modules on the UBP communications network loses power or shorts internally, communications to that module will fail.

The ISO 9141 communications network does not permit inter-module communication. When the diagnostic tool communicates to modules on the ISO 9141 communication network, the diagnostic tool must ask for all information; the modules cannot initiate communications.

The ISO 9141 communications network will not function if the wire is shorted to ground or voltage. Also, if one of the modules on the ISO 9141 communications network loses power or shorts internally, communications to that module will fail.