Principles of Operation: Notes

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

The vehicle has 4 module communications networks:

the standard corporate protocol (SCP) communications network.
the international standards organization (ISO) 9141 communications network.
the universal asynchronous receiver-transmitter (UART)-based protocol (UBP) communications network.
the controller area network (CAN).

The diagnostic tool connects to all 4 communications networks through 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 4 communications networks from one connector. The DLC is found under the instrument panel between the steering column and the radio.

The ISO 9141 communications network does not permit intermodule communication. When the diagnostic tool communicates to modules on the ISO 9141 communications network, the diagnostic tool must request all information; or initiate module commands. 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 fail.

The SCP communications network remains operational even with the severing of one of the SCP communications network circuits. Communications also continue if one of the communications network circuits are shorted to ground or voltage, or if some but not all termination resistors are lost.

The UBP communications network does not function if the circuit is shorted to ground or voltage. The UBP communications network is different from the ISO 9141 communications network in that it allows intermodule communication.

The CAN has an unshielded twisted pair cable, data plus circuit 1827 (WH/LG) and data minus circuit 1828 (PK/LG). The CAN is a communications network used for the anti-lock brake system (ABS) module and the powertrain control module (PCM) communication. The CAN remains operational even with the severing of one of the CAN circuits. Communications also continue if one of the CAN circuits are shorted to ground or voltage, or if some, but not all, termination resistors are lost.