Principles of Operation

WARNING: This page is about a different car, the 2005 Mercury Montego, 2005 Ford Freestyle, and 2005 Ford Five Hundred. However, it is still accessible from the selected car via links, so may be relevant.

The vehicle has 3 module communications networks:

high speed controller area network (CAN)
medium speed CAN
international standards organization (ISO) 9141 communications network

The high speed CAN is a high speed communications network used for the anti-lock brake system (ABS) module, the differential electronic module (DEM) (if equipped), the transmission control module (TCM) - 6-speed transaxle, TCM - continuously variable transaxle (CVT), the instrument cluster, the powertrain control module (PCM) and the restraint control module (RCM) communication. The medium speed CAN is used for the audio unit, the driver seat module (DSM) (if equipped), the dual automatic temperature control (DATC) module (if equipped), the digital versatile disc (DVD) player (if equipped), the instrument cluster, and the smart junction box (SJB) communications. The CAN remains operational with the severing of 1 of the CAN circuits or if the data minus circuit is shorted to ground. Communication may continue if one of the termination resistors is lost. The CAN does not remain operational if the data plus circuit is shorted to ground or shorted to voltage. The CAN also does not remain operational if the data minus circuit is shorted to voltage.

The medium speed CAN has an unshielded twisted pair cable, data plus circuit 2180 (WH/BK) and data minus circuit 2181 (BK/YE). The high speed CAN also has an unshielded twisted pair cable, data plus circuit 1908 (WH) and data minus circuit 1909 (BK). The ISO 9141 communications network 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 1 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 intermodule 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 fail. The ISO 9141 communications network is used to communicate with the parking aid module (PAM).