Principles of Operation: Notes

WARNING: This page is about a different car, the 2006 Mercury Montego, 2006 Ford Freestyle, and 2006 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

All 3 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 networks from 1 connector. The DLC can be found under the instrument panel between the steering column and the audio unit.

Both the high speed and medium speed CAN consist of an unshielded twisted pair cable, data plus circuit and data minus circuit. The CAN is not designed to operate under single point fault conditions. If either circuit is shorted to voltage or the CAN data plus is shorted to ground, module-to-module and module-to-tester communication is not possible. If the CAN data minus is shorted to ground or an open exists on either bus line, module-to-module and module-to-tester communication is marginal at best. The CAN bus may remain operational when one of the two termination resistors are not present. If there are issues with the CAN, there may be missing functionality and problems connecting with a diagnostic tool.