Principles Of Operation

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

The vehicle has 3 module communication networks: SCP, which is an unshielded twisted pair cable data bus plus and data bus minus, the UBP communications network, a single wire network and the ISO 9141 communications network, which is a single wire network. All 3 networks are connected to the data link connector (DLC) C251. This makes diagnosis and testing of these systems easier by allowing one diagnostic tester to be able to diagnose and control any module on the 3 networks from one connector. The DLC can be found under the instrument panel between the steering column and the radio.

The SCP communications network will remain operational even with the severing of one of the bus wires. Communications can also continue if one of the bus wires is shorted to ground or voltage, or if some but not all termination resistors are lost.

The ISO 9141 communications network does not permit intermodule 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 battery voltage. Also, if one of the modules on the ISO 9141 communications network loses power or shorts internally, communications to that module will fail.

The UBP communications network will not function if the wire is shorted to ground or voltage. The UBP network allows intermodule communication.