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Home >> Ford >> 2005 >> Mustang V6-4.0L VIN N >> Repair and Diagnosis >> Powertrain Management >> Computers and Control Systems >> Information Bus >> Testing and Inspection >> Initial Inspection and Diagnostic Overview >> Communications Network >> Principles of Operation

Principles of Operation

PRINCIPLES OF OPERATION

The vehicle has 3 module communications networks:
- high speed controller area network (CAN)
- medium speed CAN
- international standards organization (ISO) 9141 communications network

The diagnostic tool connects to the communications networks through the data link connector (DLC). The DLC allows a diagnostic tool access to diagnose and test the vehicle systems of any module on the communications networks.

All 3 communications networks are connected to the 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 3 communications networks from one common connector. The DLC can be found under the instrument panel between the steering column and the driver door.

The high speed CAN is an unshielded twisted pair cable, data plus circuit 1827 (WH/LG) and data minus, circuit 1828 (PK/LG). The high speed CAN is a high speed communications network used for the anti-lock brake system (ABS) module, the instrument cluster and the powertrain control module (PCM) communication. The ABS module, the PCM and the instrument cluster use the high speed CAN to communicate information back and forth such as engine RPM, vehicle speed and odometer. The modules on the high speed CAN are ignition switched. The high speed CAN is only on when the key is in the ON position. The high speed CAN is not fault tolerant meaning that if one of the circuits are shorted to voltage then the high speed CAN does not operate and communication is not possible. If the high speed CAN data minus circuit is shorted to ground, then communication is still possible. If the high speed CAN data plus circuit is shorted to ground, then communication is not possible. If one of the high speed CAN circuits becomes open, communication is possible on both sides of the open circuit. Communication across the open circuit is not impossible but unlikely. If there are issues with the high speed CAN, there may be missing functionality and problems connecting with a diagnostic tool. On the high speed CAN, there are 2 terminating modules which are composed of the PCM and the instrument cluster which are located at the far opposite ends of the high speed CAN wiring. Communication is possible with the loss of 1 termination resistor. Communication is not possible with the loss of 2 termination resistors.

The medium speed CAN is an unshielded twisted pair cable, data plus circuit 1847 (WH/OG) and data minus circuit 1848 (PK/OG). The medium speed CAN is used for the audio unit, the instrument cluster, and the smart junction box (SJB) communications. The SJB, the instrument cluster and the audio unit use the medium speed CAN to communicate information back and forth such as dimming, fuel level, turn signal indicator on the instrument cluster and vehicle compensated volume. The medium speed CAN can be awake when the key is in the OFF position to carry out functions such as delay accessory. In order to do diagnostics, it is necessary to have the key in the ON position. The instrument cluster behaves as a communications network gateway transferring data from the high speed CAN to the medium speed CAN and from the medium speed CAN to the high speed CAN. Some of the data that the instrument cluster gateways is vehicle speed, fuel level and AC request. The medium speed CAN is not fault tolerant meaning that if one of the circuits are shorted to voltage then the medium speed CAN does not operate and communication is not possible. If the medium speed CAN data minus circuit is shorted to ground, then communication is still possible. If the medium speed CAN data plus circuit is shorted to ground, then communication is not possible. If one of the medium speed CAN circuits becomes open, communication is possible on both sides of the open circuit. Communication across the open circuit is not impossible but unlikely. If there are issues with the medium speed CAN, there may be missing functionality and problems connecting with a diagnostic tool. On the medium speed CAN, there are 2 terminating modules which are composed of the SJB and the instrument cluster which are located at the far opposite ends of the medium speed CAN wiring. Communication is possible with the loss of 1 termination resistor. Communication is not possible with the loss of 2 termination resistors.

The ISO 9141 communication network is a single circuit communications network, circuit 70 (LB/WH). 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 short to ground, voltage or an open. Also, if one of the modules on the ISO 9141 communications network loses power or shorts internally, communications to that module fails. The ISO 9141 communications network is used to communicate with the restraint control module (RCM).
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