Principles of Operation (How Does It Work?)

PRINCIPLES OF OPERATION

The vehicle has three module communication networks: the standard corporate protocol (SCP), which is an unshielded twisted pair cable (data bus plus, [Circuit 914 (TN/OG)] and data bus minus, [Circuit 915 (PK/LB)], the universal asynchronous receiver transmitter (UART) based protocol (UBP) (Circuit 693 [OG]) communications network and the international standards organization (ISO) 9141 communications network, which is a single wire network (Circuit 70 [LB/WH]). All three networks are connected to the data link connector (DLC). This makes diagnosis and testing of these systems easier by allowing one smart tester to be able to diagnose and control any module on the three 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 will 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.

Under normal operation the powertrain control module (PCM) communicates with the diagnostic tool through the SCP communications network circuits 914 (TN/OG) and 915 (PK/LB). If one of the bus wires becomes shorted to ground or voltage, or if some but not all termination resistors are lost, communications can continue. Check circuits 914 (TN/OG) and 915 (PK/LB) between the PCM C175 and the DLC C251. Total resistance values must not be more than 5 ohms. If the resistance is more than 5 ohms there is an open in circuit 914 (TN/OG) or 915 (PK/LB), damage to the DLC C251, damage to the PCM C175, or a problem in the in-line connector C1026.

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