Data Link Communications

DTC U1000 Loss Of Communications:

CIRCUIT DESCRIPTION
The serial data circuit is used to communicate information between each system connected to the serial data line. Each system is assigned a specific recognition code (source ID). This code is used to identify which system is communicating on the serial data line. Systems communicate with others by periodically sending messages on the serial data line. These messages contain specific information (or commands) a system requires to function properly.

When the ignition is turned ON and a system receives a message it also learns the source ID of the system that sent it. This source ID is used to determine which system actually sent the message and for State of Health (SOH) monitoring. SOH monitoring determines it a system is unable to communicate properly. Systems periodically send a SOH message to each other containing their source ID. If a system tails to receive a SOH message back from a system that it communicates with, then a DTC identifying what system did not respond will set. It a system loses communication with a system before it able to learn it's source ID, then a DTC U1000.

CONDITIONS FOR SETTING THE DTC
^ A system is unable to learn the source ID of a system that it communicates with.
^ Condition must be present for 5 seconds.

ACTION TAKEN WHEN THE DTC SETS
^ Stores a DTC U100 in the system that was unable to learn a source ID.
^ The system unable to communicate will substitute default values for certain functions.
^ No driver warning message will be displayed for this DIC.

CONDITIONS FOR CLEARING THE DTC
^ This DTC requires an ignition cycle in order to change from current to history.
^ A system is able to communicate and learn the source ID of all Systems that it communicates with when the ignition is first turned ON.
^ A history DTC will clear after 50 consecutive ignition cycles if the condition for the malfunction is no longer present.
^ Use the IPC clearing DTCs feature.
^ Use a scan tool.

DIAGNOSTIC AIDS
^ If the problem is an intermittent loss of communications. Carefully inspect the serial data line and related components for the following intermittent conditions:
- There is an intermittent open or short (to ground or voltage) in the serial data line.
- Intermittent loss of communication with a system when the ignition is first turned ON.
- Damaged or loose star connector terminals.
- Poor connections/terminal tension at a harness connector.
- Intermittent communication due to a loss of power or ground to a system.
^ Communication DTCs do not necessarily indicate a malfunction. These DTCs are only intended to aid in diagnosing an intermittent communication malfunction that is accompanied by a symptom. If the vehicle does not experience a symptom related to a communication malfunction, these DTCs can be disregarded.
^ Always diagnose other communication DTCs first before diagnosing a DTC U1000. A DTC U1000 cannot determine what system it loss communications with, but other communication DTCs can. If a system has other communication DTCs stored always diagnose them first. After repairing a DTC U1000, always clear all DTCs from all systems capable of storing communication DTCs.
^ If the DTC is a history DTC, the problem may be intermittent. Perform the tests shown while moving related wiring and connectors. This can often cause the malfunction to occur.

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table:
2. This test determines if communications can be established between the scan tool and other systems on the serial data line. If communications cannot be established, refer to the applicable Diagnostic System Check for the system unable to communicate.
3. This test determines if other communication DTCs are stored in other systems. Always diagnose other communication DTCs before diagnosing a DTC U1000.
4. This test for any opens in the serial data line. It also checks for any intermittent malfunctions associated with the serial data line.