U2104

CIRCUIT DESCRIPTION
The engine control module (ECM), fuel injection module (FICM), and the transmission control module (TCM) communicate control and diagnostic information via a SAE J1939 CAN communication bus. The ECM monitors CAN operational status by expecting a constant flow of messages from the FICM and the TCM. If the ECM fails to receive an expected message from one of the other modules, DTC U1800, U2104, or U2106 will set, depending on what communication is lost.

CONDITIONS FOR RUNNING THE DTC
The ignition is ON.

CONDITIONS FOR SETTING THE DTC
The CAN message from the FICM and TCM are not received by the ECM for 45 milliseconds.

ACTION TAKEN WHEN THE DTC SETS
- The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
- The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
- The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
- A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
- A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
- Clear the MIL and the DTC with a scan tool.

DIAGNOSTIC AIDS
The modules that communicate on the SAE J1939 CAN system are wired parallel to each other until the respective circuits are spliced together outside of the ECM. An open in the CAN circuits of one module will not affect the other modules. A short to ground or short to voltage affects all modules no matter where the failure occurs.

TEST DESCRIPTION

Steps 1-2:

Steps 3-6:

Steps 7-11:

Steps 12-15:

Steps 16-23:

Steps 24-25:

The numbers below refer to the step numbers on the diagnostic table.
2. A total module failure will set multiple communication codes.
4. An open in the SAE J1939 Serial Data Bus+ circuit between the splice and the ECM will affect all communications.
5. An open in the SAE J1939 Serial Data Bus- circuit between the splice and the ECM will affect all communications.
6. A short across the two CAN circuits anywhere in the system will cause total communications failure.
7. A short to ground anywhere on the SAE J1939 Serial Data Bus+ circuit will affect all CAN communications.
10. This test isolates a shorted FICM from a shorted TCM or ECM. By reading the signal from the FICM after the wiring has been tested, without a TCM connected, an FICM signal generating problem can be diagnosed. When the CAN circuits are open between two components, both components generate a 30,000-40,000 Hertz signal. When the system is not open, the frequency is 3,000-3,500 Hertz.