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 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, Diagnostic Trouble Code (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 ms.

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.
^ Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS
The modules that communicate on the CAN system are wired parallel to each other until their circuits are spliced together outside of the ECM. An open in the CAN circuits of one module will not effect the other module(s). A short to ground or short to voltage effects all modules no matter where the failure occurs.

TEST DESCRIPTION

Steps 1-11:

Steps 12-25:

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 CAN high circuit between the splice and the ECM will effect all communications.
5. An open in the CAN low circuit between the splice and the ECM will effect 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 CAN high circuit will effect 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 Hz signal. When the system is not open, the frequency is 3,000-3,500 Hz.