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, 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 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
The numbers below refer to the step numbers on the diagnostic table.
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A total module failure will set multiple communication codes.
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An open in the CAN high circuit between the splice and the ECM will affect all communications.
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An open in the CAN low circuit between the splice and the ECM will affect all communications.
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A short across the two CAN circuits anywhere in the system will cause total communications failure.
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A short to ground anywhere on the CAN high circuit will affect all CAN communications.
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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, or more than one module is communicating on the circuit, the frequency is 2,000-8,000 Hertz.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Engine Control Module Connector End Views or Engine Controls Connector End Views | ||||
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | |
Did any communication DTCs other than DTC U1800, U2104, or U2106 set? | -- | Go to Diagnostic Trouble Code (DTC) List in Data link Communications | Go to Step 3 | |
3 |
Does the DTC fail this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions |
Did the CAN high circuit test open? | -- | Go to Step 16 | Go to Step 5 | |
Test the CAN low circuit for an open between the splice and the ECM. Refer to Testing for Continuity in Wiring Systems. Did the CAN low circuit test open? | -- | Go to Step 17 | Go to Step 6 | |
Test for continuity between the CAN high and CAN low circuits. Refer to Testing for Continuity in Wiring Systems. Does continuity exist? | -- | Go to Step 15 | Go to Step 7 | |
Probe the CAN high circuit with a test lamp connected to battery voltage. Refer to Troubleshooting with a Test Lamp in Wiring Systems. Does the test lamp illuminate? | -- | Go to Step 14 | Go to Step 8 | |
8 |
Does the test lamp illuminate? | -- | Go to Step 19 | Go to Step 9 |
9 | Probe the CAN low circuit with a test lamp connected to a good ground. Does the test lamp illuminate? | -- | Go to Step 20 | Go to Step 10 |
Probe the CAN low circuit with a test lamp connected to battery voltage. Does the test lamp illuminate? | -- | Go to Step 18 | Go to Step 11 | |
11 |
Does the frequency measure more than the specified value? | 35,000 Hz | Go to Step 12 | Go to Step 22 |
12 |
Important: If the vehicle is not equipped with a TCM, answer YES to this step.
Does the frequency measure more than the specified value? | 2,000 Hz | Go to Step 13 | Go to Step 21 |
13 | Test for an intermittent or poor connections at the ECM. Repair as necessary. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | Go to Step 23 |
14 | Repair the short to ground in the CAN high circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | -- |
15 | Repair the short across the CAN high and CAN LO circuits. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | -- |
16 | Repair the open in the CAN high circuit between the ECM and the splice. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | -- |
17 | Repair the open in the CAN low circuit between the ECM and the splice. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | -- |
18 | Repair the short to ground in the CAN low circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | -- |
19 | Repair the short to voltage in the CAN high circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | -- |
20 | Repair the short to voltage in the CAN low circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 24 | -- |
21 | Replace the TCM. Refer to Transmission Control Module Replacement in Automatic Transmission - Allison. Did you complete the replacement? | -- | Go to Step 24 | -- |
22 | Replace the FICM. Refer to Fuel Injector Control Module Replacement . Did you complete the replacement? | -- | Go to Step 24 | -- |
23 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 24 | -- |
24 |
Does the DTC run and pass? | -- | Go to Step 25 | Go to Step 2 |
25 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
