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 is 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 for the ECM. An open in the CAN circuits of one module will not affect the other module. 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.
-
IF the engine does not start and the system sets DTC U1800, the fault lies in the FICM or the FICM power and ground circuits.
-
If the fault is a short to ground, a short together, or an open between the ECM and the CAN circuit splices, DTC U2104 will also set. These failures effect the entire system.
-
If only DTC U1800 and U2106 set, The problem is a short to voltage in the CAN high circuit. This failure does not set DTC U2104.
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 | |
|
Important: If DTC U1800 was set during a FRP relay output test with a scan tool, clear the DTC and do not follow this test. Attempt to start the engine. Does the engine start? | -- | Go to Step 3 | Go to Step 6 | |
Did DTC U2104 also set? | -- | Go to DTC U2104 | Go to Step 4 | |
Did only DTC U1800 and U2106 set? | -- | Go to Step 10 | Go to Step 5 | |
5 |
Does the DTC fail this ignition? | -- | Go to Step 8 | Go to Intermittent Conditions |
6 | Inspect the fuse supplying power to the FICM ignition 1 voltage circuits. Is the fuse open? | -- | Go to Step 15 | Go to Step 7 |
7 |
Does the test lamp illuminate? | -- | Go to Step 13 | Go to Ignition Relay Diagnosis |
8 |
Did the resistance measure less than the specified value? | 5 ohms | Go to Step 9 | Go to Step 17 |
9 | Test the CAN low circuit for an open between the CAN harness splice and the FICM. Refer to Testing for Continuity in Wiring Systems. Did the resistance measure less than the specified value? | 5 ohms | Go to Step 16 | Go to Step 18 |
10 |
Does the test lamp illuminate? | -- | Go to Step 19 | Go to Step 11 |
11 |
Does the test lamp illuminate? | -- | Go to Step 20 | Go to Step 12 |
12 |
Important: If the vehicle does not have an automatic transmission, answer this step NO.
Does the test lamp illuminate? | -- | Go to Step 21 | Go to Step 22 |
13 | Test the FICM ignition 1 voltage circuits between the fuse and the FICM for an open, and test the entire FICM ignition 1 voltage circuit for high resistance and repair as necessary. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems. Did you find and correct the condition | -- | Go to Step 23 | Go to Step 14 |
14 | Test the FICM ground circuits for an open or high resistance and repair as necessary. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 23 | Go to Step 16 |
15 |
Did you find and correct the condition? | -- | Go to Step 23 | Go to Step 20 |
16 | Test for an intermittent or poor connections at the FICM. Repair as necessary. Refer to Testing for Intermittent Conditions and Poor Connections and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 23 | Go to Step 20 |
17 | Repair the open or high resistance in the CAN high circuit between the FICM and the CAN splice. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 23 | -- |
18 | Repair the open or high resistance in the CAN low between the FICM and the CAN splice circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | Go to Step 23 | -- |
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 23 | -- |
20 | Replace the FICM. Refer to Fuel Injector Control Module Replacement . Did you complete the replacement? | -- | Go to Step 23 | -- |
21 | Replace the TCM. Refer to Transmission Control Module Replacement in Automatic Transmission - Allison. Did you complete the replacement? | -- | Go to Step 23 | -- |
22 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | Go to Step 23 | -- |
23 |
Does the DTC run and pass? | -- | Go to Step 24 | Go to Step 1 |
24 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
