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Begin the diagnosis of the body control system by performing the diagnostic system check for the system in which the customer concern is apparent.
The diagnostic system check will direct you to the correct procedure for diagnosing the system and where the procedure is located.
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Test Description
The numbers below refer to the step numbers on the diagnostic table
- 2 . 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
- 3 . 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.
- 4 . If only DTC U1800 and U2106 set, The problem is a short to voltage in the SAE J1939 Serial Data Bus+ circuit. This failure does not set DTC U2104
| Step | Action | Values | Yes | No |
|---|---|---|---|---|
| Schematic Reference: See appropriate Engine Controls schematic in SYSTEM WIRING DIAGRAMS | ||||
| 1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | Go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS | |
| 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 | |
| 3 | Did DTC U2104 also set | Go to DTC U2104 | Go to Step 4 | |
| 4 | Did only DTC U1800 and U2106 set? | Go to Step 10 | Go to Step 5 | |
| 5 | 1. Observe the Freeze Frame/Failure Records for this DTC. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Does the DTC fail this ignition? | Go to Step 8 | Go to TESTING FOR INTERMITTENT AND POOR CONNECTIONS | |
| 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 | 1. Turn ON the ignition with the engine OFF. 2. Probe the fuse supplying power to the FICM with a test lamp connected the a good ground. Does the test lamp illuminate? | Go to Step 13 | Go to IGNITION RELAY Diagnosis | |
| 8 | 1. Turn OFF the ignition. 2. Disconnect the ECM and FICM harness connectors. 3. Test the SAE J1939 Serial Data Bus+ circuit for an open between the CAN harness splice and the FICM. Did the resistance measure less than the specified value? | 5 | Go to Step 9 | Go to Step 17 |
| 9 | Test the SAE J1939 Serial Data Bus- circuit for an open between the CAN harness splice and the FICM. Did the resistance measure less than the specified value? | 5 | Go to Step 16 | Go to Step 18 |
| 10 | 1. Disconnect the ECM, FICM, and TCM if equipped. 2. Turn ON the ignition, with the engine OFF. 3. Probe the SAE J1939 Serial Data Bus+ circuit at the ECM harness connector with a test lamp connected to a good ground. Does the test lamp illuminate? | Go to Step 19 | Go to Step 11 | |
| 11 | 1. Turn OFF the ignition. 2. Reconnect the FICM harness connector. 3. Turn ON the ignition, with the engine OFF. 4. Probe the SAE J1939 Serial Data Bus+ circuit at the ECM harness connector with a test lamp connected to a good ground. 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. 1. Turn OFF the ignition. 2. Reconnect the TCM harness connector. 3. Turn ON the ignition, with the engine OFF. 4. Probe the SAE J1939 Serial Data Bus+ circuit at the ECM harness connector with a test lamp connected to a good ground. 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. 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. Did you find and correct the condition? | Go to Step 23 | Go to Step 16 | |
| 15 | 1. Disconnect the FICM. 2. Test the FICM ignition 1 voltage circuits for a short to ground and repair as necessary. 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 AND POOR CONNECTIONS . Did you find and correct the condition? | Go to Step 23 | Go to Step 20 | |
| 17 | Repair the open or high resistance in the SAE J1939 Serial Data Bus+ circuit between the FICM and the CAN splice. Did you complete the repair? | Go to Step 23 | ||
| 18 | Repair the open or high resistance in the SAE J1939 Serial Data Bus- circuit between the FICM and the CAN circuit splice. Did you complete the repair? | Go to Step 23 | ||
| 19 | Repair the short to voltage in the SAE J1939 Serial Data Bus+ circuit. Did you complete the repair? | Go to Step 23 | ||
| 20 | Replace the FICM. Refer to FUEL INJECTION CONTROL MODULE Replacement. Did you complete the replacement? | Go to Step 23 | ||
| 21 | Replace the TCM. Refer to TRANSMISSION CONTROL MODULE 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 | 1. Clear the DTCs with a scan tool. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? | Go to Step 1 | Go to Step 24 | |
| 24 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | Go to DTC INDEX | System OK | |
DTC U1800
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, or more than one module is communcating, the frequency is 2,000-8,000 Hertz
| Step | Action | Values | Yes | No |
|---|---|---|---|---|
| Schematic Reference: See appropriate Engine Controls schematic in SYSTEM WIRING DIAGRAMS | ||||
| 1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | Go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS | |
| 2 | Did any communication DTCs other than DTC U1800, U2104, or U2106 set | Go to DTC INDEX | Go to Step 3 | |
| 3 | 1. Observe the Freeze Frame/Failure Records for this DTC. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Does the DTC fail this ignition? | Go to Step 4 | Go to TESTING FOR INTERMITTENT AND POOR CONNECTIONS | |
| 4 | 1. Turn OFF the ignition. 2. Disconnect the ECM , TCM-if applicable, and FICM harness connectors that contain the CAN circuits. 3. Test the SAE J1939 Serial Data Bus+ circuit for an open between the splice and the ECM. Did the CAN high circuit test open? | Go to Step 16 | Go to Step 5 | |
| 5 | Test the SAE J1939 Serial Data Bus- circuit for an open between the splice and the ECM. Did the CAN low circuit test open? | Go to Step 17 | Go to Step 6 | |
| 6 | Test for continuity between the SAE J1939 Serial Data Bus+ and the SAE J1939 Serial Data Bus- circuits. Does continuity exist? | Go to Step 15 | Go to Step 7 | |
| 7 | Probe the SAE J1939 Serial Data Bus+ circuit with a test lamp connected to battery voltage. Does the test lamp illuminate? | Go to Step 14 | Go to Step 8 | |
| 8 | 1. Turn ON the ignition. 2. Probe the SAE J1939 Serial Data Bus+ circuit with a test lamp connected to a good ground. Does the test lamp illuminate? | Go to Step 19 | Go to Step 9 | |
| 9 | Probe the SAE J1939 Serial Data Bus- circuit with a test lamp connected to a good ground. Does the test lamp illuminate? | Go to Step 20 | Go to Step 10 | |
| 10 | Probe the SAE J1939 Serial Data Bus- circuit with a test lamp connected to battery voltage. Does the test lamp illuminate? | Go to Step 18 | Go to Step 11 | |
| 11 | 1. Reconnect the FICM. 2. Remove the connector pins for the CAN circuits from the ECM harness connectors. 3. Reconnect the ECM connectors. 4. Start the engine. 5. Measure the frequency across the CAN circuits in the harness with a DMM. 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. 1. Turn OFF the ignition. 2. Reconnect the TCM-if equipped. 3. Clear any DTCs that are in the memory. 4. Start the engine. 5. Measure the frequency across the CAN circuits in the harness with a DMM. 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 AND POOR CONNECTIONS . Did you complete the repair? | Go to Step 24 | Go to Step 23 | |
| 14 | Repair the short to ground in the SAE J1939 Serial Data Bus+ circuit. Did you complete the repair? | Go to Step 24 | ||
| 15 | Repair the short across the SAE J1939 Serial Data Bus+ and SAE J1939 Serial Data Bus- circuits. Did you complete the repair? | Go to Step 24 | ||
| 16 | Repair the open in the SAE J1939 Serial Data Bus+ circuit between the ECM and the splice. Did you complete the repair? | Go to Step 24 | ||
| 17 | Repair the open in the SAE J1939 Serial Data Bus- circuit between the ECM and the splice. Did you complete the repair? | Go to Step 24 | ||
| 18 | Repair the short to ground in the SAE J1939 Serial Data Bus- circuit. Did you complete the repair? | Go to Step 24 | ||
| 19 | Repair the short to voltage in the SAE J1939 Serial Data Bus+ circuit. Did you complete the repair? | Go to Step 24 | ||
| 20 | Repair the short to voltage in the SAE J1939 Serial Data Bus- circuit. Did you complete the repair? | Go to Step 24 | ||
| 21 | Replace the TCM. Refer to TRANSMISSION CONTROL MODULE - Allison. Did you complete the replacement? | Go to Step 24 | ||
| 22 | Replace the FICM. Refer to FUEL INJECTION 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 | 1. Clear the DTCs with a scan tool. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? | Go to Step 2 | Go to Step 25 | |
| 25 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | Go to DTC INDEX | System OK | |
DTC U2104
The numbers below refer to the step numbers on the diagnostic table
- 2 .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 affect the entire system
- 4 .This test is for an open in the SAE J1939 Serial Data Bus+ circuit that will only affect the TCM communications
| Step | Action | Values | Yes | No |
|---|---|---|---|---|
| Schematic Reference: See appropriate Engine Controls schematic in SYSTEM WIRING DIAGRAMS | ||||
| 1 | Did you perform the Diagnostic System Check-Engine Controls? | Go to Step 2 | Go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS | |
| 2 | Did DTC U2104 also set? | Go to DTC U2104 | Go to Step 3 | |
| 3 | 1. Observe the Freeze Frame/Failure Records for this DTC. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Does the DTC fail this ignition? | Go to Step 4 | Go to TESTING FOR INTERMITTENT AND POOR CONNECTIONS | |
| 4 | 1. Turn OFF the ignition. 2. Disconnect the ECM and TCM harness connectors. 3. Test the SAE J1939 Serial Data Bus+ circuit for an open between the CAN harness splice and the TCM. Did the resistance measure near the specified value? | 5 | Go to Step 5 | Go to Step 8 |
| 5 | Test the SAE J1939 Serial Data Bus- circuit for an open between the CAN harness splice and the TCM. Did the resistance measure near the specified value? | 5 | Go to Step 6 | Go to Step 9 |
| 6 | 1. Disconnect the FICM. 2. Turn ON the ignition. 3. Probe the SAE J1939 Serial Data Bus+ circuit with a test lamp connected to a good ground. Does the test lamp illuminate? | Go to Step 10 | Go to Step 7 | |
| 7 | Test for an intermittent or poor connections at the TCM. Repair as necessary. Refer to TESTING FOR INTERMITTENT AND POOR CONNECTIONS . Did you find and correct the condition? | Go to Step 13 | Go to Step 11 | |
| 8 | Repair the open in the SAE J1939 Serial Data Bus+ circuit between the TCM and the CAN splice. Did you complete the repair? | Go to Step 13 | ||
| 9 | Repair the open in the SAE J1939 Serial Data Bus- between the TCM and the CAN splice circuit. Did you complete the repair? | Go to Step 13 | ||
| 10 | Repair the short to voltage in the SAE J1939 Serial Data Bus+ circuit. Did you complete the repair? | Go to Step 13 | ||
| 11 | Test the following TCM circuits for an open or a short to ground: The battery positive voltage input circuits. The battery positive voltage output circuits. The ignition voltage input circuits. The ignition voltage output circuits. The switched battery positive voltage circuits. Repair as necessary. Did you find and correct the condition? | Go to Step 13 | Go to Step 12 | |
| 12 | Replace the transmission control module. Refer to TRANSMISSION CONTROL MODULE Allison Did you complete the replacement? | Go to Step 13 | ||
| 13 | 1. Clear the DTCs with a scan tool. 2. Turn OFF the ignition for 30 seconds. 3. Start the engine. 4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? | Go to Step 2 | Go to Step 14 | |
| 14 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | Go to DTC INDEX | System OK | |
DTC U2106
| IMPORTANT | Review the system operation in order to familiarize yourself with the system functions. Refer to Body Control System Description And Operation . |
The numbers below refer to the step numbers on the diagnostic table.
6. This step tests for battery voltage on the signal circuits that are not required.
7. This step tests for no battery voltage on the required signal circuits.
8. If any ignition switch parameters that should be inactive in the present ignition switch position are active, 2 ignition switch signal circuits may be shorted together.
9. This step eliminates open circuits as the cause of the malfunction.
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See also:
• DTC INDEX
• SERVICE PROGRAMMING SYSTEM (SPS)