Circuit Description
The engine control module (ECM), electronic brake control module (EBCM), and the transmission control module (TCM) communicate control and diagnostic information via a controller area network (CAN) communication bus. The ECM monitors CAN operational status by expecting a constant flow of messages from the EBCM and the TCM. If the ECM fails to receive an expected message from one of the other modules, DTC U2104, U2106, or U2108 will set, depending on what communication is lost.
Conditions for Running the DTC
| • | The ignition is ON. |
| • | The module supply voltage is in the normal operating range of 9-16 volts. |
Conditions for Setting the DTC
The CAN message from the EBCM and TCM are not received by the ECM for 200 milliseconds.
Action Taken When the DTC Sets
The control module will not illuminate the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
Conditions for Clearing the MIL/DTC
| • | A current DTC, Last Test Failed, clears when the diagnostic runs and passes. |
| • | Clear 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 1 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.
-
A total module failure will set multiple communication codes.
-
An open in the CAN high circuit between the splice and the ECM will affect all communications.
-
An open in the CAN low circuit between the splice and the ECM will affect all communications.
-
A short across the 2 CAN circuits anywhere in the system will cause total communications failure.
-
A short to ground anywhere on the CAN high circuit will affect all CAN communications.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics Connector End View Reference: Data Link Communications Connector End Views | ||||
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls in Engine Controls - 7.8L |
Did any communication DTCs other than DTC U2104, U2106, or U2108 set? | -- | Go to Step 3 | ||
3 |
Does the DTC fail this ignition? | -- | Go to Step 4 | Go to Intermittent Conditions in Engine Controls - 7.8L |
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 |
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? | 35,000 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 low 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 EBCM. Refer to Electronic Brake Control Module Replacement in Air Antilock Brakes or Electronic Brake Control Module Replacement in Antilock Brakes. Did you complete the replacement? | -- | Go to Step 24 | -- |
23 | Replace the ECM. Refer to Engine Control Module Replacement/Restore Fuel Rate Data in Engine Controls - 7.8L. 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 | Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK | |
