Circuit Description
The engine control module (ECM), electronic brake control module (EBCM) if applicable, fuel injector control module (FICM) if applicable, and transmission control module (TCM) if applicable, communicate via a controller area network (CAN) serial data bus. Modules connected to the CAN serial data circuits monitor communications. During normal vehicle operation, information and commands are exchanged among modules. If the ECM fails to receive an expected message from one of the other modules, DTC U1800, U2106, or U2108 will set; depending on what communication is lost. If no communication is detected on the CAN serial data bus, a DTC U2104 is set. A no communication condition may result from a short to voltage, short to ground, or short between the CAN high and low circuits.
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
A no communication condition is detected on CAN serial data circuits.
Action Taken When the DTC Sets
| • | The module uses a default value for the missing parameter. |
| • | The malfunction indicator lamp (MIL) illumination depends on DTC type. For a detailed description about DTC types refer to Diagnostic Trouble Code (DTC) Types in Engine Controls - 6.6L (LB7). For information about DTC type definitions refer to Diagnostic Trouble Code (DTC) Type Definitions in Engine Controls - 6.6L (LB7). |
Conditions for Clearing the MIL/DTC
MIL or DTC clearing depends on DTC type. For a detailed description about DTC types refer to Diagnostic Trouble Code (DTC) Types in Engine Controls - 6.6L (LLY). For information about DTC type definitions refer to Diagnostic Trouble Code (DTC) Type Definitions in Engine Controls - 6.6L (LLY).
| • | DTC type C |
| • | A current DTC and MIL clear when the malfunction is no longer present. |
| • | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction. |
Diagnostic Aids
The modules communicating on the CAN serial data circuits are wired parallel to each other until the respective circuits are spliced together outside of the ECM. An open in the CAN circuits for one module will not affect other modules. A short to ground or short to voltage affects all modules no matter where the failure occurs.
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Data Link Connector Schematics Connector End View Reference: Data Link Communications Connector End Views | ||||
1 |
Does the DTC fail this ignition? | -- | Go to Step 2 | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems |
2 |
Important: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use DMM MIN/MAX function to compare intermittent conditions.
Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 3 |
|
Important: If the vehicle is not equipped with an FICM, answer YES to this step.
Does the frequency measure more than the specified value? | 35,000 Hz | Go to Step 4 | Go to Step 7 | |
4 |
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 5 | Go to Step 8 |
5 |
Important: If the vehicle is not equipped with an EBCM, answer YES to this step.
Does the frequency measure more than the specified value? | 35,000 Hz | Go to Step 6 | Go to Step 9 |
6 | Inspect for an intermittent or poor connection at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 10 |
7 | Inspect for an intermittent or poor connection at the FICM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 11 |
8 | Inspect for an intermittent or poor connection at the TCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 12 |
9 | Inspect for an intermittent or poor connection at the EBCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 14 | Go to Step 13 |
10 | Perform the module set up procedure, if required. Replace the ECM. Refer to Engine Control Module Replacement in Engine Controls - 6.6L (LB7). Did you complete the replacement? | -- | Go to Step 14 | -- |
11 | Perform the module set up procedure, if required. Replace the FICM. Refer to Fuel Injector Control Module Replacement in Engine Controls - 6.6L (LB7). Did you complete the replacement? | -- | Go to Step 14 | -- |
12 | Perform the module set up procedure, if required. Replace the TCM. Refer to Transmission Control Module Replacement in Automatic Transmission - Allison. Did you complete the replacement? | -- | Go to Step 14 | -- |
13 | Perform the module set up procedure, if required. 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 14 | -- |
14 |
Did the DTC fail this ignition? | -- | Go to Step 2 | Go to Step 15 |
15 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | -- | System OK | |
