Circuit Description
The Class 2 Serial Data Circuit is used to communicate between systems. Each system connected to the Class 2 Serial Data circuit is assigned its own recognition code (ID/address). This code is used to identify which module or systems are communicating. The Node Alive or State of Health (SOH) messages are broadcast on the serial data circuit every two seconds. If the VCM is monitoring Node Alive messages, it will reset its 5 second timer when it receives the message from the Active Transfer Case (ATC) Control Module. Any system that cannot communicate properly will also store the appropriate communication DTCs that are assigned to the system(s) it could not communicate with.
Conditions for Running the DTC
The VCM has established communications and received Node Alive/SOH messages from the ATC Control Module.
Conditions for Setting the DTC
During the same ignition cycle, the VCM has lost communications and/or has not received Node Alive/SOH messages from the ATC Control Module for 5 continuous seconds after it had been previously established.
Action Taken When the DTC Sets
| • | A DTC U1026 is stored in the VCM memory |
| • | The VCM will not illuminate the MIL |
Conditions for Clearing the MIL/DTC
| • | A history DTC will clear after 40 consecutive ignition cycles if the condition for the malfunction is no longer present |
| • | The DTCs can be cleared using a scan tool |
Diagnostic Aids
Important: Do not clear the DTCs unless directed by a diagnostic procedure. Clearing the DTCs will also clear valuable Freeze Frame and Failure Records information.
| • | Check for published service bulletins relating to the exhibited symptoms or component operation. |
| • | Inspect all related wiring and connections, including the connections at the VCM and the ATC Control Module. These connections may cause an intermittent loss of communication. |
| • | Thoroughly check any circuit that can cause an intermittent complaint for the listed items: |
| - | Backed out terminals |
| - | Improper mating of connectors |
| - | Broken connector locks |
| - | Improperly formed or damaged terminals |
| - | Poor terminal to wiring connections |
| - | Physical damage to the wiring harness |
| - | Corroded terminals or connections |
| - | Moisture in the connector |
| • | If the Class 2 Serial Data circuit is shorted to ground or shorted to voltage, all the modules or systems connected to the Class 2 Serial Data circuit will not be able to communicate properly. The systems or modules that are capable of storing loss of communication DTCs (DTCs with the letter U as a prefix) will have these codes in their memory. If multiple history U codes are stored in all the modules, the likely cause is a fault in the Class 2 circuit. If the same U code appears in all the modules, then the module identified by the code may not be able to communicate or has an open in the Class 2 circuit leading to the module. If one module has multiple history U codes, that module may not be able to communicate intermittently. The ATC Control Module must also be checked for intermittent operation due to a loss of power or ground to the module itself. After repairing the cause of a DTC U1026, clear all DTCs from the systems capable of storing this DTC. |
| • | The scan tool's Diagnostic Circuit Check can be used to: |
| - | Monitor the Class 2 Serial Data circuit for modules that are or have been communicating. |
| - | Monitor the loss of communications DTCs (DTCs with the letter U as a prefix: U1001-U1199). |
| - | Clear the loss of communication DTCs. When a Clear Codes command is issued, all codes, Freeze Frame and Failure Records information is cleared. |
| • | The scan tool's Class 2 Message Monitor will show the status of each module on the Class 2 circuit . A status of Active indicates that the module is communicating with the scan tool. An Inactive status indicates that the module previously communicated with the scan tool, but is not communicating currently. The inactive status will only appear if the loss of communication occurs while on the Message Monitor screen. If a module is not listed at all, the module never successfully established communications with the scan tool or lost communication while on another screen. Refer to Diagnostic System Check - Data Link Communications in Wiring Systems for the complete Class 2 data circuit to determine if there are any unlisted modules. |
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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This test checks if communication can be established between the scan tool and the ATC Control Module. If the scan tool cannot communicate with the ATC Control Module, refer to Transfer Case Diagnostic System Check in Transfer Case (2 Speed Automatic)
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This test checks for any opens in the Class 2 Serial Data circuit between the VCM and the ATC Control Module. It also checks for any intermittent malfunctions associated with the serial data circuit.
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Use a scan tool to clear and check for any DTCs in all the modules connected to the Class 2 Serial Data circuit (VCM, ATC, and EBCM).
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 |
Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and the Failure Records for reference. This data will be lost when the Clear DTC Information function is used. Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | ||||||||||
Does the scan tool communicate with the ATC Control Module? | -- | Go to Transfer Case Diagnostic System Check in Transfer Case (2 Speed Automatic) | ||||||||||
Check the Class 2 Serial Data circuit for the following intermittent conditions:
Did you find a problem? | -- | |||||||||||
4 |
Does DTC U1026 reset? | -- | Go to Transfer Case Diagnostic System Check in Transfer Case (2 Speed Automatic) | |||||||||
5 | This DTC is intermittent. Are any additional DTCs stored? | -- | Go to the applicable DTC table | Go to Diagnostic Aids | ||||||||
6 | Repair the circuit as necessary. Refer to Wiring Repairs or Connector Repairs in Wiring Systems. Is the action complete? | -- | -- | |||||||||
Are any DTCs displayed? | -- | System OK |
