Allison 1000 & 2000 Gen 5 Fault Code:U0074 CAN Communications BUS 2 Off

CIRCUIT DESCRIPTION:
This DTC sets when the TCM detects no communication on the CAN2 backbone harness for a period of time.

ACTION TAKEN WHEN THE DTC SETS:
1. The CHECK TRANS light is not illuminated.
2. DTC is stored in the TCM history.
3. SEM operation is not active, if applicable.
4. Default values are substituted for missing data, e.g., engine throttle percentage, engine torque, engine coolant temperature.
5. The TCM freezes shift adapts (DNA).

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The DTC automatically clears the DTC from the DTC history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
DTC U0074 indicates that a CAN2 bus hardware error has occurred. This code may be caused by:
1. Miswired or defective OEM controllers, OEM components, or OEM circuits attached to CAN1.
2. Defective TCM.

An active DTC U0074 may prevent the diagnostic tool from communicating with the TCM. The J-47276 TCM Reflash Harness may be used to confirm that the TCM is operational. Connect the J-47276 TCM Reflash Harness to the TCM and leave the OEM harness disconnected. Provide input power from the J-42455-A Load Box.

4. CAN1 and CAN2 devices must not be interconnected because they are not compatible with each other.

5. For proper datalink communications, it is necessary to have two 120 Ω resistors installed in parallel across the CAN2 High and CAN2 Low terminals. Resistance across CAN2 High and CAN2 Low should measure 60 Ω when wired correctly.

6. The diagnostic tool Data Bus Viewer may be selected from the File Menu to monitor or log SAE J 1939 datalink messages.

The OEM transmission warranty, or the extended coverage for parts and labor, does not cover the repair or replacement of OEM or bodybuilder provided components or any external wire harnesses not provided by the OEM.

Before repairing or replacing these components or circuits that caused a transmission issue, always contact the vehicle OEM call center and/or the vehicle OEM or bodybuilder service representative to verify the warranty coverage and proper payment arrangements, arrange an alternate sublet or repair plan (if needed), and get troubleshooting assistance for the repair (such as vehicle schematics, DTC trees, parts assistance, etc.).