Transmission Control Module (TCM) - 8HP45

U0121-LOST COMMUNICATION WITH ANTI-LOCK BRAKE SYSTEM (ABS) CONTROL MODULE

For a complete wiring diagram Refer to the Wiring Information.

Theory of Operation

The purpose of the diagnostic is to monitor the CAN BUS and the validity of the CAN signals. Several control units are connected to the CAN BUS which send their messages cyclically in a defined time window. The messages contain multiple signals representing vehicle information.

- When Monitored:
Continuously with the ignition on and voltage within a valid range.

No failure in CAN BUS monitoring.

No failure in CAN BUS or message monitoring.

- Set Condition:
A message was not received within its expected time frame.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. 8HP45 Pre-Diagnostic Troubleshooting Procedure

1. VERIFY DTC IS ACTIVE

NOTE: Make sure the battery voltage is between 10 and 16 volts before proceeding.

1. With the scan tool, read active DTCs.

Is the DTC active?

Yes

- Go To 2

No

- Perform the Stored Lost Communication DTCs diagnostic procedure. Stored Lost Communication DTCs

2. CHECK FOR ANY OF THE FOLLOWING ACTIVE DTCS
1. With the scan tool, read all active DTCs from all modules.

Does the scan tool display any active DTCs to the conditions listed above?

Yes

- Perform the appropriate diagnostic procedure.

No

- Go To 3

3. VERIFY THE ABS IS ACTIVE ON THE BUS
1. Turn the ignition ON.
2. With the scan tool, verify the ABS is active on the BUS.

Is the ABS active on the BUS?

Yes

- Go To 4

No

- Perform the NO RESPONSE FROM ABS diagnostic procedure. No Response From ABS (Antilock Brake Module)

4. CHECK FOR ADDITIONAL COMMUNICATION RELATED DTCS
1. With the scan tool, read all the DTCs.

Is there more than one module with active DTCs "Logged Against" the ABS?

Yes

- Replace and program the ABS module. Anti-Lock Brake System Module - Removal.
- Perform the ABS VERIFICATION TEST. ABS Verification Test.

No

- Go To 5

5. CLEAR DTC IN MODULE SETTING FAULT
1. With the scan tool, select the module setting the DTC against the ABS.
2. Clear active DTCs.

Is this DTC still active?

Yes

- Replace and program the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.

No

- Go To 6

6. CYCLE IGNITION
1. Cycle the ignition from RUN to OFF three times ending with the key in the RUN position.

Does the DTC become active?

Yes

- Go To 7

No

- The condition is not present at this time. Using the wiring diagram/schematic as a guide, inspect the wiring for chafed, pierced, pinched and partially broken wires and the wiring harness connectors for broken, bent, pushed out and corroded terminals.
- Perform the appropriate verification test for the module. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.

7. CHECK FOR ADDITIONAL LOST COMMUNICATION FAULTS
1. With the scan tool, view the modules.

Does the BCM and other CAN C BUS modules show Lost Communication with the remaining CAN C BUS modules?

Yes

- Go To 8

No

- Replace and program the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.

8. CHECK THE CAN C BUS (+) CIRCUIT FOR A SHORT TO VOLTAGE
1. Measure the voltage between the CAN C BUS (+) circuit and ground.

Is the voltage above 5.0 volts?

Yes

- Repair the short to voltage in the CAN C BUS (+) circuit. The short to voltage is present in one of the CAN C BUS circuits. Use the Star Connectors to isolate portions of the CAN C BUS in order to locate the concern.
- Perform the BODY VERIFICATION TEST. Body Verification Test.

No

- Go To 9

9. CHECK THE CAN C BUS (-) CIRCUIT FOR A SHORT TO GROUND
1. Turn the ignition OFF.
2. Measure the resistance between ground and the CAN C BUS (-) circuit.

Is the resistance below 10k Ohms?

Yes

- Repair the short to ground in the CAN C BUS (-) circuit. The short to ground is present in one of the CAN C BUS circuits. Use the Star Connectors to isolate portions of the CAN C BUS in order to locate the concern.
- Perform the BODY VERIFICATION TEST. Body Verification Test.

No

- Replace and program the module that set this DTC.
- Perform the appropriate verification test for the module being replaced. If there is no verification test for the associated module perform the BODY VERIFICATION TEST. Body Verification Test.