NAG1 Automatic Transmission

U0121-LOST COMMUNICATION WITH ANTI-LOCK BRAKE MODULE

For a complete wiring diagram Refer to Diagrams/Electrical Electrical Diagrams


Theory of Operation


The Transmission Control Module (TCM) receives information from the Engine, Anti-lock Brake System, and the Electronic Gear Shift control modules over the CAN C bus. The CAN C bus is a high speed communication bus that allows real time control capability between various controllers. Most messages are sent every 20 mSec, this means critical information can be shared between multiple controllers almost instantaneously. The CAN C bus is a two wire bus with a CAN (+) and a CAN (-) circuit. To reduce the potential of radio and other electrical noise interference, the CAN Bus circuit wiring is manufactured using a twisted pair configuration within the electrical wiring harness.


- When Monitored:

Continuously with the ignition on.

- Set Condition:

The DTC will set if a valid CAN ID was not received in the required time from the ABS.







Always perform the NAG1 Pre-Diagnostic Troubleshooting procedure before proceeding. NAG1 - Pre-Diagnostic Troubleshooting Procedure


Diagnostic Test


1. CHECK THE FCM FOR CAN C BUS COMMUNICATION DTCS

With the scan tool, check for FCM DTCs.

Does the FCM report the DTC U0001-CAN C BUS DTC or U0121-LOST COMMUNICATION WITH ANTI-LOCK BRAKE MODULE?

Yes

- Refer to Communications and perform the appropriate symptom. Perform the diagnostic procedure for U0001-CAN C BUS first if it is present. U0001
- Perform NAG1 TRANSMISSION VERIFICATION TEST. NAG1 Transmission Verification Test

No

- Go To 2


2. CHECK IF THE DTC IS CURRENT


NOTE: When the Transmission Control Module (TCM) detects and stores a DTC, the TCM also stores the vehicles operating conditions under which the DTC originally set. This information is located in the scan tool under Environmental Data. Before erasing any stored DTCs, record any available data to assist in duplicating the conditions in which the DTC originally set.

Ignition on, engine not running.

With the scan tool, erase Transmission DTC's.

Using the Environmental Data recorded earlier, try to duplicate the conditions in which the DTC originally set. It may be necessary to test drive the vehicle.

With the scan tool, read Transmission DTC's.

Did the DTC U0121-LOST COMMUNICATION WITH ANTILOCK BRAKE MODULE reset?

Yes

- Using the schematics as a guide, check the Transmission Control Module (TCM) terminals for corrosion, damage, or terminal push out. Pay particular attention to all power and ground circuits. If no problems are found, replace the TCM.
- Perform NAG1 TRANSMISSION VERIFICATION TEST. NAG1 Transmission Verification Test

No

- Go To 3


3. INTERMITTENT WIRING AND CONNECTORS

The conditions necessary to set this DTC are not present at this time.

Using the schematics as a guide, inspect the wiring and connectors specific to this circuit.

Wiggle the wiring and connectors while checking for any possible open or shorted circuits.

Check for any Service Information Tune-ups or Technical Service Bulletins that may apply.

Were there any problems found?

Yes

- Repair as necessary.
- Perform NAG1 TRANSMISSION VERIFICATION TEST. NAG1 Transmission Verification Test

No

- Test Complete.