Jeep Grand Cherokee WK. Manual — part 1452
U0121-LOST COMMUNICATION WITH ANTI-LOCK BRAKE MODULE
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AUTOMATIC TRANSMISSION NAG1 - ELECTRICAL DIAGNOSTICS
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U0121-LOST COMMUNICATION WITH ANTI-LOCK BRAKE MODULE (CONTINUED)
For the Transmission circuit diagram (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - NAG1 - SCHE-
MATICS AND DIAGRAMS)
For a complete wiring diagram Refer to Section 8W
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.
Possible Causes
TRANSMISSION CONTROL MODULE
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 21 - TRANSMIS-
SION/TRANSAXLE/AUTOMATIC - NAG1 - STANDARD 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 8-ELECTRICAL - ELECTRONIC CONTROL MODULES - ELECTRICAL DIAGNOSTICS cate-
gory and perform the appropriate symptom. Perform the diagnostic procedure for U0001-CAN C BUS
first if it is present.
Perform NAG1 TRANSMISSION VERIFICATION TEST - VER 1.
No
>> Go To 2
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AUTOMATIC TRANSMISSION NAG1 - ELECTRICAL DIAGNOSTICS
21 - 619
U0121-LOST COMMUNICATION WITH ANTI-LOCK BRAKE MODULE (CONTINUED)
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 dupli-
cating 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) pins, terminals, and
connectors for corrosion, damage, or terminal push out. Pay particular attention to all power and ground
circuits. If no problems are found, replace the TCM per the Service Information. Refer to 8 - ELECTRI-
CAL/ELECTRONIC CONTROL MODULES/TRANSMISSION CONTROL MODULE for the appropriate
service procedure.
Perform NAG1 TRANSMISSION VERIFICATION TEST - VER 1.
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 - VER 1.
No
>> Test Complete.
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AUTOMATIC TRANSMISSION NAG1 - ELECTRICAL DIAGNOSTICS
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U0141-LOST COMMUNICATION WITH FRONT CONTROL MODULE
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AUTOMATIC TRANSMISSION NAG1 - ELECTRICAL DIAGNOSTICS
21 - 621
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