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DTC U1849

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    • Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC U1849 00: Drive Motor Control Module 1 Lost Communication with Transmission Control Module

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

B+

U1849*

U1849*

--

--

Ignition

U1839*

U1849*

--

--

Accessory Wake Up Serial Data 2

P06E4*

U1849*

--

--

High Speed GMLAN Serial Data (+)

U0073, 1

U0073, U1849*

U0073, 1

--

High Speed GMLAN Serial Data (-)

U0073, 1

U0073, U1849*

U0073, 1

--

Ground

--

U1849*

--

--

* Other modules/DTCs may set with this fault.

1. Scan Tool Does Not Communicate With High-Speed GMLAN Device.

Circuit/System Description

Control modules connected to the serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the control modules. The control modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module.

Conditions for Running the DTC

    • The system voltage is at least 10 V.
    • The vehicle power mode master requires serial data communication to occur from this specific control module.

Conditions for Setting the DTC

A supervised periodic message that includes the transmitter module availability has not been received.

Action Taken When the DTC Sets

The DTC U1849 is a type B DTC.

Conditions for Clearing the DTC

The DTC U1849 is a type B DTC.

Diagnostic Aids

    • Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
       - A control module on the data communication circuit was disconnected while the communication circuit is awake.
       - Power to one or more control modules was interrupted during diagnosis.
       - A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
       - Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialise at the same time.
       - If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
    • A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U-code.
    • Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.
    • Communication may be available between the control module and the scan tool with one or more GMLAN serial data systems inoperative. This condition is due to the control module using multiple GMLAN busses.
    • Use Data Link References to determine what GMLAN serial data communications the non communicating control module uses.
    • The control module may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and the B+ or ignition circuit is not shorted to earth, ensure none of the control circuits are shorted to earth before replacing the non communicating control module.

Reference Information

Schematic Reference

    •  Data Communication Schematics
    •  Control Module References

Connector End View Reference

Component Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

  1. Verify that DTC U0101 is not set.
  2. If the DTC is set, refer to DTC U0100-U02FF .
  3. Verify that DTC U0073, P06E4, U1814, U2099, B1428, B1370, B1380, B1440, B1441, B1325, or P0562 is not set.
  4. If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle .

Circuit/System Testing

    Note: Use the schematic to identify the following:

       • Control module location on the high-speed GMLAN serial data circuits
       • The control module B+, ignition, earth, accessory wake-up serial data 2 and serial data circuit terminals

  1. Vehicle OFF, disconnect harness connector at the K71 transmission control module (TCM).
  2. Test for less than 10 Ω between each ground circuit terminal and ground.
  3. If greater than the specified value, test the ground circuit for an open/high resistance.
  4. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between each B+ circuit terminal and earth.
  5. If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K71 TCM for a short-circuit to earth. If the circuits test normal, replace the K71 TCM.
  6. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between each ignition circuit terminal and earth.
  7. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K71 TCM for a short-circuit to earth. If the circuits test normal, replace the K71 TCM.
  8. If equipped, vehicle in Service Mode, verify that a test lamp illuminates between the accessory wake-up serial data 2 circuit terminal and earth.
  9. If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K114B hybrid powertrain control module 2, for an open-circuit/high resistance or short-circuit to earth. If the circuits test normal, replace the K114B hybrid powertrain control module 2.
  10. Vehicle OFF, disconnect the harness connector at the T6 power inverter module.
  11. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 2 Ω in each high-speed GMLAN serial data circuit between the K71 TCM and the T6 power inverter module.
  12. If greater than the specified range, test the serial data circuit for an open-circuit/high resistance between the K71 TCM and the T6 power inverter module.
  13. If all circuits test normal, replace the K71 TCM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  GMLAN Wiring Repairs
    • Control Module References for control module replacement, programming and setup
   


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