Transmission Tips
Rough gear movements may be attributed to driver operation.
Some owners of Chevrolet or GMC trucks or SUVs may point out rough shifting, surging, jerking or lurching – particularly when accelerating from a stop out of 4LO.

The concern, which has been reported on 2014 to 2020 models, often leads to the illumination of the Malfunction Indicator Lamp or to a Service 4WD message in the Driver Information Centre (DIC).  
When servicing the vehicles, technicians may find the following DTCs set in the Engine Control Module (ECM):
  • P17D4: Transfer Case Neutral Range Detected in High or Low Range
  • P215B: Vehicle Speed – Output Shaft Speed Not Plausible
  • P279A: Transfer Case High Range Incorrect Ratio
  • P279B: Transfer Case Low Range Incorrect Ratio

The condition may be caused by the customer improperly shifting the transmission out of neutral or applying the gas pedal before the transfer case shifts completely out of 4LO. Direct customers to the owner’s manual, which provides the correct procedure for shifting out of 4LO.
Technicians should follow the correct repair procedure, which is as follows:

  1. Engage Ignition On
  2. Engage Park Brake
  3. Engage Transmission Neutral
  4. Clear DTCs with Scan tool
  5. Switch vehicle to power mode OFF and wait for 2 minutes to allow for the BUS and ECM to sleep
  6. Start vehicle
  7. Confirm no codes or Service 4x4 message on DIC is present
  8. Shift Transfer Case through 2HI, 4HI, 4LO, N and back to 2HI to confirm the Transfer Case Range shift is functioning properly
If the transfer case cannot shift through all the modes noted above, or if there are any abnormal conditions present, perform the diagnostics within Service Information (SI). Note that shifting out of neutral with the engine running may cause a grinding sound or a loud bang. It also may not shift if the transmission output shaft speed is too high due to clutch drag. These are all normal if the engine is running.
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