21-NA-120
Service Bulletin Details
Public Details for: 21-NA-120
This service bulletin provides a diagnostic tip for noise, vibration, and/or harshness issues/clunk noise heard during coast down event after the transfer case was removed from the transmission and reinstalled during a recent repair.
- 2024 - 2023 - 2022 - 2021 - 2020 -
Models from 2024
2024 CHEVROLET SILVERADO 2500 |
2024 CHEVROLET SILVERADO 3500 |
2024 GMC SIERRA 2500 |
2024 GMC SIERRA 3500 |
Models from 2023
2023 CHEVROLET SILVERADO 2500 |
2023 CHEVROLET SILVERADO 3500 |
2023 GMC SIERRA 2500 |
2023 GMC SIERRA 3500 |
Models from 2022
2022 CHEVROLET SILVERADO 2500 |
2022 CHEVROLET SILVERADO 3500 |
2022 GMC SIERRA 2500 |
2022 GMC SIERRA 3500 |
Models from 2021
2021 CHEVROLET SILVERADO 2500 |
2021 CHEVROLET SILVERADO 3500 |
2021 GMC SIERRA 2500 |
2021 GMC SIERRA 3500 |
Models from 2020
2020 CHEVROLET SILVERADO 2500 |
2020 CHEVROLET SILVERADO 3500 |
2020 GMC SIERRA 2500 |
2020 GMC SIERRA 3500 |
Bulletin No.: Service Bulletin Date: 21-NA-120 January, 2024 TECHNICAL Subject: Diagnostic Tip for Noise, Vibration and/or Harshness Issues/Clunk Noise Heard During Coast Down Event Brand: Model: Chevrolet Silverado 2500HD/ 3500HD GMC Sierra 2500HD/ 3500HD Model Year: from to 2020 2024 VIN: from to Engine: Transmission: L5P, L8T MGM, MGU, MKM Involved Region or Country North America, Middle East, Israel Additional Options (RPOs) NQF (TRANSFER CASE-ELECTRIC SHIFT CONT, TWO SPEED, ALUM) NQH (TRANSFER CASE-ACTIVE, TWO SPEED, SWITCH ACTIVATED, ALUM) Condition Some technicians or customers may comment on one or more of the following conditions after the transfer case was removed from the transmission and reinstalled during a recent repair: • Harsh downshift during a coast down event • Noise during a coast down event • Vibration during a coast down event • Harshness issues such as a clunk noise heard during coast down event Cause Copyright 2024 General Motors LLC. All Rights Reserved. This condition may be caused by a misalignment between the transmission output shaft and the transfer case input. ⇒ Transfer case to transmission misalignment could cause the transfer case shift collar to slip out of engagement. Page 2 January, 2024 Bulletin No.: 21-NA-120 Correction 5411622 1. Connect the GDS Scan Tool and monitor transmission shift data. 2. Drive the vehicle and duplicate the condition while monitoring transmission shift data to determine if the noise/clunk/roughness occurs during the downshift or if the noise/clunk/roughness occurs randomly during the de-acceleration event. ⇒ If it is determined that the noise/clunk/roughness does not occur at a transmission downshift, shift point follow published Service Information for Transfer Case Assembly Replacement (with MGM, MGU, MKM Automatic Transmission). Note: The transfer case does not need to be completely removed from the vehicle. The attaching bolts must be loosened (not removed) to allow the transfer case to be separated from the transmission a minimum of a ½ (13mm) but no more than 1 inch (25mm). The use of transmission jack or adjustable jack stand may provide support in holding the transfer case to the transmission. Proper transfer case to transmission bolt installation, tightening sequence and proper torque must be followed. If the noise/clunk/roughness is occurring during a transmission downshift, shift point, refer to SI for further diagnostics. Important: Service agents must comply with all International, Federal, State, Provincial, and/or Local laws applicable to the activities it performs under this bulletin, including but not limited to handling, deploying, preparing, classifying, packaging, marking, labeling, and shipping dangerous goods. In the event of a conflict between the procedures set forth in this bulletin and the laws that apply to your dealership, you must follow those applicable laws. Version 4 Modified Released May 24, 2021 Revised November 17, 2022 - Added 2022 and 2023 Model Years. Revised September 19, 2023 - Added 2024 Model Year. Revised January 17, 2024 - Added L8T to Engine RPO section. GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information. WE SUPPORT VOLUNTARY TECHNICIAN CERTIFICATION