Noise, Vibration, and/or Harshness Issues/Clunk Noise Heard During Coast Down Event – 2020-2021 Chevrolet Silverado & GMC Sierra 2500HD/3500HD

NHTSA ID Number: 10198888

Manufacturer Communication Number: 21NA120



This technical bulletin provides the tech with a procedure to check if the transmission is the cause of the concern by checking the data.



4 Affected Products


GMC SIERRA 2500 2020-2021



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



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.



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 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.




1 Associated Document

Manufacturer Communications

Bulletin No.: 21-NA-120
Date: May, 2021

MC-10198888-9999.pdf 1684.294KB




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Last update on 2023-05-13 / Affiliate links / Images from Amazon Product Advertising API

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