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.

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