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

SEOCONTENT-START

Service Bulletin Bulletin No.: 21-NA-120
Date: May, 2021
TECHNICAL
Subject: Diagnostic Tip for Noise, Vibration, and/or Harshness Issues/Clunk Noise Heard
During Coast Down Event
Brand: Model:
Model Year: VIN:
Engine: Transmission:
from to from to
Chevrolet Silverado 2500HD/
3500HD
2020 2021 L5P MGM, MGU
GMC Sierra 2500HD/
3500HD
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
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 May, 2021 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 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.
Version 1
Modified Released May 24, 2021
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

SEOCONTENT-END