990420002L

Bulletin No.:
Service Bulletin
Date:
99-04-20-002L
June, 2021
INFORMATION
Subject:
Information on Driveline Clunk Noise
Models:
2021 and Prior GM Passenger Cars and Trucks (including Medium Duty)
Attention: This bulletin also applies to any of the above models that may be Export from North
America vehicles.
This bulletin has been revised to add the 2021 Model Year. Please discard Corporate Bulletin
Number 99-04-20-002K.
Some customers of vehicles equipped with automatic
transmissions may comment that the vehicle exhibits a
clunk noise when shifting between Park and Drive,
Park and Reverse, or Drive and Reverse. Similarly,
customers of vehicles equipped with automatic or
manual transmissions may comment that the vehicle
exhibits a clunk noise while driving when the
accelerator is quickly depressed and then released.
Tipping into the throttle after deceleration can also
result in some level of clunk as the driveline is loaded in
one direction (coast) then with throttle reapply, the
driveline gets loaded in the opposite direction (drive).
On manual transmission vehicles, depressing the
clutch while in a deceleration immediately releases load
on the driveline and may produce a clunk noise as the
driveline unloads.
Note: Compare this complaint vehicle to a like vehicle.
If the results are the same, this is a normal condition.
For additional diagnostic information, refer to the
appropriate Service Information.
Whenever there are two or more gears interacting with
one another, there must be a certain amount of
clearance between those gears in order for the gears to
operate properly. This clearance or freeplay (also
known as lash) can translate into a clunk noise
whenever the gear is loaded and unloaded quickly, or
whenever the direction of rotation is reversed. The
more gears you have in a system, the more freeplay the
total system will have.
The clunk noise that owners sometimes hear may be
the result of a buildup of freeplay (lash) between the
components in the driveline.
For example, the potential for a driveline clunk would
be greater in a 4-wheel drive or all-wheel drive vehicle
than a 2-wheel drive vehicle. This is because in
addition to the freeplay from the rear axle gears, the
universal joints, and the transmission (common to both
vehicles), the 4-wheel drive transfer case gears (and
their associated clearances) add additional freeplay to
the driveline.
In service, dealers are discouraged from attempting to
repair driveline clunk conditions for the following
reasons:
• Comments of driveline clunk are almost never the
result of one individual component with excessive
lash, but rather the result of the added affect of
freeplay (or lash) present in all of the driveline
components.
Because all of the components in the driveline
have a certain amount of lash by design, changing
driveline components may not result in a
satisfactory lash reduction.
• While some owners may find the clunk noise
objectionable, this will not adversely affect
durability or performance.
• For additional diagnostic information, refer to the
appropriate Service Information.
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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