99-04-20-002I

File in Section:
Service Bulletin
Bulletin No.:
Date:
04 - Driveline/Axle
99-04-20-002I
November, 2014
INFORMATION
Subject:
Information on Driveline Clunk Noise
Models:
2015 and Prior GM Passenger Cars and Light Duty Trucks
This Bulletin has been revised to add the 2015 Model Year and remove the Important
statement. Please discard Corporate Bulletin Number 99-04-20-002H.
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.