File in Section:
Bulletin No.:

Service Bulletin

Date:

07 - Transmission/Transaxle
03-07-29-004K
April, 2016

INFORMATION
Subject:

Manual Transmission Operating Characteristics

Models:

2017 and Prior GM Passenger Cars and Light Duty Trucks
2009 and Prior Chevrolet and GMC Medium Duty Trucks
Equipped with Manual Transmission

Attention: This Bulletin also applies to any of the above models that may be Export vehicles.
This Bulletin has been revised to add the 2017 Model Year and update the Skip Shift section.
Please discard Corporate Bulletin Number 03-07-29-004J.

Important: Even though this bulletin attempts to cover
operating characteristics of manual transmissions, it
cannot be all inclusive. Be sure to compare any
questionable concerns to a similar vehicle and if
possible, with similar mileage.
The purpose of this bulletin is to assist in identifying
characteristics of manual transmissions that repair
attempts will not change. The following are
explanations and examples of conditions that will
generally occur in all manual transmissions. All noises
will vary between transmissions due to build variation,
type of transmission (usually the more heavy duty, the
more noise), type of flywheel and clutch, level of
insulation, etc.
Basic Information
Many transmission noises are created by the firing
pulses of the engine. Each firing pulse creates a
sudden change in angular acceleration at the
crankshaft. These changes in speed can be reduced
with clutch damper springs and dual mass flywheels.
However, some speed variation will make it through to
the transmission. This can create noise as the various
gears will accel and decel against each other because
of required clearances.
Cold Operation
Manual transmission operation will be affected by
temperature because the transmission fluid will be
thicker when cold. The thicker fluid will increase the
amount of force needed to shift the transmission when
cold. The likelihood of gear clash will also increase due
to the greater time needed for the synchronizer
assembly to perform its function. Therefore when the
transmission is cold, or before it has reached operating
temperature, quick, hard shifts should be avoided to
prevent damage to the transmission.

Copyright 2016 General Motors LLC. All Rights Reserved.

Gear Rattle
Rattling (not to be confused with a missed shift type of
grinding, also described as a combustion knock type of
noise) type noises usually occur while operating the
engine at low RPMs (lugging the engine). This can
occur while accelerating from a stop (for example, a
Corvette) or while operating at low RPMs while under a
load (for example, Kodiak in a lower gear and at low
engine speed). Vehicles equipped with a dual-mass
flywheel (for example, a 3500 HD Sierra with the
6-speed manual and Duramax®) will have reduced
noise levels as compared to vehicles without (for
example, a 4500 Kodiak with the 6-speed manual and
Duramax®). However, dual-mass flywheels do not
eliminate all noise.
Neutral Rattle
There are often concerns of rattle while idling in neutral
with the clutch engaged. This is related to the changes
in angular acceleration described earlier. This is a light
rattle, and once again, vehicles with dual mass
flywheels will have reduced noise. If the engine is shut
off while idling in neutral with the clutch engaged, the
sudden stop of the engine will create a rapid change in
angular acceleration that even dual mass flywheels
cannot compensate. Because of the mass of all the
components, this will create a noise. This type of noise
should not be heard if the clutch is released (pedal
pushed to the floor).
Backlash
Backlash noise is created when changing engine or
driveline loading. This can occur when accelerating
from a stop, coming to a stop, or applying and releasing
the throttle (loading and unloading the driveline). This
will vary based on vehicle type, build variations, driver
input, vehicle loading, etc. and is created from the

Page 2

April, 2016

necessary clearance between all of the mating gears in
the transmission, axle(s) and transfer case (if
equipped).
Shift Effort
Shift effort will vary among different style transmissions
and synchronizer designs. Usually the more heavy duty
the transmission, the higher the shift effort because of
the increased mass of the components. Shift effort can
also be higher in cold weather because the fluid will be
thicker. Medium duty transmissions will not shift as
quickly as a Corvette transmission. To reduce shift
effort, do not attempt to rush the shift – allow the
synchronizers to work as designed. Shifting harder will
only increase the chance of rushing past the
synchronizer leading to grinding while shifting.
Non-Synchronized Gears
Some light duty truck transmissions in 1st gear
(creeper-gear) and reverse gears in various
transmissions, along with all gears in some medium
duty transmissions, may be non-synchronized. This
means there is not a mechanism to match input and
output shaft speeds to allow for a smooth shift. This
function is left up to the driver. This can be noticed if a
shift into 1st or reverse is attempted while the vehicle is
rolling or before the input shaft stops rotating leading to
a gear grind. The grinding can be reduced by coming to
a complete stop and pausing for a moment before
shifting into the 1st or reverse gear. Some slight
grinding can be expected.
In medium duty non-synchronized transmissions, the
driver must match input shaft (engine) speed to output
shaft (driveshaft) speed with every shift. This can be

Bulletin No.: 03-07-29-004K

accomplished by double clutching, or by using other
methods. If the driver is not able to perform this function
properly, there will be gear grinding with each
improperly completed shift. Driver training may be
required to correct this condition.
Clutch brakes are used in medium duty
non-synchronized transmissions to allow a shift into
gear at a stop. The clutch brake is used to stop the
input shaft from spinning, allowing a shift into gear at a
stop without grinding. The clutch brake is activated by
pressing the clutch pedal all the way to the floor. When
the clutch brake is used, it is possible to have a blocked
shift with the vehicle stationary. If this occurs, engage
the clutch slightly to rotate the input gear to allow the
shift. The clutch brake is intended to only be used while
at a stop. Care must be taken to not activate the clutch
brake while shifting between gears. This could lead to
excessive grinding or a blocked or missed shift.
Skip Shift
Currently, the Cadillac CTS-V, Pontiac GTO, Chevrolet
Corvette, Chevrolet Camaro and Chevrolet SS, ALL V8
(other models may follow) equipped with the 6/7-speed
manual transmission have a feature referred to as a
"skip-shift.” This feature only allows a shift from 1st to
4th gear when the indicator lamp is illuminated on the
dash. Dealers cannot disable this feature as it was
established to help meet fuel economy standards. The
conditions for this feature are: engine coolant at normal
operating temperature, vehicle speed of 24-31 km/h
(15-19 mph), 21% or less throttle being used (refer to
Service Information or the Owner Manual for more
details.)

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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