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Service Bulletin

Bulletin No.:
Date:

07 - Transmission/Transaxle
03-07-29-004I
December, 2014

INFORMATION
Subject:

Manual Transmission Operating Characteristics

Models:

2015 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 2014-2015 Model Years. Please discard Corporate
Bulletin Number 03-07-29-004H.

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. Even though many of the conditions are described as characteristics and may not be durability issues, GM
may attempt to improve specific issues for customer satisfaction.
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.
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 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 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 and Camaro SS (other models may follow)
equipped with the 6-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.)