03-07-29-004H Manual Transmission Operating Characteristics - (May 30 2012) Manual
|Subject:||Manual Transmission Operating Characteristics|
|Models:||2013 and Prior GM Passenger Cars and Light Duty Trucks|
|2009 and Prior Chevrolet and GMC Medium Duty Trucks|
|Equipped with Manual Transmission|
This bulletin is being revised to add model years. Please discard Corporate BulletinNumber 03-07-29-004G (Section 07 – Transmission/Transaxle).
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 similarvehicle and if possible, with similar mileage. Even though many of the conditionsare described as characteristics and may not be durability issues, GM may attemptto improve specific issues for customer satisfaction.
The purpose of this bulletin is to assist in identifying characteristics of manualtransmissions that repair attempts will not change. The following are explanationsand 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, levelof insulation, etc.
Many transmission noises are created by the firing pulses of the engine. Each firingpulse creates a sudden change in angular acceleration at the crankshaft. These changesin 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 noiseas the various gears will accel and decel against each other because of required clearances.
Manual transmission operation will be affected by temperature because the transmissionfluid will be thicker when cold. The thicker fluid will increase the amount of forceneeded to shift the transmission when cold. The likelihood of gear clash will alsoincrease due to the greater time needed for the synchronizer assembly to perform itsfunction. Therefore when the transmission is cold, or before it has reached operatingtemperature,quick, hard shifts should be avoided to prevent damage to the transmission.
Rattling (not to be confused with a missed shift type of grinding, also describedas a combustion knock type of noise) type noises usually occur while operating theengine at low RPMs (lugging the engine). This can occur while accelerating from astop (for example, a Corvette) or while operating at low RPMs while under a load (forexample, Kodiak in a lower gear and at low engine speed). Vehicles equipped with adual-mass flywheel (forexample, a 3500 HD Sierra with the 6-speed manual and Duramax®) will have reducednoise levels as compared to vehicles without (for example, a 4500 Kodiak with the6-speed manual and Duramax®). However, dual-mass flywheels do not eliminate all noise.
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.