Transmission Operation Explained

How Manual, Automatic, FWD, RWD, & 4WD Transmissions Operate

Auto Service experts Local Transmission Repair Shop in San Antonio

A transmission includes several components which work together to transfer power from a vehicle's engine to its wheels. These components differ depending on whether the transmission is manual or automatic, and whether the vehicle is front wheel drive (FWD), rear wheel drive (RWD), or four wheel drive (4WD or 4X4).

 2004 Chevrolet Trailblazer 4.2 Automatic Transmission

This 2004 Chevy Trailblazer 4.2 automatic transmission has been removed and is ready to be rebuilt.

Why your Car or Truck Cannot Operate Without a Transmission!

Internal combustion engines only rotate in one direction and operate within a narrow range of speeds. The transmission implements gear ratios which allow the wheels to travel at a wide range of different speeds while the engine speed remains constant. The transmission also allows the vehicle to travel in both forward and reverse.


Transmission Functions:

    * Shift Gears.
          Automatically – controlled by vehicle’s computer system
     * Adjust Speed -Torque Ratio.
          Torque - Power to wheels.
     * Supply the wheels with sufficient power depending on the speed of the car.
      * Implement Gear Ratios
          Gear Ratios - Reduce the engine’s high rotational speed output while increasing the amount of power delivered to the wheels, or torque


Automatic Transmissions

Shifting Gears With an Automatic Transmission:

To shift gears in a car or truck with an automatic transmission, the driver selects park, reverse, or drive. When the car is in drive the automatic transmission automatically chooses the correct gear based on the engine speed and shifts to that gear. Automatic transmissions are connected to the engine via the torque converter. These systems are much more intricate in design than standard transmissions, therefore it requires more specialized knowledge and equipment to repair automatic transmissions. They perform the functions of selecting and shifting to the ideal gear under current driving conditions, and changing gears automatically when necessary. Multiple sensors throughout the vehicle collect and transmit information regarding driving conditions. This information is gathered and processed by the electronic control module (ECM).


Automatic Transmission Components:

Torque Converter - The torque converter is a hydraulic fluid-filled component which includes an impeller, turbine, and stator.
Impeller - The impeller connects to the crankshaft, which is spun by the rotation of the engine, and transfers torque to the turbine.
Turbine - The turbine connects to the transmission.
Stator - The stator controls the amount of torque transferred from the impeller to the turbine.


Automatic Transmissions Rely on Transmission Fluid - Hydraulic Systems

In automatic transmission systems, the transmission fluid and filter are located within an oil pan. Automatic transmissions are hydraulic systems, meaning that the force of the transmission fluid is used to power the shifting of gears. Because the operation of automatic transmissions relies heavily on the transmission fluid, it is important to have your vehicle’s transmission fluid changed according to manufacturer recommendations. Fluid may become dirty or contaminated with debris or may deteriorate to a dangerously low level. These conditions can cause many transmission problems such as transmission slipping, and even complete transmission failure.

Automatic Transmission Service includes draining and replacing the transmission fluid and replacing the pan gasket and filter (if applicable). An Auto Service Experts Transmission mechanic will also thoroughly check for transmission leaks, and test drive your vehicle to ensure proper shifting. Regularly servicing automatic transmissions helps avoid major repairs down the road!


Manual or Standard Transmissions

Shifting Gears With a Standard or Manual Transmission:

When shifting gears in a vehicle with a manual transmission the driver first presses down on the clutch pedal which disengages the transmission from the engine. The driver then uses the manual shift lever to change gears. The different positions of the gear shift lever engage corresponding pairs of gears. These gears operate at different gear ratios and propel the wheels at different speeds.


Manual transmissions are mechanical systems, therefore, transmission fluid is not involved in gear changing. Manual transmissions do not have a filter to remove the contaminants and debris from the transmission fluid. This makes it extremely important to have the transmission fluid changed regularly. As other parts wear and break down, they can shed particles of metal which can contaminate transmission fluid and damage internal components. In many cases, a lack of maintenance leads to complete transmission rebuilds or other costly repairs.

**The Automatic Transmission Rebuilders Association states that roughly 90% of automatic transmission failures are due to overheating which could have been avoided by replacing the transmission fluid.

Transmission Components

Transmission Soft Parts – clutch components, gaskets, valves, O-rings, seals, bands, and filters. These parts experience a lot of wear and tear and are all replaced during a transmission rebuild.
Transmission Hard Parts – converters, pumps, shafts, drums, gears, and casing. These parts are more durable and do not have to be replaced as often.


Gearbox/ Differential:

The differential controls which of several different sized gears the vehicle operates on. The gears are sized differently to allow optimal car performance under changing driving conditions such as speed, direction, or degree of incline. Cars, like bicycles, use different gears depending upon how much effort is being exerted, and how fast they are traveling. The differential delivers power to a set of two driven wheels and also permits them to turn at independent speeds from one another. This allows us to control and steer the car or truck around curves or turns. Differential repair can often be avoided by simply having the fluid changed according to the manufacturer recommended service schedule.


Manual (or Standard) Gearbox: 

The driver decides which gear allows the best vehicle performance according to current speed and driving conditions. A shifting mechanism inside the vehicle cabin allows the driver to manually select the proper gear, and change gears while driving, as conditions change. Standard transmissions are not as complicated to rebuild as automatics, however, standard / manual transmission service is incredibly important to maintain optimal performance and avoid major repairs.



A driveshaft connects the differential to one of the driven wheels. Power is transferred from the engine first to the differential, then throughout the drive shafts to the wheels responsible for propelling the vehicle.


Electronic Control Module (ECM):

The ECM acts as the brain of the automatic transmission. It is a computer which gathers and processes information from electronic sensors within the automobile. 


FWD Transmissions

Most cars today are front wheel drive, meaning that power from the engine is only transferred to the front two wheels. The FWD design is the simplest of transmission types and consists of the differential (or gearbox) and two driveshafts.

To Be Continued....

ASE master technician Carlos Rodriguez

(210) 495-6688
Auto Service Experts

13050 San Pedro Ave. San Antonio, TX 78216

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