Automotive Q&A

Learn about your Transmission

What is automatic transmission service?

There are two main differences between an automatic and manual transmission. In a manual transmission there is a clutch pedal and a gearshift, whereas an automatic transmission has neither. Automatic transmissions are very complex systems. Basically a transmission is responsible for transferring the power from the engine to the drive wheels, whether you have front- or rear-wheel drive. Transmissions allow an engine to operate at a few speeds or gears, while your vehicle can travel at a large range of speeds. There are many different parts in an automatic transmission including bands, clutches, a hydraulic system, and a gear pump, but it all starts with the planetary gear set. The planetary gear set creates all the different possible gear ratios your vehicle uses. The planetary gear set contains three parts: the sun gear, two or more planet gears and the planet gears’ carrier, and the ring gear. The sun gear is located in the center with each planet gear surrounding and in contact with it. Then the ring gear is surrounding and in contact with each of the planet gears. So, from inside out, it’s the sun gear, then planet gears, and then the ring gear. Each part can be one of three things: the input, output, or stationary. It is what role it has (input, output, stationary) that determines the gear sets’ gear ratio. In one of the planetary gear sets, the ring gear has 72 teeth and the sun gear has 30 teeth; so, there are thousands of possible gear ratios. To explain how different gear sets will change your vehicle’s gears and speeds, I will explain how your vehicle enters first gear, third (or above) gear, and reverse. For your vehicle to enter first gear, the ring gear will be the input, and the planet gears will be the output. The sun gear will remain stationary. What happens in first gear is the ring gear will be set in motion moving the planet gears around the stationary sun gear. In this instance, the output (planet gears) will move slower than the input (ring gear), which means that the gear ratio is a reduction. Another possible scenario includes unlocking the sun gear and locking any other two parts, meaning they are stationary. This will cause all three parts (ring gear, planet gears, and sun gear) to move at the same speed. This would mean that the input would be moving at the same speed as the output and that your vehicle is operating in third gear or higher. The last scenario involves keeping the planet gear stationary. Then, when the ring gear (input) is set in motion, it will force the sun gear (output) to move in an opposite direction, which will put your vehicle in reverse gear. Another very important part in automatic transmissions is the torque converter. The torque converter works just like the clutch in a manual transmission vehicle. Torque converters transmit the necessary power to the engine to keep it running while your vehicle is stopped. Torque converters also supply more torque to your wheels to allow your vehicle to speed up quicker after sitting still. In order to supply this power, torque converters use transmission fluid.Transmission fluid is used to lubricate, clean, and keep all of the moving parts in an automatic transmission cool. Transmission fluid is sent throughout a transmission by a complex hydraulic system. This hydraulic system is responsible for providing a constant supply of transmission fluid to the transmission and torque converter. The hydraulic system uses a pump, called the gear pump, to provide the necessary pressure to send the transmission fluid throughout the transmission. ow that we know how an automatic transmission works, we can discuss what occurs during an automatic transmission service. In order to keep your automatic transmission running as efficiently as possible, this necessary service is required. Over time, transmission fluid wears down and must be replaced. It is also important to change your transmissions’ filter and pan gasket.