An automatic transmission changes gears on its own using planetary gear sets, clutches and bands that are applied by fluid pressure. A torque converter connects it to the engine so the car can sit still in gear, a pump creates hydraulic pressure, and a valve body, controlled by solenoids and a computer on modern transmissions, decides which clutches to apply for each gear.
The main parts
- Torque converter: a fluid coupling that lets the engine idle in gear, multiplies torque when pulling away and locks up at cruise. See what does a torque converter do?
- Pump: driven by the converter, it pressurizes the transmission fluid.
- Planetary gear sets: each set has a sun gear, planet gears on a carrier and an outer ring gear. Holding or driving different members gives different ratios, including reverse.
- Clutch packs and bands: friction elements that hold or connect parts of the gear sets to select each gear.
- Valve body: a maze of passages and valves that routes fluid to the right clutch or band.
- Solenoids and control module: electrically operated valves and the computer that commands them; see transmission solenoid symptoms.
How it shifts
- Sensors report vehicle speed, engine speed, throttle position and load.
- The control module chooses the gear and commands the solenoids.
- The solenoids direct fluid pressure through the valve body to apply one set of clutches or bands and release another.
- The planetary gear set locks into the new ratio, and the torque converter clutch locks up once the vehicle is cruising.
Older transmissions such as the 47RE and 48RE use more hydraulic control, while electronically controlled units like the 4L60E, 6L80E and 10R80 rely on the computer for every shift. More gears let the engine stay near its most efficient speed; compare ratios on our transmission gear ratios chart.
Why fluid matters so much
The same fluid transmits power through the converter, applies the clutches, lubricates the gears and carries heat away, so low, burnt or wrong fluid affects everything. See transmission fluid color and how to check transmission fluid.
Automatic vs manual and CVT
A manual uses a driver-operated clutch and gear selector. A continuously variable transmission (CVT) uses a belt or chain between variable pulleys instead of fixed gears. A conventional automatic like the ones in most trucks uses fixed gear ratios selected by clutches, as described above.
Replacement automatic transmissions
If yours has failed, find it by code on our transmissions page; every unit we carry comes with a torque converter. What transmission do I have? explains how to identify it.
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