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The automatic transmission explained

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Updated 23 Nov 2020

Jason Unrau

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One of the most complex parts in any vehicle is the transmission, particularly if it’s an automatic transmission.

It’s a difficult system to understand for even the most mechanically minded person.

Knowing a little bit about how the automatic transmission works in your car can help you know why it’s important to maintain it well, as well as how to identify when something isn’t working quite right.


A transmission’s purpose is to literally transmit torque to the drive wheels.

As the engine runs, it spins the crankshaft, producing power that needs to be put into use to propel your car.

An automatic transmission uses a few unique processes to make transmitting torque and shifting gears easier for the driver.

Namely, the torque converter, clutches, and hydraulic systems are what differentiate an automatic transmission from a manual gearbox while ultimately achieving the same purpose – controlled movement!


An automatic transmission can’t receive uncontrolled energy directly from the engine.

There would be no way to stop movement without a buffer. The torque converter serves that role.

It’s filled with fluid and uses centrifugal force to engage or disengage the transmission from the engine, depending on the engine’s speed.

At low RPMs, little to no power is channelled into the transmission while at higher RPMs, the engine and transmission’s rotations are effectively locked together.

With power entering the transmission and the gearshift in gear, the transmission uses hydraulic fluid pressure to control which gear is engaged.

A valve body opens and closes tiny channels that direct fluid pressure to the appropriate location.

Sensors determine where to direct fluid pressure to allow the right gear to be selected for the engine speed and the speed you are travelling.

When your car starts out from a standstill, the automatic transmission engages the lowest gear – 1st gear.

However, to accelerate to higher speeds, the transmission needs to change gears.

This keeps engine RPMs lower and aids in fuel economy.

To change gears while your car is in motion essentially requires disconnecting one gear to switch to the next. Clutches serve that purpose.

Unlike a manual transmission where one clutch disc disengages the whole transmission from the engine, an automatic transmission has several clutch packs.

A variety of gears inside an automatic transmission create the different gear ratios needed to accelerate smoothly and powerfully.

And when reverse gear is selected, a sun gear rotates the mechanism backwards.


Automatic transmissions have hundreds of moving parts, some more prone to problems than others. There are several symptoms you can experience with an auto transmission such as these:

  • Hard shifting. When you’re accelerating from a stop or decelerating, you might notice harsh engagement of the next gear up or down. This is often caused by low fluid levels or from a clogged or malfunctioning shift solenoid.
  • Slipping gears. During acceleration or holding a constant speed, the transmission may feel like it’s slipped out of gear. If your auto transmission isn’t holding a gear, it could indicate burnt or worn clutches, or low trans fluid.
  • Delayed engagement. If you’ve shifted into Drive but your car doesn’t move until you rev the engine up, it’s a clear sign of low fluid pressure inside the transmission. It could be due to leaking seals, low fluid, or other components that have developed a fault.
  • Limp mode. A transmission failure could show itself by not shifting beyond second gear. Many carmakers have a failsafe built into the transmission to prevent severe damage by restricting the transmission’s function to first and second gear. It could be caused by an electrical issue, internal fault, or a fluid problem.
  • No movement. Obviously, if your car won’t drive when you’re in gear, there’s a problem. It could be anything from a catastrophic internal breakdown to all the fluid leaking out of the transmission.
    Regardless which symptom you have, it’s vital to have any transmission issues inspected and fixed right away. It could be the difference between a massive repair bill and your car out of service for days, and a quick, modest fix.


The cost to repair an automatic transmission varies wildly depending on the issue.

A transmission cooler hose leak, for example, might be just $150 to $250 to correct.

A faulty solenoid or valve body can be repaired with the transmission still in your car, saving you money on labour costs.

The heftiest repair bills are associated with fixes that require transmission removal and installation.

A transmission overhaul or rebuild can easily reach $2,500 to $4,000 depending on the make and model.

An automatic transmission replacement is commonly $3,000 to $7,000 with some specialty models considerably higher yet.

Head to AutoGuru to get quick and easy quotes for your automatic transmission repair.

Choose from a great range of local, high-quality mechanics!

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Written By

Jason Unrau

Jason is a Canadian automotive content writer with a background in the auto service industry, but he’s been hooked on cars and mechanics since childhood.

One of his first cars was an ’80 Mazda RX-7 that’s sorely missed to this day. A ’68 Ford Torino GT, a ’66 Ford Country Squire Woodie station wagon, and a ’96 Suzuki GSX-R 750 have spent time in his fleet of cars, bikes, and trucks over the past two decades.

Jason’s pride and joy is under construction – a turbocharged ’88 Mazda RX-7 convertible. Also on his resume is CASCAR official certification.