A lot of folks just don’t like shifting gears, so some smart cookies started figuring out ways to get a car to move by just selecting a position and applying the throttle.

To begin with, most cars weigh several thousand pounds, and an engine doesn’t have the power to move a car without the advantage of gears.  Anybody who has ridden a bicycle with more than one gear knows that it’s a lot easier to pedal the bike in a lower gear.  In First Gear on a bicycle, the pedals make lots of turns and are easy to operate, but the bike doesn’t move very fast.

It’s the same with your car.  In lower gears, the engine turns lots of revolutions, but the car isn’t moving very fast until the momentum enables the engine to drive the wheels faster through higher gears.

On a manual transmission, you start with first, build some speed,  release the clutch, shift to second, build more speed, release the clutch, shift to third, and so on.  Fifth gear is actually a ratio where the drive shaft is turning faster than the engine – that’s for gas mileage, and it’s called “Overdrive.”

In an automatic transmission, there’s a device connected to the engine called a ‘torque converter.’  That torque converter is the coupling between the engine and the transmission.  So how does it work?

Imagine having two fans facing each other.  We’re working with air, right?  Okay, fan number one is connected to power and begins to blow air across fan number two. Fan number two is connected to the shaft that will be doing the work. As fan number one picks up speed, fan number two begins to respond to the air and turn as well, and that action turns the shaft that does the work.

The two fans represent a torque converter in its simplest form.  A torque converter, however, is filled with transmission fluid.  The fan (called an ‘impeller’) that moves the fluid is connected to the engine – that impeller is permanently connected to the inside the shell of the torque converter, which is what you see when you look at an automatic transmission somebody has removed from a vehicle.  What you can’t see is the other fan (called a ‘turbine’) that is inside the shell – that fan is connected to a shaft (called the ‘turbine shaft’) that runs through the center of the transmission and as it turns, it applies power to the components in there.  When the transmission is in neutral or park and the engine is running, the shaft is free to turn without doing much of anything else. 

Okay, pay attention to this next part, because it’s really important.  There is a third part that looks like a smaller fan, and it is sandwiched between the impeller and the turbine. That part is called a ‘stator,’ but it spins on a special clutch that only allows it to turn in one direction. What that means is that it can spin freely in one direction, but it can’t turn at all in the other direction.  That stator provides torque multiplication by making sure the fluid is redirected to strike the turbine blades with the greatest possible force.

The torque multiplication provided by the stator helps your car take off faster than it would if the stator wasn’t there or if the stator’s one-way clutch couldn’t hold it still.  The inside of the stator’s one-way clutch is connected to an immoveable part with long teeth (called ‘splines’).

The torque converter produces a lot of heat while it’s shearing the fluid, particularly when you’re pulling a heavy load or sitting with the car in gear and with your foot on the brake, and for that reason the fluid is pumped through a cooler that is either mounted in front of the radiator or is inside the radiator.

The torque converter shell is connected to a fluid pump in the very front of the transmission. That fluid pump sucks transmission fluid up out of the transmission oil pan and fills the torque converter. It also provides fluid pressure to the transmission’s internal components and that pressure is used for a variety of jobs.

The Gears 

The heart of an automatic transmission is the ‘planetary gear set.’  That sounds complicated, doesn’t it? Well, you have a ring gear and a sun gear, with planet gears in between.  This planetary gear set is connected to various components that are applied by fluid pressure so that any of the three parts of the planetary gear set can be held or driven.   Holding and driving different members of the planetary gear set is a mechanical function that provides different gear ratios (first, second, third, reverse, etc.).

 Ford Automatic Overdrive Transmission (AOD)

To hold or drive different parts of the gear set, clutches (steel plates with teeth on the outside and friction disks with teeth on the inside in alternation)  are applied by hydraulic pistons.  Some holding members are in the form of wide ‘bands’ with friction material that stops a ‘drum’ from turning.  The drum will be connected to a part of the planetary gear set and may be either driven when its internal clutches are applied or held when its band is applied.   There are also one-way clutches in the transmission, usually only one or two, and they become holding members when a particular component tries to turn against their action.

The part of the transmission that directs fluid to the driving and holding members is a complicated part known as a ‘valve body.’  If you’ve ever seen an automatic transmission with its oil pan removed, most of the time you’ll be looking at the valve body, which contains spool valves and springs that are moved by fluid pressure this way and that to redirect fluid to the various members inside the transmission.

What Happens in Drive

So, you’ve placed your transmission in gear.  There is a valve in the valve body that is connected to your gear selector lever – I like to call the gear selector lever a PRNDL stick.  Anyway, when you move the main valve that is connected to your PRNDL stick, fluid pressure is applied to some of the valves, moving them against their springs, and fluid flows through carefully designed passages to apply the necessary driving and/or holding members that will cause the planetary gear set to produce your lowest gear.

Some transmissions have solenoid valves that are operated electrically to facilitate the redirection of the fluid, and the car’s computer (Powertrain Control Module, or PCM) operates the solenoids.

Until you apply the throttle, the torque converter is not producing enough turning force on the turbine shaft to move the vehicle.  When you release the brake, the torque converter turbine begins to turn its shaft and the driven part of the planetary gear set, which transmits power to the driveshaft or axles, depending on whether you have front wheel drive or rear wheel drive.

As you gain speed on older transmissions, there is a hydraulic valve called the 'governor' operated by centrifugal force (mounted on the output shaft on Fords but driven by a gear on the output shaft on GM cars) that moves some valves and increases pressure to move valve body valves against their little springs, redirecting fluid to apply different clutches and holding members.  This in turn drives and holds different elements of the planetary gear set to provide higher gear ratios.

On newer transmissions, there are speed sensors to tell the PCM how fast the turbine shaft and the output shaft (drive shaft or axle) is spinning (it knows how fast the engine is spinning already), and it operates its little solenoids to redirect fluid pressure and bring about gear shifts.

Now, how does the transmission know you’re taking off fast and hard?  It won’t shift the same way on a full throttle takeoff as it will when you take off gently, will it? 

Older transmissions handled this in a couple of different ways. On early GM and Ford cars, a vacuum device called a ‘modulator’ gets information from a vacuum line running from the engine’s intake manifold. The vacuum modulator is connected to a valve inside the transmission.  With lower engine vacuum (which the PCM interprets as higher load), the modulator valve opposes governor pressure and forces the transmission to hold each gear for a longer period of time.

On Chrysler automobiles, there was a piece of linkage connected on one end to the throttle and on the other end to a valve in the transmission that does about the same job as the vacuum modulator’s valve.  A steeper throttle angle means higher load and later shifts. Ford and GM adopted this same strategy eventually on almost all of their vehicles.

On the most recently manufactured vehicles, the Powertrain Control Module (PCM) takes inputs from throttle position and speed sensors to regulate the pressures and valves in the transmission.  For those people who don’t like to feel the transmission shift (although it’s better if you DO feel it), some automakers designed the PCM to remove some of the engine’s power during transmission shifts.  This makes the transmission shift smoother and it prevents the clutches from slipping and burning.

Okay, so how do you keep your transmission running smoothly?  Here are a few simple tips:

        1. If your transmission has a dipstick (some auto transmissions don’t have one) find out where it is and check the fluid with the engine running and the transmission in park.  The bottom of the cross hatch area is ½ quart on a transmission.  Realize that the transmission fluid will be higher on the stick with a hot transmission than it will with a cold one.

        2. Have regular fluid changes – check your owner’s manual – on most vehicles it’s about 30,000 miles between fluid changes (some Mercedes cars have lifetime fluid). The full fluid exchange is a good idea, but it’s pretty pricey and requires a machine.

        3. If you pull heavy loads, change your transmission fluid more regularly, else it’ll get gummy and cause varnish to form inside the transmission. 

        4. If you’re sitting in traffic for extended periods of time, realize that the torque converter makes a lot of heat and you can overheat your transmission if you hold the brake with the transmission in gear for a long time, particularly in hot weather.  When you’re in the drivethrough, put it in part or neutral until time to move forward.

        5.  Treat the transmission gently – don’t slam it in drive while it’s still rolling backwards after backing out.

Automatic transmissions can be very expensive to repair.  Treat yours right, because if you hire somebody to rebuild it or replace it, you’ll think the parts are made of gold!  R.W.M.