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Interactive 3D guide

How a Transmission Works

Most people never see inside an automatic transmission until something goes wrong. This is the same view our technicians have on the bench. Install the three parts customers ask about most, spin the model around, and watch it run. Then keep reading for the whole story: why a car needs a transmission, what happens on every shift, every part inside the case, and what usually fails.

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From the team at Rohnert Park Transmission & Auto Repair, 305 Laguna Dr, Rohnert Park. Family owned, ASE Certified technicians. Published September 1, 2026.

What does a transmission do?

A transmission is the gearbox between your engine and your drive wheels. It multiplies the engine's turning force, called torque, so the car can pull away from a stop, then changes gear ratios as you speed up so the engine can settle into a relaxed, efficient speed on the highway. It also gives you reverse, neutral, and park.

An automatic transmission does all of that without a clutch pedal. Three systems handle the work: a torque converter that passes engine power into the transmission through fluid, planetary gear sets that create each gear ratio, and a hydraulic control system, the valve body and its solenoids, that applies the right clutch at the right moment. Those are the three parts you install in the model below.

Build the transmission

Three parts, one see-through case. Install them in any order, then watch the model run through a drive cycle: idling at a light, pulling away, shifting, and cruising in lockup. Tap a part after it is in to read what it does, how it fails, and what we do about it in the shop.

Parts installed
0 / 3
Loading the 3D workbench
Drag sideways to rotate

Build in any order.

  • Torque Converter

    The fluid handshake between your engine and your transmission.

  • Planetary Gear Set

    Where your gears actually live.

  • Valve Body

    The control center that decides when you shift.

How this works

Build it yourself

The see-through case on the workbench is an automatic transmission with its three most talked-about parts removed. Tap any Install button to drop a part into place and read what it does, how it fails, and what we do about it. Build in any order. Drag the model to look at it from any side.

  1. Torque Converter

    The fluid handshake between your engine and your transmission.

  2. Planetary Gear Set

    Where your gears actually live.

  3. Valve Body

    The control center that decides when you shift.

As soon as a part is in, the model runs through a full drive cycle: waiting at a light, pulling away, shifting, and cruising in lockup. Use Watch it run and Pause to control it, and Exploded view to pull the parts apart while they work.

Why your car needs a transmission

A gasoline engine only makes useful power in a narrow band, roughly 1,000 to 6,000 rpm. Your drive wheels need to turn anywhere from zero to about 900 rpm at 70 mph. Nothing bolted straight to the crankshaft could cover that spread, and the engine cannot spin slower than idle without stalling. The transmission bridges the gap.

It does that with gear ratios. In first gear the engine turns about three times for every turn of the transmission's output shaft, so its torque is multiplied about three times, which is what gets two tons of car moving. In top gear the ratio flips into overdrive, around 0.7 to 1, so the engine can loaf near 2,000 rpm while the car holds highway speed. The differential behind the transmission multiplies everything again, usually three to four times, before it reaches the wheels.

Think of the gears on a bicycle. A low gear lets you climb Sonoma Mountain Road with easy pedaling, but you cannot go fast. A high gear is hard to start from a stop but keeps your legs at a comfortable cadence on the flat. A transmission changes gears for the same reason, and an automatic does it for you.

How does an automatic transmission work? Step by step

This is the same drive cycle the model runs when you press Watch it run. Follow along with the 3D view open and you can see each step happen.

  1. Idling at a light in Drive

    Your engine is spinning the front half of the torque converter, the pump side, at idle. The back half, the turbine, is connected to the transmission and is nearly stopped because you are holding the brake. Fluid slips between the two halves, which is why the engine keeps running in gear and why the car creeps when you ease off the pedal. Meanwhile the converter hub is driving the transmission's own oil pump, so line pressure is already up and first gear is already applied.

  2. Pulling away

    As the engine speeds up, the fluid coupling tightens and the stator between the two halves redirects the fluid so the converter multiplies torque, roughly doubling it from a standstill. That torque goes into first gear, where the ring gear is held still and the sun gear drives the planet carrier at a big reduction. Two multiplications in a row is what gets two tons of car rolling without any drama.

  3. The first shift

    Speed sensors on the input and output shafts, throttle position, engine load, and fluid temperature all feed the transmission control module. When the numbers say it is time, the module fires a shift solenoid in the valve body. Pressurized fluid is redirected: one clutch pack releases while another applies, with a brief overlap so the car never freewheels. A different planetary member is now held, and the ratio drops.

  4. Up through the gears

    Each gear brings the ratio closer to 1 to 1, then into overdrive. A modern 6-, 8-, or 10-speed automatic reaches all of those ratios by stacking two or three planetary sets and applying clutches in different combinations. The more speeds, the smaller the step between them and the closer the engine can stay to its sweet spot.

  5. Lockup at cruising speed

    Once you are cruising, usually above 35 to 45 mph with light throttle, the module applies the lockup clutch inside the torque converter. Engine and transmission now turn as one solid piece. There is no slip, no wasted heat, and better fuel economy. Most converters spend the majority of their highway miles locked.

  6. Slowing down, reverse, and park

    Lift off the throttle and the lockup clutch releases while the transmission downshifts on the way to a stop. In Drive a one-way clutch lets the car coast freely in first gear, which is why you feel engine braking in Low but not in Drive. Reverse holds a different member of the planetary set so the output turns backward. Park drops a steel pawl into a notched wheel on the output shaft, and that pawl is the only thing holding the car when you park without the brake.

How does a transmission know when to shift?

Older automatics used a governor spun by the output shaft to measure road speed and a throttle cable or vacuum modulator to measure engine load, and those two pressures pushed the shift valves in the valve body. Every automatic built in the last few decades uses sensors and a computer instead. The transmission control module reads vehicle speed, input and output shaft speed, throttle position, engine load, the brake switch, the gear selector, and fluid temperature, then commands shift and pressure control solenoids in the valve body.

It also adapts. The module learns how quickly each clutch fills and trims pressure over time, which is why a transmission can feel different for a few days after a fluid service, a valve body replacement, or a battery disconnect. Part of our road test after any of those jobs is letting the module relearn.

The three parts in the model, explained

These are the three parts customers ask about most, because they are behind most of the diagnoses we deliver at the counter. Everything below is also in the model: install a part and tap it.

Part 1 of 3

Torque Converter

The fluid handshake between your engine and your transmission.

Where it lives: Bolted to the engine's flexplate, inside the bell housing at the very front of the transmission.

Your engine never stops spinning, but your wheels do. The torque converter is what lets an automatic sit at a red light in Drive without stalling. Instead of a clutch pedal, it uses transmission fluid to pass power from the engine into the transmission.

Inside the donut-shaped housing, two finned wheels face each other in a sealed bath of fluid. The engine spins one wheel, the fluid it throws spins the other, and a third wheel in between called the stator redirects that fluid so you get extra pulling power from a stop. That built-in slip is also why the car creeps forward when you lift off the brake.

At cruising speed a lockup clutch inside the converter clamps both sides together so no power is wasted as heat. When that clutch starts to wear, it is usually the first thing you feel.

How it fails

  • A shudder or rumble-strip vibration around 35 to 50 mph as the lockup clutch grabs and slips.
  • Engine RPM climbs but the car does not pull the way it should.
  • The transmission runs hot, or the fluid looks dark and smells burnt.
  • A whine in gear that rises and falls with engine speed.

What we do in the shop

We start with a fluid inspection and a road test, because worn-out fluid can imitate converter trouble and is the simpler fix. Replacing the converter itself means removing the transmission, so we confirm the diagnosis before we recommend it.

Part 2 of 3

Planetary Gear Set

Where your gears actually live.

Where it lives: In the middle of the main case, riding on the center shaft behind the pump and the clutch packs.

A manual transmission uses rows of separate gears. An automatic packs its gears into a planetary set: a sun gear in the center, several planet gears orbiting it on a carrier, and a ring gear with teeth on the inside wrapped around all of them. All three are always meshed together.

Hold one member still and drive another and you get a different ratio out the back. That is how one compact set can deliver several forward gears plus reverse without any gear ever sliding in or out of mesh. Clutch packs and bands do the holding, and most modern automatics stack two or more planetary sets to reach 6, 8, or 10 speeds.

Because everything stays meshed, the set itself rarely wears out on its own. When it does fail, something else usually let it slip first.

How it fails

  • Slipping: the engine flares between gears before the car catches up.
  • A grinding or metallic whine that changes with road speed rather than engine speed.
  • A check engine light with a gear ratio code such as P0730, incorrect gear ratio.
  • Metal flakes or glitter in the fluid at service time.

What we do in the shop

Gear set damage almost always traces back to worn clutch packs, low or burnt fluid, or a control problem that let the set slip. We find that cause first. Because the gear set lives deep inside the case, repairing it is rebuild territory, and we tell you honestly whether a rebuild or a remanufactured unit makes more sense for your vehicle.

Part 3 of 3

Valve Body

The control center that decides when you shift.

Where it lives: Bolted to the bottom of the case, hidden under the oil pan.

The valve body is a metal maze of fluid channels, spring-loaded valves, and electric solenoids. Pressurized fluid from the pump enters it, and the solenoids, taking orders from the transmission computer, open and close passages so fluid reaches the right clutch pack at exactly the right moment.

Every shift you feel is the valve body redirecting fluid. When a valve sticks, a solenoid gets lazy, or a passage starts leaking pressure, shift quality is the first thing to change, long before anything mechanical breaks.

It is one of the few parts of an automatic that can often be serviced without pulling the transmission out of the car, which is why we always check it early.

How it fails

  • Harsh or banging shifts, or a delay when moving from Park into Drive or Reverse.
  • Shifting at the wrong time, hunting between gears, or skipping gears.
  • Stuck in one gear or in limp mode after a warning light comes on.
  • A check engine light with solenoid or pressure codes, such as P0700 or the P0750 to P0770 range.

What we do in the shop

Valve body problems are common on higher-mileage vehicles and often do not call for a full rebuild. We scan for codes, check fluid pressure, and test the solenoids, then service or replace the valve body with the transmission still in the car whenever the vehicle allows it.

What you are looking at

The see-through shell on the workbench is drawn from the outside of a typical rear-wheel-drive automatic, engine side on the left and driveshaft side on the right. Front-wheel-drive transmissions fold the same parts into a shorter, wider case, but the jobs each part does are the same.

We built it so our team can pull it up at the front counter and show you exactly where a problem lives before any work starts. If a technician tells you the shudder you feel is the torque converter, you can see why that means the transmission has to come out, and why a valve body problem often does not.

Bell housing
The wide, rounded front end that bolts to the engine. The torque converter spins inside it.
Main case
The long aluminum body that holds the pump, clutch packs, and the planetary gear sets on the center shaft.
Oil pan
The shallow pan on the bottom that holds the fluid. Drop it and the valve body is right there.
Tail housing
The narrow end where the output shaft leaves the transmission and heads for the driveshaft.

Parts of an automatic transmission and what each one does

The model shows three parts. A real automatic has a few dozen, and the ones below are the ones that matter when something goes wrong. They are listed in the order power flows through them, engine side first.

Flexplate
A thin steel disc bolted to the engine's crankshaft. The torque converter bolts to it, so it is the physical link between engine and transmission, and it carries the starter ring gear.
What goes wrong: Cracks cause a knock or rattle at idle that is easy to mistake for an engine noise.
Torque converter
Passes engine power into the transmission through fluid, multiplies torque from a stop, and locks solid at cruising speed.
What goes wrong: A worn lockup clutch causes shudder at 35 to 50 mph. A failed stator clutch causes weak acceleration. A converter that comes apart sends metal through the whole unit.
See it in the model
Front pump
Driven by the converter hub, it builds the line pressure that applies every clutch and fills the converter. It runs whenever the engine runs.
What goes wrong: A worn pump means low pressure, a whine that rises with engine speed, and delayed engagement into gear.
Input shaft
Splined into the converter turbine, it carries power from the converter into the gear train.
What goes wrong: Rarely fails on its own. Stripped splines usually follow a converter failure.
Planetary gear sets
Sun gear, planet carrier, and ring gear in constant mesh. Holding one member and driving another creates each forward ratio and reverse.
What goes wrong: Damage almost always follows something else letting it slip: burnt clutches, low fluid, or a control fault.
See it in the model
Clutch packs
Stacks of alternating steel and friction plates inside a drum. Hydraulic pressure squeezes the stack to hold or drive a planetary member.
What goes wrong: Burnt friction material is the classic slipping transmission. Dark fluid with a burnt smell is the giveaway.
Slipping guide
Bands and servos
A steel strap lined with friction material that a hydraulic servo tightens around a drum to hold it still. Common on older and simpler automatics.
What goes wrong: Out of adjustment or worn, the band lets the unit slip going into that gear, often second.
One-way clutches
Also called sprag or roller clutches. They let a member turn freely one way and lock the other, like the rear hub on a bicycle.
What goes wrong: A failed sprag gives no drive in first or a flare on the 1-2 shift, and no engine braking where there should be some.
Valve body
The hydraulic control center under the oil pan. Its valves and passages route pressure to the right clutch for each shift.
What goes wrong: Sticking valves and worn bores cause harsh, delayed, or mistimed shifts long before anything mechanical breaks.
See it in the model
Solenoids
Electric valves on the valve body. Shift solenoids pick the gear, pressure control solenoids set how firmly clutches apply, and the TCC solenoid controls converter lockup.
What goes wrong: A lazy or shorted solenoid causes wrong-gear starts, limp mode, and codes in the P0750 to P0770 range.
Valve body and solenoid repair
Transmission control module and sensors
The computer that decides every shift, fed by input and output speed sensors, throttle position, fluid temperature, and the range sensor on the shifter.
What goes wrong: A bad sensor or a wiring fault causes erratic shifting and a P0700 with more codes stored behind it.
P0700 code guide
Transmission fluid
Carries power through the converter, applies the clutches under pressure, lubricates everything, and carries heat to the cooler. The wrong type behaves like the wrong part.
What goes wrong: Old, low, or wrong fluid is behind most of the shudder, slipping, and overheating we see.
Fluid service
Filter and oil pan
The pickup filter screens fluid on its way to the pump. The pan holds the fluid and, on many vehicles, the drain plug.
What goes wrong: A clogged filter starves the pump. A leaking pan gasket is one of the most common drips we fix.
Cooler and cooler lines
A portion of the fluid is routed to a cooler in the radiator, sometimes with an extra auxiliary cooler, and back through steel lines.
What goes wrong: A restricted cooler cooks the transmission, and a leaking line fitting can empty it.
Cooler service
Case, seals, and gaskets
The aluminum housing and the seals that keep fluid in: the front seal behind the converter, the rear seal at the output, and axle seals on front-wheel-drive units.
What goes wrong: Seals harden with age and heat. Most external leaks trace to a seal, a gasket, or a cooler line.
Output shaft, park pawl, and final drive
The output shaft sends power to the driveshaft or, in a front-wheel-drive transaxle, to the differential built into the same case. The park pawl locks the output shaft when you shift into Park.
What goes wrong: A worn pawl can let the car roll or click in Park. Always set the parking brake too.
Differential repair

Automatic, manual, CVT, and dual-clutch: what changes

Every type has to solve the same problem: match a narrow engine speed range to a wide wheel speed range. We service all four in-house, and here is what is different inside each one.

Conventional automatic

Torque converter, planetary gear sets, and hydraulic control, exactly what the model shows. Six to ten speeds are normal today. This is what most cars, trucks, and SUVs on the road use.

Automatic transmission repair

Manual

Rows of gear pairs on parallel shafts, a driver-operated clutch instead of a torque converter, and synchronizers that match gear speeds so you can shift without grinding. Simpler inside, but the clutch is a wear item.

Manual transmission repair

CVT

No fixed gears. A steel belt or chain runs between two variable-width pulleys, so the ratio changes continuously. Many still use a torque converter. Fluid is even more critical, and the belt and pulleys wear differently than a planetary unit.

CVT repair

Dual-clutch

A computer-shifted manual with two clutches, one for the odd gears and one for the even, so the next gear is already selected. Very fast shifts, but clutch wear and control faults show up as shudder and hesitation at low speed.

Dual-clutch problems guide

What usually fails in a transmission, and what it feels like

Symptoms point somewhere, and after nearly thirty years of Sonoma County transmissions on our lifts the pattern is consistent. Use this to understand what a technician is likely to check first, not to diagnose it yourself.

What you feelWhere it usually pointsRead more
Shudder or rumble-strip vibration around 35 to 50 mphTorque converter lockup clutch, very often started by worn fluidShudder guide
Engine flares between gears, or the car is slow to catch upWorn clutch packs, low or burnt fluid, or low line pressureSlipping guide
Long pause going from Park into Drive or ReverseLow fluid, a weak pump, or a sticking valve in the valve bodyDiagnostic
Harsh, banging, or mistimed shiftsValve body wear, a pressure control solenoid, or adaptive settings that need a relearnValve body repair
Stuck in one gear or limp mode with a warning lightA solenoid, a speed sensor, or wiring, reported as P0700 plus the codes behind itP0700 guide
Whine that rises and falls with engine speedFront pump or torque converterTransmission repair
Check engine light with a gear ratio codeA clutch pack slipping, or gear set damage after it slippedP0730 guide
Red or brown fluid on the drivewayPan gasket, front or rear seal, axle seal, or a cooler lineTransmission repair
Burnt smell and dark fluidOverheated clutches, usually from old fluid or a restricted coolerCooler service

The number one killer of transmissions is heat

Transmission fluid is happiest around 175 to 200°F. A long-standing rule of thumb in the trade is that every 20 degrees above that roughly halves its life. Old fluid loses the friction modifiers that let clutches grab cleanly, so they slip, slipping makes more heat, and the cycle feeds itself until the friction material is gone.

Towing, stop-and-go traffic on Highway 101, a restricted cooler, and simply never changing the fluid are the four ways we see it happen most. The fix is boring: the exact fluid your vehicle specifies, changed on time.

How we find the real cause

We do not guess from a symptom. A transmission concern at our shop starts with a road test to feel the problem ourselves, a scan of the transmission module for stored and pending codes, and a look at the fluid's level, color, and smell. If the story is still unclear we check line pressure and command the solenoids from the scan tool.

Only then do we talk about a repair, and we tell you which of three buckets it falls into: a service or an in-car repair such as a valve body, a rebuild of your own unit, or a remanufactured replacement. Our rebuild vs. replace guide walks through how we make that call.

Transmission questions we hear every week

How does an automatic transmission work?

An automatic transmission takes the spinning power of the engine and delivers it to the wheels at the right gear ratio without a clutch pedal. The torque converter passes engine power into the transmission through fluid, the planetary gear sets create the different gear ratios, and the valve body, directed by the transmission control module, sends pressurized fluid to the clutches that select each gear.

How does a transmission know when to shift?

The transmission control module reads vehicle speed, input and output shaft speed, throttle position, engine load, and fluid temperature many times a second. When those numbers cross the shift schedule programmed for your vehicle, it energizes a shift solenoid in the valve body, which redirects hydraulic pressure to release one clutch and apply the next. Older automatics did the same job with a governor and a throttle cable or vacuum modulator instead of sensors.

What does a torque converter do?

The torque converter connects the engine to the transmission using fluid instead of a clutch. It lets the engine keep running while the car is stopped in gear, multiplies torque when you pull away from a stop, and locks up at cruising speed so no power is wasted. A worn lockup clutch inside it is a common cause of shudder between 35 and 50 mph.

What is a planetary gear set?

A planetary gear set is a compact group of gears with a sun gear in the center, planet gears orbiting it on a carrier, and a ring gear around the outside. Holding one member and driving another produces different gear ratios, which is how automatics change gears without sliding gears in and out of mesh. Most modern automatics stack two or more of them.

What does a valve body do in a transmission?

The valve body is the hydraulic control center of an automatic transmission. Its channels, valves, and solenoids route pressurized fluid to the right clutch pack at the right moment, which is what produces each shift. Harsh, delayed, or mistimed shifting often points to the valve body, and it can frequently be serviced with the transmission still in the car.

What are the main parts of an automatic transmission?

Torque converter, front pump, input shaft, planetary gear sets, clutch packs, bands and one-way clutches, the valve body with its solenoids, the transmission control module and its sensors, the fluid, filter and pan, the cooler and its lines, the case and seals, and the output shaft with the park pawl. Front-wheel-drive transaxles also carry the differential inside the same case.

What usually fails in an automatic transmission?

The parts that wear are the ones designed to slip: the torque converter's lockup clutch, the clutch packs, and the seals that hold pressure. Behind them, the most common failures we repair are valve body wear and solenoid faults, which cause shift problems without any mechanical damage. Fluid that is old, low, or the wrong type is the root cause more often than any single part.

What is the number one killer of transmissions?

Heat. Fluid that runs too hot breaks down, clutches start to slip, and slipping makes more heat. Old or low fluid, towing, stop-and-go driving, and a restricted cooler are the usual reasons a transmission runs hot. Changing the fluid on schedule with the exact type your vehicle specifies prevents more transmission failures than anything else you can do.

How do I know if my transmission is bad?

Watch for slipping, where the engine revs but the car does not pull; a shudder at highway speed; delayed or harsh engagement into Drive or Reverse; shifts that come late, early, or not at all; a burnt smell or dark fluid; red fluid under the car; and a check engine light with a P07 code. One symptom does not mean the transmission is finished. Many of these are fluid or valve body problems that are repaired with the transmission still in the car.

Is it better to rebuild or replace a transmission?

It depends on what failed and what the rest of the unit looks like. A rebuild replaces the worn parts inside your own transmission and keeps the case, which is often the right call when the damage is limited to clutches, seals, and the valve body. A remanufactured replacement makes more sense when the case is damaged, hard parts are scattered through the unit, or the design has known factory updates. We inspect first, then explain both paths before any work starts.

Should I put my automatic in neutral at a traffic light?

No, it is not necessary. The torque converter is designed to slip at idle in gear, and the small amount of heat it makes at a light is well within what the cooler handles. Shifting in and out of Neutral at every light adds an engagement cycle each time for no benefit. Leave it in Drive with your foot on the brake, and use Park for long waits like a train crossing.

Keep learning

Transmission Shudder Guide

Why the torque converter lockup clutch causes that rumble-strip feeling, and what fixes it.

Transmission Slipping Guide

What slipping feels like, why it happens, and when it is safe to keep driving.

Valve Body Repair Explained

The in-car repair that fixes most shift-quality complaints without pulling the transmission.

P0700 Transmission Code Guide

The code that tells you the transmission computer has stored a fault, and what to do next.

Transmission Fluid Guide

Why the exact fluid type matters, how to check it, and how often it really needs changing.

Rebuild vs. Replace

How we decide between rebuilding your transmission and installing a remanufactured unit.

Have a transmission question?

Bring the symptom, not the diagnosis. Our ASE Certified technicians road test, scan, and check the fluid before anyone talks about pulling a transmission. Rohnert Park, Cotati, Petaluma, and Santa Rosa drivers are all a short drive away.

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