Key Takeaways
- The transmission transfers engine power to wheels; automatic uses torque converter, manual uses clutch.
- Brake systems use hydraulic pressure: master cylinder, brake lines, calipers (disc) or wheel cylinders (drum).
- Steering systems include rack-and-pinion (most common) and recirculating ball (trucks/older vehicles).
- Suspension systems (springs, shocks, struts) absorb road irregularities and maintain tire contact.
- Exhaust systems reduce noise and emissions: manifold, catalytic converter, muffler, tailpipe.
Last updated: January 2026
Automotive Systems
Understanding how major vehicle systems work together is crucial for the ASVAB Auto Information subtest.
Transmission System
The transmission transfers power from the engine to the drive wheels at different gear ratios.
Transmission Types
| Type | How It Works | Advantages |
|---|---|---|
| Manual | Driver shifts gears using clutch and shifter | More control, better fuel economy, less expensive to repair |
| Automatic | Hydraulic system shifts gears automatically | Easier to drive, smooth shifting |
| CVT | Continuously variable; no fixed gears | Optimal efficiency, seamless acceleration |
Manual Transmission Components
| Component | Function |
|---|---|
| Clutch | Connects/disconnects engine from transmission |
| Flywheel | Connects to clutch; stores rotational energy |
| Clutch Disc | Friction disc that engages flywheel |
| Pressure Plate | Presses clutch disc against flywheel |
| Throw-out Bearing | Releases clutch when pedal pressed |
| Gear Shift | Lever for selecting gears |
| Synchronizers | Match gear speeds for smooth shifting |
Automatic Transmission Components
| Component | Function |
|---|---|
| Torque Converter | Fluid coupling; replaces clutch; provides torque multiplication |
| Planetary Gears | Set of gears providing multiple ratios |
| Valve Body | Hydraulic control center; directs fluid for shifting |
| Transmission Fluid | Lubricates, cools, and transmits hydraulic pressure |
| Bands and Clutches | Hold or release gear elements for shifting |
Drivetrain Configurations
| Configuration | Description |
|---|---|
| FWD (Front-Wheel Drive) | Engine and transaxle in front; powers front wheels |
| RWD (Rear-Wheel Drive) | Engine in front; driveshaft to rear axle |
| AWD (All-Wheel Drive) | Power to all wheels; often computer-controlled |
| 4WD (Four-Wheel Drive) | Selectable; transfer case splits power front/rear |
Brake System
Brakes convert kinetic energy (motion) into thermal energy (heat) through friction.
Hydraulic Brake Components
| Component | Function |
|---|---|
| Brake Pedal | Driver input; applies force to master cylinder |
| Master Cylinder | Converts pedal force to hydraulic pressure |
| Brake Lines | Carry brake fluid to each wheel |
| Brake Fluid | Hydraulic medium; DOT 3, 4, or 5.1 ratings |
| Brake Booster | Uses vacuum to multiply pedal force |
Disc Brakes (Most Common)
| Component | Function |
|---|---|
| Brake Rotor (Disc) | Rotating disc attached to wheel hub |
| Brake Caliper | Houses pistons and pads; squeezes rotor |
| Brake Pads | Friction material that contacts rotor |
| Caliper Piston | Pushes pads against rotor when pressure applied |
Drum Brakes (Rear Wheels/Older Vehicles)
| Component | Function |
|---|---|
| Brake Drum | Rotating cylinder attached to wheel |
| Brake Shoes | Curved friction material inside drum |
| Wheel Cylinder | Pushes shoes outward against drum |
| Return Springs | Pull shoes back when pressure released |
Anti-Lock Braking System (ABS)
- Prevents wheels from locking during hard braking
- Uses sensors to detect wheel speed
- Rapidly pulses brake pressure to maintain traction
- Allows driver to steer while braking hard
Steering System
The steering system allows the driver to control vehicle direction.
Rack-and-Pinion Steering (Most Common)
| Component | Function |
|---|---|
| Steering Wheel | Driver input device |
| Steering Column | Connects wheel to steering gear |
| Pinion Gear | Small gear connected to steering shaft |
| Rack | Horizontal bar with teeth; moves side to side |
| Tie Rods | Connect rack to steering knuckles |
| Steering Knuckle | Connects to wheel hub; pivots for turning |
Power Steering Types
| Type | Description |
|---|---|
| Hydraulic | Uses power steering pump and fluid |
| Electric (EPS) | Electric motor assists steering; more efficient |
| Electro-Hydraulic | Electric pump provides hydraulic pressure |
Recirculating Ball Steering (Trucks/SUVs)
- Uses gear box with recirculating ball bearings
- More durable for heavy-duty applications
- Less direct feel than rack-and-pinion
- Common on trucks and older vehicles
Suspension System
The suspension absorbs road shocks and maintains tire contact with the road.
Suspension Components
| Component | Function |
|---|---|
| Springs | Support vehicle weight; absorb bumps |
| Shock Absorbers | Dampen spring oscillation; control bounce |
| Struts | Combine spring and shock in one unit |
| Control Arms | Connect suspension to frame; allow wheel movement |
| Ball Joints | Pivot points allowing steering and suspension movement |
| Sway Bar | Reduces body roll during turns |
Spring Types
| Type | Description | Common Use |
|---|---|---|
| Coil Springs | Spiral wound steel | Most passenger cars |
| Leaf Springs | Stacked steel plates | Trucks, heavy-duty vehicles |
| Torsion Bars | Twisted steel bar | Some trucks and SUVs |
| Air Springs | Air-filled rubber bladders | Luxury vehicles, adjustable ride |
Suspension Types
| Type | Description |
|---|---|
| Independent | Each wheel moves separately; smoother ride |
| Solid Axle | Both wheels connected; simpler, more durable |
| MacPherson Strut | Common front independent; strut is part of steering |
| Double Wishbone | Two control arms; excellent handling |
Exhaust System
The exhaust system removes combustion gases, reduces noise, and controls emissions.
Exhaust System Components
| Component | Function |
|---|---|
| Exhaust Manifold | Collects exhaust from all cylinders |
| Exhaust Pipe | Carries exhaust gases toward rear |
| Catalytic Converter | Converts harmful gases (CO, HC, NOx) to less harmful substances |
| Muffler | Reduces exhaust noise through baffles and chambers |
| Resonator | Further reduces specific sound frequencies |
| Tailpipe | Exits exhaust at rear of vehicle |
| Oxygen Sensor | Monitors exhaust for fuel mixture adjustment |
Catalytic Converter Function
The catalytic converter contains precious metals (platinum, palladium, rhodium) that:
- Convert carbon monoxide (CO) to carbon dioxide (CO2)
- Convert hydrocarbons (HC) to water and CO2
- Convert nitrogen oxides (NOx) to nitrogen and oxygen
Test Your Knowledge
In an automatic transmission, what component replaces the clutch used in manual transmissions?
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D
Test Your Knowledge
What is the function of the master cylinder in a brake system?
A
B
C
D
Test Your Knowledge
What is the purpose of a catalytic converter?
A
B
C
D
Test Your Knowledge
Which component prevents a vehicle from bouncing excessively after hitting a bump?
A
B
C
D