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  Your car’s transmission works in a very similar way, but it’s housed in a big block of metal attached to the engine. Inside are gears connected to several spinning shafts that ultimately spin the wheels of your car.

  A block of metal houses your car’s transmission system.

  Inside that metal housing are gears not unlike the ones on a bicycle, switching from low settings (above left) to high settings (below left).

  The Four Types of Transmission Systems

  The type of transmission system in your car will affect the vehicle’s cost, along with the frequency and cost of maintenance and repair, so you should familiarize yourself with the options.

  Use this handy key to decipher the diagrams on the following pages.

  The wheels the transmission connects to and spins are called the drive wheels. Some transmissions connect to only the front wheels of the car, while others connect to the rear. Larger trucks and SUVs have all four wheels connected to the transmission. All transmission systems use a main transmission, which is attached to the engine, and several mini transmissions or gear boxes that help direct and control the power from the engine to the wheels. These gear boxes are called differentials and transfer cases.

  Front-Wheel Drive

  Front-wheel drive is the type of two-wheel-drive transmission you’ll find on most coupes, sedans, and minivans, as well as on some crossover SUVs and light trucks. This is the most basic transmission system, composed of the fewest parts and requiring the least maintenance.

  A single transmission is used in a FWD system—the main transmission (also called a transaxle), which is attached to the front wheels by axle shafts. The rear wheels follow along once the front wheels begin to spin.

  + Pros: The FWD transmission is the most cost-effective at purchase. It puts the driving wheels at the front of the car, giving the tires better traction, which is particularly helpful if you are driving or stuck in snow, sand, or mud. (Because the engine and transmission are located under the hood, the front of the car carries most of the car’s weight—hence its improved traction.)

  - Cons: The front tires of a front-wheel-drive vehicle do the accelerating, the steering, and most of the braking, so they wear faster than the rear tires. That annual tire rotation becomes increasingly important. Front-wheel-drive cars aren’t as fun to drive if you like speed, driving around curves, and performance.

  Rear-Wheel Drive

  Rear-wheel drive is a two-wheel-drive transmission system found on smaller, sportier cars and two-wheel-drive trucks. Rear-wheel drive handles better around corners and for quick acceleration or braking.

  In an RWD system, the main transmission and the differential drive the rear wheels on the car together. The main transmission is attached to the two rear wheels by a driveshaft, U-joints, the differential, and axle shafts.

  + Pros: Better handling in dry road conditions than an FWD transmission, and faster acceleration or “pickup.” When you accelerate in any car, the weight of the vehicle shifts backward—and with RWD you’ve got the driving wheels picking up the slack. (The extreme version of this is a wheelie, which only a power engine can do, when the front tires lift off the ground.)

  - Cons: More expensive at purchase. May cost more in gas. Since most of the weight of the car is in the front, the rear tires don’t have as much traction as on an FWD vehicle. They do not perform as well when driving in snow, sand, or mud as an FWD. If you have an RWD vehicle, take extreme caution in poor road conditions to avoid getting stuck.

  Four-Wheel Drive

  Now we’re getting a little fancy! Front-wheel-drive and rear-wheel-drive systems are great for coupes and sedans, but trucks and SUVs require something a little more robust. These vehicles carry heavy loads, tow cargo, and have large parts and tires that require more engine power, and that’s what a 4WD transmission helps provide—along with great traction in adverse and off-road driving conditions.

  + Pros: Great road traction to keep you safe when driving in adverse weather conditions or off-road. More power for towing cargo or another car.

  - Cons: More expensive to buy, maintain, and repair. Requires more gas, and more frequent tire rotations.

  In a 4WD system, the main transmission, a transfer case, and two differentials (one front and one rear) drive the front and rear wheels at the same time.

  Part-Time Four-Wheel Drive

  A part-time four-wheel-drive transmission system doesn’t need to be activated at all times. Full-time 4WD vehicles are also known as AWD vehicles, covered in the following section; part-time 4WD systems have you switching between 2WD and 4WD, and between high and low gears, depending on the circumstances. Here’s how and when to use each.

  • 2WD High, aka 2H, 2Hi: During normal driving conditions. Essentially allows the car to run on an RWD transmission system.

  • 4WD High, aka 4H, 4Hi: If you need more traction when driving in ice, snow, sand, or other common off-road conditions.

  • 2WD Low, aka 2L, 2Lo, or 2Low: If you are towing something heavy.

  • 4WD Low, aka 4L, 4Lo, or 4Low: If you are towing something heavy in ice, snow, or sand, or if you are driving in extreme off-road conditions.

  In part-time 2WD, the transmission reverts to rear-wheel drive, with the transfer case spinning only the rear differential and rear wheels.

  All-Wheel Drive

  Once limited to Subarus, AWD transmission systems are popping up on average cars everywhere. Now some sedans and many SUVs and trucks come equipped with AWD transmissions. An AWD vehicle is essentially running in 4WD High (useful if you need more traction, when driving in ice, snow, or sand, or other common off-road conditions) at all times.

  + Pros: Better handling around corners. Great in all driving conditions, on- and off-road. Most people buy AWD vehicles specifically for off-road conditions.

  - Cons: More expensive to buy, repair, and maintain. Should be considered a luxury item for added safety and drivability unless you live in an area that gets a lot of snow and ice. You will have to take better care of your tires and match all four tires for wear and brand. That means if you replace one worn-out tire, you’ll likely have to replace all four.

  Most all-wheel-drive systems use the main transmission along with three differentials (situated in the front, rear, and center of the car) to drive both the front wheels and the rear wheels at the same time.

  Your Wheels, Your $$$: Don’t Spend Money on Unnecessary Transmission Power

  If you don’t live in a place that gets a lot of snow, don’t need to carry large loads, and/or don’t plan on driving off-road, there’s really no advantage to buying an AWD or 4WD vehicle. You’ll spend more up front and on maintenance and gas.

  But if snow and ice are common during your winter season, you should consider buying an AWD or 4WD vehicle; common doesn’t mean you see snow a handful of times per winter and typically get less than 3 inches.

  The least expensive systems to buy, maintain, and repair, FWD transmissions are perfectly fine for coupes, sedans, minivans, or crossover SUVs, especially if you don’t frequently encounter a lot of snow.

  Four-Wheel Drive and All-Wheel Drive and Your Tires

  A difference in diameter of less than half an inch between front and rear tires on a 4WD or AWD vehicle can mean trouble for your transmission system. And as tire tread wears down, tires shrink. A new tire is rounder (and therefore taller) than a tire that has been driven for many miles. A bigger, newer tire travels more distance in a single revolution than a smaller, used tire. So to compensate, a smaller-diameter tire will spin faster than a bigger-diameter tire. On a 4WD or AWD vehicle, heat buildup from a smaller tire spinning faster can cause stress to axles, shafts, joints, and differentials, also confusing the systems that monitor transmission and wheel speed. The powertrain control module may think that the transmission is slipping or that conditions are snowy or rainy, and automatically shift gears or adjust the transmission.

  For this reason, when a tire needs to be replaced on a 4WD ve
hicle, it is best practice to replace the tire on the opposite side as well, and to match tire brand while you’re at it. If a tire needs to be replaced on an AWD vehicle, it is more than likely that you must replace all four tires.

  Check your owner’s manual or call your PCT to find out. Ignoring this advice may result in costly damage to some of the most important driving components of your car, including the transmission.

  TRANSMISSION MAINTENANCE AND REPAIRS

  PART

  ISSUE

  FIX

  TIMING

  COST

  Automatic Transmission Fluid

  Not maintaining the automatic transmission fluid as needed can cause hard shifting or slipping—when the gears inside the transmission aren’t catching as they shift. While driving, it will feel like the car wants to speed up as you press on the gas pedal, but it will hesitate before the gear catches and the car accelerates. If you can check the transmission fluid, it will smell like burned crayons when your system is in need of a flush

  On some vehicles, needs very occasional fluid flushes and filter changes

  See your owner’s manual, but we’re talking once every five to ten years

  For most cars $250 or less

  Differential Oil or Fluid

  Oil or fluid breaks down over time, creating wear on the transmission’s gears

  Oil or fluid may need an occasional change-out

  See your owner’s manual

  For most vehicles $150 or less

  Main Transmission

  Transmissions will fail if they are poorly driven or maintained. Some just fail from use over time. One classic way that all of us hurry-up-and-wait types wear out our transmissions is by trying to accelerate faster than our cars’ engine power can manage

  If the car won’t shift, doesn’t drive in reverse or in drive, or exhibits extreme slipping, the transmission will likely need replacing

  Variable

  Expect to spend at least $2,000 to replace a failed main transmission with a used model

  Constant Velocity (CV) Axle Shafts and CV Joints

  Located on the front of the vehicle to connect the main transmission to the wheels, CV axle shafts attach to the wheel bearings and fail from use over time. The point of failure most often seen is the CV joint boot, a rubber covering designed to keep dirt and air from damaging the lubricated joints

  A PCT will replace a CV axle shaft when the boot is torn or ripped; if the joint fails, the shaft will need to be replaced

  When a CV axle shaft needs to be replaced, you will hear thumping noises coming from the wheels

  Often, used CV axles can be installed for some savings, but make sure they come with a warranty. Less than $350 per wheel

  Driveshafts and U-Joints

  Driveshafts and U-joints on 4WD and AWD vehicles will fail from use over time, from corrosion, or from driver abuse (accelerating too quickly, not replacing tires in pairs or sets); U-joints often make bumping noises when going from drive to reverse or vice versa when they need to be replaced, with the sound coming from under the car seats

  Replace; sometimes the driveshaft will simultaneously need to be replaced

  Varies depending on what needs to be replaced

  $500+

  Differentials

  Differentials on 4WD and AWD vehicles will fail if they are not properly maintained. Signs of failure include grinding noises from the rear of the car while cruising or turning

  When a rear differential needs to be replaced, the entire rear axle or rear end (including the differential and axle shafts) will have to be replaced as well

  Variable

  Greater than $1,000

  Transfer Case

  Transfer cases don’t normally fail, unless they aren’t properly maintained through occasional automatic transmission fluid flushes (see page 71) or suffer from driver abuse

  Replace

  Variable

  Greater than $1,000

  Clutch

  On manual (aka stick-shift) transmissions, the clutch will wear out from use over time and from driver abuse, and the car will slip when you try to shift gears

  Replace

  If driven well, a clutch will last 100,000 miles or more and may need to be replaced only once during a car’s lifetime

  Clutch replacement is an involved job, requiring removal of the transmission; greater than $750

  Common Auto Airhead Mistakes: Transmission

  Transmission systems may be low-maintenance, and you gotta love ’em for that. But a transmission issue is a real pain and quite an expense, so avoid the following common mistakes to keep yours in tip-top condition:

  • Ignoring problems during acceleration and when shifting gears.

  • Failing to maintain the automatic transmission fluid and differential fluids. Failing to flush the automatic transmission fluid can cause hard shifting, slipping, and damage to the differentials and transmission.

  • Incorrectly using part-time 4WD.

  • Failing to replace tires properly on vehicles with AWD transmission systems.

  • Failing to rotate tires at least once a year.

  • Driving a four-cylinder engine like it’s a V-6. Ease up on that gas pedal, speed demons.

  Brakes: The Most Important System in Your Car

  Now it’s time to get down to business about what might be the most important system in your entire vehicle: the miracle of engineering that prevents a 4,000-pound car from crashing into another car, person, or tree. Slowing down or stopping a moving object of that weight takes a lot of force, much more than our little bodies and right feet can generate. It involves physics, and processes like energy, gravity, friction, and hydraulics (a fancy word for moving a liquid around to do work).

  Brake Repair Time Line

  The most common questions I see online are about how often drivers should replace their brakes. There’s no definitive answer—because it all depends on how you drive. If you live in a city or a suburban area and often drive in stop-and-go traffic, you’ll need to replace your brake pads and other parts of your brake system more frequently than if you drive mostly highway miles. On the highway, you’re braking less, so the pads and other parts of the brake system don’t wear as fast. Either way, brakes give you lots of audible and palpable feedback about their condition, so they will let you know when it’s time to replace them, even if nothing surfaces during your regular maintenance checkup. Brakes often last at least two years, unless you drive for a living.

  Front brakes, especially on front-wheel-drive vehicles, wear at a quicker rate than rear brakes, both because the engine and transmission weigh down the front of the car and because the weight of the car and its cargo shifts forward as you brake.

  One way to make a DIY assessment of the condition of your brakes is to check your brake fluid (see page 172 for instructions). Your brake system is a closed system, meaning the fluid stays inside and is not affected by any outside forces or matter (air, other fluids). That means if the brake fluid is low, you don’t add fluid. Low brake fluid is an indication that you need new brake pads (more likely) or you have a leak (less likely). If it’s the former, new brake pads or shoes with thicker friction material will cause brake fluid levels to rise back to full. Do not top off your brake fluid!

  The intricate brake system recruits hydraulic power and friction to generate the force required to slow or stop a car.

  The Parts and Processes of a Brake System

  There are lots of little actions going on between the moment you hit the brake pedal and the slowdown at the wheels. Here’s a snapshot of the parts and processes involved. Follow along on the big picture illustration (opposite) to get a sense of how it all comes together.

  Pedal. The central command station for the braking system, the pedal is set off by your foot applying force to it.

  Brake Booster. It takes at least 800 and up to 2,000 pounds of pressure to slow down a 4,000-pound car. Humans need an assist, and fortunately the brake
pedal is connected to a gadget called a booster. For you Super Mario Bros. fans, a brake booster is a mushroom that powers you up to a bigger, stronger Mario or Princess Peach, doubling the force generated by your foot. Most standard cars have vacuum boosters, while luxury cars have what’s called a hydraulic booster. Either way, boosters involve complex engineering and aren’t common points of failure.

  Hydraulic System. Once it leaves the pedal, the boosted force initiated by your foot connects with a gadget under the hood called a master cylinder, topped by the plastic reservoir that holds your brake fluid. Boosted force pushes that fluid into the master cylinder and through the metal brake lines (pictured opposite), then down to each wheel.

  Wheel Brakes. Most cars come with disc brakes at all four wheels, but some cars have disc brakes at the front wheels and drum brakes in the rear (see opposite).

  Disc brakes (most common)

  Side view of a rotor

  With disc brakes, boosted braking force comes in contact with a caliper, which squeezes or clamps brake pads against a spinning rotor—a flat metal disc attached to the car’s wheels. The caliper straddles the rotor and houses the brake pads.

  The friction caused by contact between nonmoving parts (the brake pads) and a moving object (the rotor) causes the spinning rotor to slow down and eventually stop. Because the rotor is attached to the wheel, when the rotor slows down, the wheel slows down.

  Drum brakes (less common)

  With drum brakes, boosted force comes in contact with a wheel cylinder, which pushes brake shoes up and out against a rotating drum. Drum brakes work very similarly to disc brakes—using friction to slow down a spinning wheel—but their components are housed inside that round drum, and metal springs are used to hold the shoes in place.