Rick's Free Auto Repair Advice

Four wheel drive 4WD

What is 4WD?

Four wheel drive is usually abbreviated with this: 4WD. The term refers to a vehicle drive system that delivers power equally to each of the four wheels. 4WD systems can be part-time or full-time.

How 4WD works

The vehicle transmission takes power and torque from the engine and changes the gear ratios to maintain the lower engine RPM and improve gas mileage. A transfer case is connected to the output shaft from the transmission. It’s job is to transfer the power coming from the transmission and deliver it equally to a front and rear differential.

The differential then transfer the power to two front or rear axles, yet allows one wheel to rotate more than the other as the vehicles changes lanes and makes turns. If the vehicle is equipped with a limited slip differential, it automatically provides power to both wheels when one is slipping.

Advantages of 4WD

Because 4WD vehicles deliver engine power to all four wheels, you’ll get better acceleration in snow, ice, sand and slippery conditions because all four wheels are moving the vehicle instead of just two. The increased ability to accelerate often makes 4WD owners think they can drive faster than 2WD vehicles.  However, 4WD doesn’t provide any advantage when braking or turning in those same slippery conditions, so that over confidence can causes 4WD owners to wind up in a ditch far more often than 2WD drivers who must exercise more caution.

Disadvantages of 4WD

• higher vehicle cost

• lower MPG than the same vehicle equipped with 2WD

• more frequent and higher repair and maintenance costs than a comparable 2WD vehicle

What is part time 4WD?

Part time 4WD vehicles have a transfer case but no center differential. That’s an important distinction because the front and rear wheels rotate at different rates during a turn. If there’s no center differential and the vehicle is driven on dry pavement, that rotation difference can cause substantial damage to the drive axles. In fact, driving a part time 4WD vehicle on dry pavement can cause the vehicle to “hop,” bind, break axles and even destroy the front or rear differential. Part time 4WD can only be driven safely on snow, ice or wet roads or in mud or sand where the wheels can slip to accommodate differences in rotation between front and rear differentials.

part time 4WD diagram

What is full-time 4WD?

Vehicles with full time 4WD have a center differential or a viscous coupling that transfers power to the front and rear differentials but still allows for different rotation rates between the front and rear wheels.

full time 4WD diagram

What’s the difference between 2-Lo, 4-Lo and 4-High?

2-Lo and 4-Lo gears are designed to deliver maximum pulling power at very slow speeds. You would use either of those gears when pulling a heavy load, like 4WD shiftera boat, out of the water at slow speeds. Obviously the 4-Lo gear position would provide torque to each of the four wheels, while the 2-Lo gear would only provide torque to two wheels. The 4-High gear position is used when traveling at higher speeds where you want engine power delivered to all four wheels.

What is shift on the fly?

Older 4WD vehicles required you to stop the vehicle completely before shifting into or out of 4WD mode, or 4wd shift on the flyshifting between 2-Lo, 4-Lo or 4-high. Late model vehicles with shift on the fly allow you to shift into and out of those gears while the vehicle is moving.

Should you buy a 4WD or 2WD?

First, let’s take a look at the difference in price between a brand new 2WD vehicle versus the same vehicle with 4WD. I’m using a 2017 Ford Fusion SE as the comparison. Both have the 2.0 turbo engine and the same accessory package. Adding 4WD bumps the price by $1,750 and reduces MPG by 2MPG. The lower MPG adds approximately $1,200 to your operating costs every 100,000 due to the lower MPG rating, making the total price at least $2,950 more in the first 100,000 miles, not including increased maintenance and repair costs.

The 2017 Ford Fusion is a front wheel drive vehicle, so one money saving option is to invest in a set of winter tires and an extra set of wheels and tire pressure sensors. With winter tires, you’ll have slightly less acceleration from a stop than you’d have if it were 4WD. But you’ll have shorter stopping distances and better control in turns due to the winter tires’ better traction. Four winter tires, along with wheels and sensors cost around $1,000.

So you get almost the same traction leaving stop signs, but much better handling in all other driving conditions. Plus, a FWD drivetrain is less costly to maintain and repair. Winter tires save a minimum of $2,000 while giving better overall performance.

©, 2017 Rick Muscoplat




Posted on by Rick Muscoplat



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