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The Difference Between 4WD High and 4WD Low


Published October 24, 2018

Most 4WD pickup trucks and more off-road oriented SUVs on the market will have a dual-range transfer case that provides the driver with multiple driving ranges, typically 2H, 4H, and 4L. The selection of any of these three options engages a different set of gears in the transfer case, typically mounted directly behind the transmission, which delivers power via the wheels based on the option the driver selected. The selection of the drive mode is either by the old-school lever on the floor or a more modern switch on the dash or center console.
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Before we go much deeper, it's important to understand how a car, truck or SUV goes around a corner. Imagine a snowy day and the car in front of you is turning onto a street or driveway that not only hasn't been plowed but hasn't yet been driven on. One of the things you'll notice is that the inside two wheels (the right side for a right-hand turn) travel a shorter distance than the outside wheels (a difference in the radius of the two sides). In order for the rear axle to allow the two driven wheels to achieve these two different radii, they cannot be locked together. Instead, what we have is an open differential, which allows the driven wheels to operate at different speeds. The bad news is that an open differential, by its nature, will route power to the wheel with the least amount of traction, which is why you can find yourself in a situation where one wheel is spinning furiously in the snow, ice, or mud, while the other just sits there.

You may have read about a Limited Slip Differential being fitted to more performance-oriented cars and trucks, which allows the two drive wheels to operate at different speeds but splits the power more equally between the two drive wheels. This is done largely to improve acceleration in a performance car to get the power down onto the road. Now that we have an understanding of how a powered vehicle gets around a turn we can jump into the different modes of a 4WD or 4x4 system.

As mentioned, you'll be faced with three options: 2H, 4H, and 4L. 2H is used for normal road driving. Two wheels are driven, usually the rear, in high-range, the normal gear ratio used for everyday driving. It will typically be fitted with a limited slip differential for added traction even in two-wheel drive mode. Some vehicles may be constant 4WD or all-wheel drive, in which case they won't have the 2H option.

4H is for when you need extra traction at regular speeds. These situations might include dirt roads, ice- or snow-covered streets, or hard-packed sand. Both front and rear wheels are engaged in high-range, using the same gear ratio as 2H for normal driving speeds. The front wheels aid in traction, making the vehicle more stable on loose surfaces. On 4H the wheels operate independently so you're able to turn a corner without causing mechanical damage to your vehicle.

4L is for when you need maximum traction and maximum power: sharp inclines and declines, deep mud or snow, loose sand, and clambering over extremely rocky surfaces. Both front and rear wheels are driven in low-range, which uses a lower gear ratio. The wheels turn much more slowly per revolution than they would in high-range, resulting in slower road speed but higher torque, which assists with traction and steep ascents. The lower gearing gives the vehicle better engine braking, making it easier to control on the descent. In 4L all four wheels are locked together to provide maximum traction. It's not to be used except for low speeds as when you turn a corner the system is literally fighting itself to negotiate the curve. That's why your vehicle and owner's manual both warn against driving in 4L at speeds or on dry surfaces.

If your 4WD vehicle has manual locking hubs, located in the center hub of the front two wheels, you'll have to engage them when you're using either 4H or 4L. These lock the hub and wheel to the front differential output shafts. Some vehicles are equipped with self-locking hubs or hubs that can be locked from inside the cabin. Even with 4H or 4L engaged, the front axles won't transmit power unless the hub is in the locked position.
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