Hill Descent Control Systems

steep descent
Hill descent control is designed to make it safer to descent steep inclines in rough terrain. Image courtesy of Andy Arthur, via Flickr (Creative Commons 2.0)

Hill descent control is a car safety feature that’s designed to facilitate safe travel down steep grades. The feature is primarily intended for use in rough terrain, but it can be used whenever a driver wants to descend slowly down a steep hill. Unlike cruise control, which typically only works above a certain speed, hill descent control systems are usually designed so that they can can only be activated if the vehicle is moving slower than 15 or 20 mph.

The specifics vary from one OEM to the next, but it is generally a low-speed technology..

The History of Hill Descent Control

Bosch developed the first hill descent control system for Land Rover, which introduced it as a feature of its Freelander model. The Freelander lacked the low range gear box and differential locking features of the Land Rover and other 4x4 off road vehicles, and HDC was billed as a fix for that situation. However, the initial implementation of the technology suffered from a few drawbacks, such as a preset speed that was too high for many situations. Later implementations of hill descent control, both by Land Rover and other OEMs, either set a “walking pace” speed or allow the driver to adjust the speed on the fly.

Low Speed Cruise Control for Rough Terrain

Like many other automotive safety features, and advanced driver assistance systems, hill descent control automates a task that a driver would normally have to do manually.

In this case, that task is controlling the speed of a vehicle on a down slope without losing traction. Drivers typically accomplish that by downshifting and tapping the brakes, which is also the same basic method utilized by hill descent control systems.

The way hill descent control works is very similar to the way that traction control and electronic stability control work.

Just like those systems, HDC has the ability to interface with the ABS hardware and pulse the brakes without any input from the driver. Each wheel can be independently controlled in this manner, which allows the hill descent control system to maintain traction by locking up or releasing individual wheels as the need arises.

How Do You Use Hill Descent Control?

Hill descent control systems are offered by a number of OEMs, and the precise operation of each system is slightly different. In every case, the speed of the vehicle must be below a specific threshold before hill descent control can be activated. Most OEMs require the vehicle to be below about 20mph, but there are some exceptions. In some cases, such as the Nissan Frontier, the speed threshold changes depending on the gear setting. The vehicle typically also has to be in either forward or reverse gear and on a grade before hill descent control can be activated. Most vehicles with HDC have some type of indicator on the dash that shows when all of the conditions are met and the feature is available.

When all of the prerequisites are met, hill descent control can be activated by pressing a button. Depending on the OEM, the button may be located on the center console, below the instrument cluster, or elsewhere.

Some OEMs, such as Nissan, use a rocker switch instead of a simple button.

After hill descent control has been activated, each system is operated a little differently from the others. In some cases, the speed of the vehicle can be controlled by the cruise control buttons. In other cases, the speed can be increased by tapping the gas and decreased by tapping the brake.

Who Offers Hill Descent Control?

Hill descent control was originally introduced by Land Rover, and it’s still available on models like the Freelander and Range Rover. In addition to Land Rover, a number of other OEMs have also introduced similar features on SUVs, crossovers, station wagons, sedans, and trucks.

Some of the other OEMs that offer hill descent control include Ford, Nissan, BMW and Volvo, but more look at adding it somewhere in their line each year.

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