While the multitude of safety and driver assist systems such as HDC (Hill Descent Control) fitted to especially SUV’s, crossovers, and almost any other vehicle that has even a modicum of off-road capability today no doubt improves the driving experience, all these systems add layers of complexity to vehicles that even experienced technicians find difficult to untangle sometimes.
However, since many of these systems are tacked onto the ABS system, understanding how the ABS system works removes much of the perceived mystery that surrounds these systems and as a result, diagnosing issues in systems such as HDC becomes much easier. In this guide, we will briefly discuss what HDC systems are, how they work, why they fail, and how to diagnose common issues, starting with this question-
The first system was developed in 1993 by Bosch for use by Land Rover on the Freelander model range, primarily to replace the low range gear set and transfer case that was previously used by most applications to control a vehicle’s speed while descending steep hills.
However, while the new HDC system managed to control a vehicle’s speed effectively when travelling downhill, many users considered this speed to be too high to be safe. To address these concerns, subsequent iterations of the system included features that allowed a driver to select an appropriate speed, while at the same time limiting the maximum downhill speed to 30 km/hour.
Modern-day applications of the system include many other features, such as automatic activation/deactivation, and the integration of the HDC system into GPS-based Cruise Control, Electronic Brake Force Distribution, and even Autonomous Braking systems. Note however that not all manufacturers include all options on all models.
While all major car manufacturers offer HDC on at least some of their products (and use different names for their versions), all versions work in the same way, in the sense that all versions use the ABS brake system to control the vehicle’s speed by applying braking forces selectively when the HDC System is activated. In practice, a HDC system will control a vehicle’s speed while descending a hill without any braking inputs from the driver.
In simple terms, when the HDC system is activated – provided that all enabling conditions are met – the wheel speed sensors supply the ECU and the ABS control module (and the Transmission Control Module on automatics), with input data on the rotational speed of each wheel. Based on the speed selected by the driver, the ABS system will then maintain braking force on the wheels to keep the vehicle at the selected speed until the system is deactivated by either the driver, or automatically, when the system determines that the vehicle is again on an even keel.
Moreover, on automatic vehicles, the ECU will also assume control of the transmission to prevent gearshifts that can cause issues with traction and vehicle stability during the descent, as well as control of the throttle to limit throttle inputs by the driver to those that are required to either select a different speed, or to deactivate the HDC system. However, it must be noted that under some conditions, some versions of the technology will force the automatic transmission to shift to a lower gear, and engage the torque converter lock-up clutch to increase engine braking.
Note that during the descent the ABS system will like during normal ABS operation, release brake pressure on any wheel that locks up until that wheel again matches the rotational speed of the other wheels. Note also that whether or not deactivation happens automatically depends on the make and model of the vehicle.
Nonetheless, some versions of the technology allow the driver to increase or decrease the vehicle’s speed during the descent, either by using the cruise control system’s switches / controls, or by momentarily tapping the brake or accelerator pedals while the system is in operation.
On automatic vehicles, the transmission must be in “D”, “R”, or in any other manually selected gear. However, most systems also demand that the vehicle’s motion be below the maximum speed that can be selected when the HDC system is in operation, which is typically below 30 km/hour, before the HDC system can be activated. On manual transmissions, either reverse or first gear must be selected.
While a HDC system is a very useful feature to have especially in the off-road environment, the usefulness of these systems on conventional vehicles is severely limited. Unlike hybrids and electric vehicles that use a variety of braking strategies that include increasing levels of regenerative braking in addition to friction braking in their HDC systems, conventional applications have to rely on friction and engine braking only.
In practice, HDC systems are intended to be used on short, steeply inclined driveways and off road obstacles that can be negotiated in less time than it takes for the brakes to overheat. In fact, on many applications, brake temperature is one of the parameters that is monitored when the system is in operation and in a fully functional system, excessively high brake temperatures will cause the system to be deactivated automatically.
As a practical matter, there is a direct correlation between the mass of a vehicle, the speed at which the HDC system is set to control the vehicle’s speed, the angle of the descent, and the time it takes for the brakes on that vehicle to overheat. Therefore, in the case of a heavily laden vehicle, or a vehicle that is towing a trailer, the system may be automatically deactivated due to excessively high brake temperatures before the bottom of the decline is reached- and quite possibly end up with damaged or destroyed brakes.
One other disadvantage of some HDC systems is that not all systems illuminate the brake lights on the vehicle when the system is in operation, which creates an obvious safety risk on applications that do not illuminate the brake lights.
In addition to the above, HDC systems are not safe to use on loose /sandy surfaces, soft mud, or extremely steep declines that are not paved. In these conditions, activating the system may cause a complete loss of traction which may result in the vehicle falling off a cliff, in a manner of speaking.
Even though all HDC systems work on the same basic principle, diagnostic and repair procedures are for the most part manufacturer specific. However, since all HDC systems use the ABS and traction control systems to control a vehicle’s speed when travelling downhill, any fault, malfunction, defect, or failure in the ABS system will almost certainly affect all safety and driver assist systems that depend on the ABS system to work.
It should be noted however that despite the similarities between HDC systems, there are significant differences between HDC systems in terms how they are activated /deactivated, and which operating parameters are monitored. Moreover, code-setting parameters also differ between systems, as do the levels of control over the system a driver can exercise when the system is in operation.
In practical terms though, the most common warning message you will see on an HDC-equipped vehicle will read, “System Unavailable”, “System Disabled” (or similar), which may or may not be accompanied by a flashing ABS, and/or Traction Control warning light. However, the good news is that since one or more warning lights are illuminated, there will be one or more trouble codes stored as well, which may relate to anything from issues with one or more failed or failing wheel speed sensors, to a failed brake light switch, to a failed ABS control module or hydraulic unit, or to communication issues between two or more control modules.
Nonetheless, successfully diagnosing issues with HDC systems depends on two things. The first is to understand how the system works in terms of understanding the roles of the various controls, switches, and push buttons that control the HDC system: the second is not forgetting that any HDC system is merely an extension of the ABS system, and resolving the underlying ABS issue will therefore almost certainly resolve the HDC system issue as well.
In conclusion, the only other advice this writer can offer when you encounter issues with HDC systems is this-
