Hill Start Assist: What You Need to Know

Hill Start Assist: What You Need to Know

You may have grown up driving a manual transmission vehicle and remember the fear of having to stop on a hill at a stoplight. When the lights changed, you would quickly release the clutch and the brake and press the accelerator, hoping that you wouldn’t roll back into another car.

Rolling back into the car behind you isn’t only a fear for stick-shift drivers. People who drive automatics face this problem, too. But thanks to a technological feature called “Hill Start Assist,” rolling backward is a problem of the past. What is Hill Start Assist? Read on to find out.

What is Hill Start Assist?

Hill Start Assist is a technology that helps you smooth out starts on hills without rolling backward. But how does it work exactly?

When you stop your vehicle on a hill, a pitch sensor alerts the vehicle’s engine control unit (ECU) to momentarily hold the brake while you move your foot to the gas pedal, so you don’t roll backward. Gravity loses out to technology in this battle.

You may have recently discovered Hill Start Assist, but it’s been around since 1936. It was invented by Wagner Electric, and installed as a standard feature for Studebaker’s premier model, The President. The following year, the device was named “NoRol” and was available on the Hudson, the Nash, and other cars. At that time, the Hill Start Assist activated when a steel ball would roll back and block brake fluid return to the brake master cylinder. A bypass valve attached to the clutch pedal would release the brakes when you fully released the clutch.

Hill Start Assist may have been invented in the ‘30s, but it was only recently adopted on a broader scale. With updated technology, VW included it in its 2005 car lineup. These original 2005 Hill Start Assist models were available only with automatic transmissions. Because of vehicle manufacturers’ adoption of electronic stability controls, Hill Start Assist is now also available with manual transmissions. You can even get it on a $13,400 Chevy Spark.

How to use Hill Start Assist

If not for learned anxiety, you may not have been aware that Hill Start Assist was working. There is very little for you to do other than drive as you normally do. But that’s not to diminish the value of this feature, especially for manual shifting vehicles.

To start a manual shift car, you always step on the clutch to disengage the engine’s transmission. But disengaging the clutch also prevents you from using the engine’s power to brake the car. And if you aren’t using the brakes, the vehicle will roll freely on an incline. Let’s check out the process for using Hill Start Assist in manual and automatic transmission vehicles.

With a manual transmission

While stopped on an incline:

  1. Hold the brake and keep the clutch pedal pressed using the same pressure you normally would.
  2. When the light changes, shift your foot away from the brake and then the clutch while pressing the accelerator. The car should remain stopped instead of rolling back until it accelerates.

With an automatic transmission

When stopped on a hill:

  1. Hold the brake using the same pressure as you normally would.
  2. When the light changes and you shift your foot away from the brake pedal momentarily to press the accelerator, the car should remain stopped instead of rolling back slightly.

It really is that simple. If your car’s Hill Start Assist is working properly, you shouldn’t really notice it at all.

How does Hill Start Assist work?

Advanced systems work to prevent your vehicle from rolling forward and backward. As simple as its operation may sound, there are complicated systems to make sure it seamlessly works every time. Let’s check them out.

Hill Start Assist control systems

The following systems are common components of a Hill Start Assist feature, though you won’t find all of them on every vehicle.

Incline detection

The incline sensor can detect an incline that is more than a set amount and communicate to the control unit that there is a potential to roll backward.

Torque detection

This torque sensor determines if the engine produces enough torque to move the car and, if so, disables the Hill Start Assist.

Brake detection

Brake detection senses whether or not the brakes are in use and if they’re exerting sufficient force to hold the car in place.

Clutch detection

Clutch detection senses the activation and release of the clutch. In some models, the speed of the action is also monitored.

Backward roll detection

This sensor automatically determines if the car is in danger of rolling backward through other sensors’ information and whether the brake has been applied. If the brake has not been applied, the system will automatically activate the brakes to keep the car stationary.

Forward roll detection

When on a decline, you must select the reverse gear, which lets the system know that you’re trying to back up a hill without rolling forward. After you take your foot off the brake or clutch, the Hill Start Assist will hold for around two seconds and then smoothly release when you accelerate.

Again, not every vehicle will include every system. Depending on your manufacturer and, especially, your model, most Hill Start Assist systems in today’s vehicles fall into one of three categories:

Basic Hill Start Assist systems

Sensors used in the most basic systems include:

  • Brake pedal travel sensor
  • Master cylinder pressure sensor
  • Wheel speed sensors
  • Throttle position sensor

The basic system’s sensors will not include an incline or decline sensor.

Medium-level Hill Start Assist systems

These systems are more complex. They include similar sensors to those used by the basic-level systems, plus:

  • Clutch position switch (for manual transmissions)
  • Longitudinal acceleration sensor
  • Reverse gear sensor

Premium-level Hill Start Assist systems

The premium-level system is similar to the mid-level system, but generally includes the following extra sensors:

  • Clutch travel sensor for determining the speed with which the clutch pedal is pressed or released
  • Hill-decline sensor that allows for reversing the vehicle when stopped on a downhill angle

Hill Start Assist technology

The ECU is the brain of your vehicle and receives communication from sensors around the car, including the Hill Start Assist group of sensors. Using that data, the ECU then decides when it should hold the brakes. It then collects data about your vehicle’s weight and the hill’s angle to apply the needed pressure that holds your car before accelerating the car uphill. Let’s see what sensors are involved and what they do.

Pressure sensors

These measure weight distribution across the vehicle’s suspension to detect a hill. In some models with active suspensions, pressure sensors detect the car’s total weight and vertical positioning. This data detection functions while the vehicle is in motion or stationary.

Wheel speed sensors

These detectors, often part of the anti-lock braking system (ABS), can determine the speed and direction the wheels are turning.

Angle sensors

Angle sensors alert the ECU when your vehicle’s on an incline and the amount of slope angle.

Brake actuator

The brake actuator converts a signal from the ECU, telling it to trigger the brakes. It quickly activates the brake valves, and fluid flows to the brakes as if you’ve continued pressing the pedal. For a hybrid, the electric motor will offset the force of gravity with enough power to stay stationary.

Torque sensor

Torque is the force generated by the engine to accelerate your vehicle from a stop. The torque sensor monitors how much power is transmitted to the wheels and informs the ECU when it’s time to release the brake.

Does Hill Start Assist work on every hill?

No. Hill Start Assist is a useful safety technology, but it does have a few important limitations.

Hill Start Assist doesn’t improve traction

Since Hill Start Assist’s only purpose is to help you transition from being stationary to accelerating smoothly, it won’t directly improve your traction. If the road is slippery, the car’s traction control will limit any wheelspin when accelerating, but Hill Start Assist only attempts to prevent you from rolling backward. If the road is icy and steep enough, you may slide backward regardless of the system’s attempt to stop it.

Hill Start Assist is not an indefinite hold

Unlike Auto Hold technology available in some vehicles, Hill Start Assist will hold only for a few seconds before it releases. Once you take your foot off of the brake and clutch, you have a few seconds to accelerate before the brakes release. Auto Hold keeps your car stationary without your braking assistance. It will hold the vehicle for an unlimited time or until you touch the accelerator.

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At Bumper, we are on a mission to bring vehicle history reports and ownership up to speed with modern times. A vehicle is one of the most expensive purchases you'll likely make, and you deserve to have access to the same tools and information the pros use to make the right decisions.

About Steven Mitchell

Steve’s first car had pedals, and he’s been filled with passion for four wheels ever since. Steve lives and breathes the auto industry as a technologist, futurist, and writer while keeping up with the whys, hows, and plans of the automotive industry. His automotive journey began with a fully restored Triumph TR3b that he sadly never appreciated until decades later. Ownership of X-1/9’s, BMW’s, an SVT and a Dodge Ram Hemi have given way to an Audi Sport.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.