Every day in every way, electronics are taking over mechanical tasks. Thanks to alleged unintended acceleration, almost everyone now knows about throttle by wire. But parking brakes by wire still seems like a secret for many, even professionals.
Yet, the technology is not so new. Electronic parking brakes have been around since the mid-2000s. The 2004 Audi A8 had them. Since the 2009 model year, all Audi vehicles have electronic parking brakes. So do some Volkswagens, Volvos, Cadillacs, Buicks, Jaguars, Lexuses and more.
There are two basic designs. The first, and simplest, still uses cables to actuate the parking brakes in the traditional manner. Cables from the wheels attach to a motor that pulls and releases similar to the way a manual lever pulls and releases. But an electrical switch replaces the lever. Lexus, for example, uses this design.
Closing the circuit sends a signal to the parking brake module that, in turn, turns the reversible motor to apply the brakes. Upon close examination, you may think that the motor looks a lot like a window regulator motor, and you would be right. Of course the torque is multiplied substantially by a transmission.
With interior real estate growing more valuable as more and more amenities and infotainment features are added, this switch for the electronic parking brake on the Caddy CRS-V takes up far less space than the mechanical lever or pedal.
The second design has no cables. Instead, the actuators are attached directly to the rear calipers. A signal to the controller relays power to the motors to clamp the brake pads against the rotors. The distance the pads have to move is minimal, but must be maintained through regular adjustment, which is done automatically. Volkswagen, for example, uses this design.
In most cases, a parking brake switch, either a pull on/push off or a push on/push off, actuates the parking brakes. In some cases, the brakes are actuated when the transmission is shifted into park. On systems with the caliper-mounted motors, the electronic parking brake can double as a hill-hold for vehicles with a manual transmission.
Of course, these command circuits have been added to the vehicle CAN bus which has been on virtually all cars sold in the U.S. since it was made mandatory with the 2008 model year.
Single motor/cable system
This Lexus system is typical of the single motor system that actuates traditional shoe-in-hat parking brakes. The motor and control module are not integrated and both are tucked away to reduce the likelihood of damage. Note also the option of applying the brake upon demand or allowing it to be applied automatically when the vehicle is at rest and the transmission is in park.
Let’s take a look at the 2009 Lexus LS 460 as an example of the single motor/cable system.
In the manual mode, the parking brakes are set by activating a switch while in the automatic mode; the brakes are activated when the transmission is shifted into and out of park, while pressing the brake pedal.
As long as the car is not moving, pressing the switch runs the motor to tighten the parking brake cable. The tension will be increased to a value depending on the tilt of the vehicle (hills) based on input from the yaw rate and acceleration sensor. Once it reaches the desired clamping force, the motor stops and maintains the tension.
When the switch is pulled up for release, the motor turns in the opposite direction until the slack reaches a predetermined value. All values are orchestrated by the control module.
If the switch is pressed while the car is moving, the brakes will apply for as long as the switch is held, then will release as
Based on input from the yaw rate and deceleration sensor, the electronic parking brakes are applied to provide hill-hold ability.
soon as the switch is released.
If the battery dies, the parking brake system won’t operate. To release the parking brakes manually, a special tool is included along with the jack tools. Install the correct bit on the tool, remove the plug from the spare tire well, insert the tool and turn counterclockwise to release.
If power was totally lost, or the control module or motor replaced, the electronic parking brake system will have to be re-initialized. Of course, a scan tool is required. Incidentally, the parking brake operates even if the CAN system quits.
Servicing the brake pads, calipers and parking brake shoes does not require any special techniques.