Getting vehicles to stay put

Electronic parking brkaes are gaining ground and opening the way to brake-by-wire.

undercar brakes brake by wire electronic parking brakes repair shop training technician training A/C training automotive aftermarket It's no secret to regular readers of Motor Age that cars are becoming increasingly electronic. The addition of the Controller Area Network (CAN) protocol has played a role in speeding up the process because of its open architecture and the ease with which it allows designers to add additional systems to those already sharing the bus. There are distinct advantages to the trend of electrification of the automobile: less weight, more room and less cost to manufacture. The difficulty for us lies in staying in touch with these changes and adjusting our workshop mentality to accommodate the new procedures and precautions we need to follow. If we don't, we can ruin some very expensive parts at the least, and at the worst, we can risk serious injury.

Electro-hydraulic braking systems aren't new, and neither are cars equipped with electronic parking brake systems. But they might be new to you as the fleet ages and starts showing up outside your door instead of dealerships. Here are some things you need you to know about these systems when that happens.

Mercedes Benz Sensotronic Braking System

This is an electro-hydraulic system that eliminates the need for a vacuum brake booster and allows each individual wheel to receive the exact braking force needed to maintain vehicle control. The heart is the Brake Operating Unit, or BOU. In addition to the routine fluid reservoir and tandem master cylinder, the BOU incorporates a brake pedal pressure simulator for the driver and the SBC pedal value sensor (made up of two Hall Effect sensors) that supplies the controller with the needed driver input.

The hydraulic unit consists of an electric pump and accumulator that supplies the fluid pressure and volume needed for braking under a variety of conditions. Should the system itself fail, the front calipers still can be operated conventionally.

The system performs a pre-drive check to test for pressure in the system reservoir (which it will correct if low), as well as an operational test of the system and each individual wheel's pressure sensor. This pre-drive check is triggered whenever the SBC module gets a wake up call: the door is opened, the central locking system is operated, the brake pedal is depressed, the key is turned to position 1 or the parking brake is operated.

It is vital that the system be deactivated using a special tool or dedicated scan tool before the hydraulic system is serviced and before any routine service is performed including pad replacement or rotor service. Deactivating the system will discharge pressure in the reservoir and prevent the charge motor from starting, which it might if the system goes into its predrive check.

Here's the scenario. You have the calipers hanging and properly secured and forget for a moment what you're working on. You open the driver's door or accidentally trigger the key fob that's in your pants pocket. The system wakes up, pressurizes the system and the caliper pistons are shot out like mini-mortars under, if I recall the numbers correctly, 2,500 psi of pressure.

That could ruin your day. And if you're standing near by, or holding one of those calipers at the time, it would seriously ruin your day.

I've read of some techs successfully disabling the SBC system without a tool, and if you do enough Googling, you'll find the same info I did. But if you do so, you must be prepared mentally and financially to deal with the consequences if you make a mistake along the way.

Electronic Parking Brakes

What does a parking brake really have to do? It's primary purpose is to keep the vehicle still when not in use, and for years it's been a fairly simple matter of operating a ratcheting lever that in turn pulled on a set of cables to apply the brake shoes on drum brake systems. Even the dedicated shoes used on some rear disc systems and those that manipulate the caliper pistons themselves are operated by mechanical cable.

Well, not all.

Electronic parking brake (EPB) systems first started appearing around 2001 with Renault offering the first. Today, there are two basic types of EPB. The first substitutes the mechanical actuation of a traditional cable system with an electric motor/transmission unit that is controlled by a dedicated module. The second type moves the electric motor assembly directly to the rear calipers where they actuate a normal floating screw/nut mechanism to move the piston in and out.

Here's a simplistic idea of how a typical EPB functions. When the driver starts the engine, the engine control module (ECM) sends a wake up call to the EPB controller. As the driver applies throttle and begins to release the clutch pedal, the EPB gradually releases the parking brake pressure until the wheels are free to turn, releasing its hill hold feature. The needed inputs can be hardwired to the EPB controller or received over the data bus. When the driver comes to a stop, the system may apply automatically or have to be manually engaged.

In some models, applied force is precisely controlled using a variety of inputs fed across the bus data network. Cable torque and vehicle inclination sensors may be incorporated to provide data input and feedback, and might be housed in the actuator or control module assembly.

A Land Rover Example

You'll find EPB cable designs on models offered by Lexus, Ford, Jaguar, some BMWs and Land Rover. The Land Rover actuator assembly contains an electric motor/transmission and a force sensor that provides the feedback to the EPB control unit.

The brake is applied by pulling up on the console switch. While it can be applied with the key off, the key must be on and the brake pedal depressed to release the parking brake. In "drive away" mode (key on, transmission in gear and the accelerator depressed), the brake release is automatic. Like many vehicles equipped with EPB, the system also offers a hill hold feature that automatically can apply the parking brake to keep the vehicle from rolling backwards at a stop. Some also are programmed to automatically engage when the ignition key is removed.

The cable force sensor inside the actuator housing allows the control module to accurately control brake application, and most EPBs are designed to apply enough force to hold the vehicle steady on a grade of 30° or so. Once engaged, the control unit monitors the sensor for up to 20 minutes (or if awakened) and will run the motor as needed to maintain the programmed level of force.

Typically, there are no serious concerns when servicing this version of EPB. Most require that the system be deactivated before removing the rear rotors or system components. This often is as easy as removing a fuse or two or disconnecting the battery, but returning the system to service might require a recalibration of the control module. Check the service information (SI) for the specific procedures required for any EPB service before beginning work.

Electronic Calipers

Some manufacturers, in particular VW and Audi, use a caliper designed with the electronic actuator built in. This eliminates the need for the cables, levers and mounting hardware. It uses a similar forcing screw and floating nut as that used in the mechanically operated calipers, and the motor/transmission assembly can be removed and serviced separately.

The control unit measures clamping force and the system also can track brake pad wear. A dedicated tool or capable scan tool is required to disable and initialize the system when any service work is performed. This includes motor replacement, pad/rotor service or system bleeding. Forcing the caliper pistons in without retracting the EPB motors first, or trying to jumper the motors directly will result in damage to the system.

Integrated electronic parking brake systems eliminate the need for a separate ECU, and instead incorporate the EPB into existing electronic stability control (ESC) systems (a feature, by the way, that will be mandatory on all models effective in the 2012 model year). This opens the door to future brake-by-wire designs that would further enhance passenger safety not only in ESC, but any of the growing number of onboard collision avoidance systems making their way onto American roadways.

Yes, the car is becoming more and more of a rolling electronic network and many of us will wonder how we'll deal with advancing technology. But speaking from past experience, having watched techs in the early 1980s stare in wonder at Single Board Engine Controllers (SBEC) and the first electronic ignitions, we'll do what we've always done: adapt, learn and continue to keep our customers on the road.