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The Push-button Parking Brake

Tuesday, July 1, 2008 - 00:00

To many, electrically operated parking brakes are the answer to a question nobody asked. These systems replace parking brake cables and the lever or pedal with a switch, a processor, wiring and actuators. They started coming into common use about six years ago – not long, but sufficient for some trends to be evident.

Why are they here? A couple of reasons. One is the fact that (as you've noticed) more and more vehicle control has been taken over by electronics. It's relatively simple for a manufacturer that already has effectively circled a vehicle with a Controller Area Network (CAN) to use the existing CAN circuitry to control an electronic parking brake. That has the potential to lower costs. Also, according to TRW, a supplier of Electric Parking Brake (EPB) systems, they can be used in conjunction with other systems, like stability control, without involving hydraulics. There's also some discussion of incorporating EPBs in true "emergency" brake systems. In the case of total hydraulic failure, the electric actuators could be used to apply the parking brakes.

There's another reason many vehicle interior designers like EPBs. They don't need to find a place to locate a pedal or a handle. In today's accessory-rich passenger compartments, there's often precious little space available for those assemblies. Electric parking brakes made their initial appearance on some pricey cars and trucks: Jaguar, Land Rover, Volvo, Audi, BMW, Lincoln and Lexus, to name a few. These vehicles tend to have more features than more mundane models. They routinely feature seat heat and dual HVAC as well as satellite navigation systems. Some control systems, notably Rover's Terrain Response System and BMW's controversial I-Drive, park a large knob between the seats. That's space traditionally occupied by the parking brake handle.

But EPBs just require a button to engage and disengage. The downside for drivers is that if they're new to a vehicle that someone else has parked, they may have to search for the button before they can release the brake and drive off. Then again, that's not always a problem. Many EPB systems are programmed to self-release when the shifter's been moved from park to drive or reverse and the TPS signals that the accelerator has been applied.

How It Works

Sure, there's a computer involved, but EPB operation is not rocket science. Step one of application is for the driver to operate the button. There's a potential for confusion here. Typically, console-mounted buttons apply the brake when they're pulled up and release when pushed down – in effect copying the action of the floor- or console-mounted lever they replace. But some of the dash-mounted EPB switches we've encountered work the other way.

No matter which direction is appropriate, flipping the button the right way sends a signal via the car's CAN bus to the EPB module. The module then sends current (typically 12 volts) via dedicated wires to the actuators on the calipers. Each actuator consists of a motor and gearbox, driving a shaft threaded into a spindle. Depending on which direction the motor is turning (based on the polarity of current feed) the shaft/spindle assembly either expands, pushing out the piston and applying the brake, or retracts, releasing it.

At least that's how an EPB works when everything is operating normally. But if you service vehicles, you see them when they're not operating normally. And what about the electronics/controls? Are you going to need to spring for costly, new tools in order to service these new systems?

Getting Pushed Around

One obvious set of questions arises when it's time to move a disabled car equipped with an EPB. Rolling a battery-less vehicle around the shop or pulling a damaged car onto a flatbed are relatively simple matters with a conventional parking brake: turn the key, take the shifter out of park, release the brake and steer/push (or pull) the car where you need it. But what if it has an EPB? If its electrics are intact, you may be home free by dropping in a battery and releasing it. But what if doing so isn't feasible or doesn't get the brake to release? In the shop, the wheels on your floor jack are about to get a workout. On the road, your dollies are about to see more work than before. Either that, or lots of rear tires are going to be dragged around.

But EPBs also affect routine servicing. The reason is simple: You need to retract the caliper pistons at pad-replacement time, as you always have. But unlike pistons you've already encountered, you can't drive these in with a piston retractor or C-clamp, or retract them by screwing them in. The prescribed method is to use a bidirectional scan tool that communicates with the car and commands the pistons to retract.

That's fine if you have the right scanner, but a problem if you don't. One technician we know, who planned to do a "routine" pad replacement on a late-model Audi, went "Uh-oh," when he had the car in the air, wheels off, and realized it had the electric parking brake and none of his scan tools would talk with the Audi.

Our friend was able to determine the parking brake motors on this car's rear calipers operated on 12 volts. So he disconnected the harness from the actuator assembly, brought over a battery and "commanded" the pistons to retract by applying power directly. He finished the job.

We don't recommend this as the way to install pads in EPB-equipped vehicles, and the technician we interviewed plans to buy the right scanner before servicing any more of them. But his story illustrates two major points. First, new technologies – even "simple" ones – present new challenges. Second, when a smart technician understands how a system works, he can often come up with a way to work around the challenges it presents.

Calibrating Jaguar EPB

Most procedures for servicing electric parking brakes aren't complicated, but some require fancy footwork. Take calibrating the EPB in an S-Type Jaguar. It's necessary anytime you remove the rear calipers, otherwise premature rear pad wear is likely.

Start by resetting the EPB module and then switching off the ignition. Then disconnect the battery for 30 seconds, reconnect it, start the engine and put on your dancing shoes. Here's the rest of the procedure:

1. Firmly apply the brake pedal and release it five times.

2. Look at the Message Center on the dash. It should say "NOT CALIBRATED" or "APPLY FOOT AND PARK BRAKE." These messages mean calibration mode has been entered.

3. Lightly apply the footbrake. Keep your foot down.

4. Lift the parking brake switch to apply it, then push it down to release the parking brake.

5. Release the service brake pedal.

6. Confirm that the "BRAKE" warning light on the dash is off and that the Message Center no longer displays the message referenced in Step 2.

7. Apply and release the electric parking brake five times to verify normal operation.

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