Electronic park brakes
Guide to troublesome park brakes on Discovery 3 and early Sport models
The electronic park brake (EPB) is here to stay. It’s fitted to every current model of Land Rover except, of course, Defender where too much re-design would be involved and it would be objectionably out of character anyway. The EPB certainly makes life easy. A mere flick of the lightweight switch lever applies the park brakes sufficiently to render the vehicle immovable on the steepest slopes.
It’s a similar wimp’s effort to release it, and it will even release for you if you forget when driving off. But do we need this, and what was wrong with pulling a traditional lever and pressing the button to set it? Nothing, of course. It’s a matter of competitive vehicle manufacturers keeping up with technology, keeping weight down for emissions, monitoring costs, and de-cluttering and safety-smoothing the passenger compartment. The traditional handbrake lever is destined to become another curiosity of classic Land Rovers.
How the EPB works
On Discovery 3 and 4 and Range Rover Sport 1 models the electronic park brake system comprises four main parts: the driver’s switch in the cab, the brake module mounted in the centre of the chassis infront of the spare wheel, the cables that run from the module to the wheel brakes to operate them, and the brakes themselves, which comprise a pair of shoes inside a drum machined into the inboard side of each rear brake disc.
The driver’s switch movement replicates that of a conventional handbrake lever: up for on, down for off, which sends signals to the park brake module. A circuit board control unit inside the module activates an electric motor which, through a gearbox, drives two cables to operate the two rear drum brakes.
The Discovery 3 park brake module (body removed here) is mounted on the chassis above the rear diff, where it’s exposed to dirt and water.
Here is a clean module with leads and cables. The plastic casing is sealed, but the mounting frame and bolts rust, making removal difficult sometimes.
This diagram shows the cables –connected to an electric motor inside the module, operate each park drum brake at the rear wheels.
To operate the cables, the electric motor, via its gearbox, rotates a splined shaft which is hollow and has an internal screw thread. A threaded connector on the left hand brake cable is located into the hollow spline, but it cannot rotate so, when the splined shaft rotates, the cable is forced in or out by the screw thread to apply or release the brake. The right hand brake cable is attached (via a force sensor) to the opposite end of the splined shaft.
Because the splined shaft can move axially, the movement applied to the left brake cable is shared with the right cable, applying each brake equally. The force sensor detects the load on the cables, signalling the motor to stop when it feels the brakes are sufficiently applied. The sensor also initiates tension adjustments in the cables to maintain the optimum park brake force.
Each operating cable leads into the inboard side of the brake backplate where it is connected to the park drum brake assembly.
Turning the brake disc around, the inside of the disc casting is machined out to form a brake drum on which the park brake shoes operate.
The park brake shoes need to be set in position manually using the gear-toothed adjuster between the shoes (top above spring).
The park brake module communicates with other vehicle electronic systems, in particular the ABS, which influences the conditions allowing the park brake to be applied, and facilitates ‘drive away release’ in which the park brake automatically releases when a given wheel speed is detected. The module sends signals to the facia instrument panel via the CAN bus (a wiring communication system running throughout the vehicle).
This activates the red warning lamp to indicate the park brake is applied (and which stays on for a few minutes after stopping the engine). It will also flash this red lamp and illuminate the park brake’s orange fault lamp when certain faults are detected or if the park brake does not fully operate when requested by the driver’s switch. If the brakes fail to release, an emergency release cable is available inside the vehicle under the console below the park brake switch.
Current models use rear disc brakes for parking. Each brake is operated by an actuator mounted on the caliper.
Improved EPB system
The electronic park brake system on Discovery 3 and Range Rover Sport models continued into the Discovery 4. A different and superior EPB is used on later models including the current Range Rover and Sport, Evoque, later Freelander 2 and Discovery Sport. On these models the brake module is a smaller item safely housed inside the vehicle behind the rear loadspace trim, rather than underneath.
The biggest design change is the absence of the cable-operated drum brake arrangement. Instead, the new system employs the rear disc brakes for parking, applying the disc brakes via an actuator mounted directly onto each brake caliper. The actuators contain an electric motor, which moves a spindle against the caliper piston to apply the disc brake. The module controls the actuators, adjusting the force of application according to the gradient of the surface the vehicle is parked on.
The park brake system is automatically applied when the ignition is switched off or if auto transmission ‘park’ is selected. It will automatically release when driving away, and does so in a controlled manner, according to the gradient and the driver’s use of the accelerator and transmission.
After heavy braking during driving, the brake discs can become hot and will contract as they cool when parked. So to maintain the optimum park brake force, the park brake automatically resets while parked. The new system’s components are simpler and (for the sake of emissions) lighter in weight than the earlier Discovery 3 and 4, and Sport 1 version. It is efficient and very reliable.
Current Land Rover models use the rear disc brakes for parking. Each brake is operated by an actuator.
Here is the actuator (black, and arrowed) mounted to the inboard side of the rear brake caliper on a Range Rover Evoque.
Problems with early park brakes
The original electronic park brake has caused a few expensive problems during its life. But when introduced on the Discovery 3 it was new technology (at least on a Land Rover) and so there was much to learn about potential problems and best maintenance and servicing practices. Just as the ground-breaking P38 Range Rover gained an unfair reputation for unreliability due to lack of understanding outside of dealerships and good specialists, so too did the early electronic park brake.
The systems and their foibles are now well understood and good specialist Land Rover garages know how to maintain and adjust them to avoid trouble. But others can get it wrong, leading to costly, even dangerous trouble, as we’ll see next month.
As with most systems, they work fine with correct maintenance and use. Below, are a few typical concerns. Park brake shoes binding or dragging: evidenced by noise from the rear, excess heat felt at the road wheel and/or excessive brake dust on the wheel. The friction lining on the shoes may have been damaged by the heat. If so they, and any other heat-affect parts including the brake discs, will need to be renewed. Either way, dust is likely to have accumulated in the brake drums and needs to be removed from the drum, the shoes, and from all other internal components.
Brake shoe retaining clips need to be renewed if they are at all suspect or damaged (a new type of clip was introduced which is, by now, likely to have been fitted to all vehicles). The clips are renewed as a matter of course when new shoes are fitted. A park brake service kit includes re-designed brake shoes and new improved retaining clips.
The brake shoe guides on the back plate also need to be cleaned, and a very light smear of grease applied. Screech noise during operation of the park brake: if accompanied by a warning lamp (which doesn’t always happen), a fault code will have been logged. This noise indicates a significant problem with the mechanical parts of the system.
If possible, the park brake should not be used until the system has been checked because the mechanism in the module has overrun its normal amount of movement. This can happen if the cable is detached, shoe linings broken up or if the shoes are excessively worn and out of adjustment. If the mechanism has not jammed, it soon will do, though it may be possible to clear this using the ‘unjam procedure’, otherwise the module may need to be replaced.
Squeal from rear brakes during normal braking: this won’t be caused by the park brake system, but is more likely due to the rear disc pads vibrating. The pads have since been improved, so this is only likely to occur on very low mileage vehicles that have their original pads still fitted.
• Park-brake fault-finding and most repairs need diagnostic equipment. • It’s necessary to put the system into ‘mount mode’ to test the module and to reset the brake shoes. • The system needs to be electrically isolated 20 minutes before being worked on (including removing a rear brake disc for access), to avoid the possibility of the brake automatically re-applying, with possible subsequent seizure of the module.
It is important for the park brake shoes, drums and internal components to be periodically cleaned and the shoes reset if necessary. Adjustment of the shoes requires accuracy, and the setting can be lost if brake dust collects and becomes baked onto the shoes by heat, effectively altering the diameter of the shoes, and thus the clearance to the drum.
It’s worth checking electrical connectors and cables, and also the steel guide hoops on the suspension lower links, which locate the cables and can wear through by abrasion. The checks are due at the routine 12 month/15,000 miles service, or more frequently in arduous conditions, and especially if driven in deep mud or muddy water.
The toothed cylinder between the brake shoes here is used to set the clearance between the shoes and the drum.
The shoes are set with the disc/drum fitted in place, levering the toothed adjuster through a hole in the disc/drum.
• Ensure the park brake checks are included in the annual/15,000 miles service, carried out by a main dealer or a good independent specialist. • Have the park brake cleaned and checked more frequently if regularly driving in deep mud or in a mixture of dusty and wet conditions. • Occasionally, when parked with the engine off, open the door and lean out while operating the park brake. You’ll hear the motor running, but notice if the noise later becomes unusually loud or a screech is heard. If so, have a garage check the system before damage is done. • Occasionally, when parked, feel for heat coming from the rear wheels which suggest's a brake is binding. That needs sorting quickly. • Remember to release the park brake before driving off. • Once the brake is correctly set and maintained correctly, there is no particular reason why an electronic park brake should give any trouble. • There’s nothing that quite beats mechanically sympathetic driving, and good maintenance.
Setting the park brake shoes
The running clearance between the brake shoes and their drums inside the discs needs to be set accurately, to avoid eventual damage to the components, including the remotely mounted module. The system needs to be electronically
put into mount mode before adjustments are made. Each brake has two manual adjusting mechanisms which are manipulated through a hole in the brake disc. A toothed-wheel adjuster sets the shoe-to-drum clearance, and a wedge adjuster operated by a hexagon key sets the cable adjustment.
Renewing brake shoes
After replacement of the brake shoes or brake discs, a bedding in procedure must be performed to ensure the drum brakes operate satisfactorily.
Diagnostic tests should be performed to confirm the module is at fault before considering replacement of this expensive item. The equipment is also needed to program a new module to the vehicle. Many have been replaced unnecessarily, even due to cable problems when the cables can be replaced individually and without removing the module. As we said earlier, failure of the module is usually the result of the driver ignoring symptoms and warnings. Prompt action can avoid damaging it.