Brakes (Front Brakes): Description and operation

Mechanical Brake Switch A mechanical brake pedal switch is used to illuminate the stop/brake lamps on the vehicle because of its high current carrying/switching capabilities. It is also used to input the status of the brake pedal to the ABS ECU. This switch is a double contact switch where the Brake Lamp Switch (BLS) contacts are open and the Brake Test Switch (BTS) contacts are closed when the brake pedal is at rest. When the pedal is depressed, the BLS contacts close and BTS contacts open, supplying 12 volts to the brake/stop lamps and indicating to the ABS ECU that the pedal has been operated. When the pedal is depressed there is a time when both the BLS and the BTS contacts are closed, which allows the ABS ECU to perform a plausibility check on the switch. The switch used is a carry over from the Range Rover.

A Hall effect brake pedal position sensor is installed adjacent to the mechanical brake switch. This is not used by the brake system but by other system ECU's which are not compatible with the outputs from a mechanical switch.

Brake Fluid Level Switch The Brake Fluid Level Switch (BFLS) is a Reed switch located in the brake fluid reservoir. The BFLS is connected to the ABS ECU and to earth. The BFLS is closed when the brake fluid level is above the minimum limit. If the brake fluid level decreases below the minimum limit, the BFLS opens and the ABS ECU sends a CAN message to the instrument pack to activate the brake warning lamp.

Hill Descent Control Relay and Switch The HDC relay is located inside the engine compartment fusebox. The HDC switch is a latching switch mounted on the gear lever surround of automatic vehicles and the gear lever for manual vehicles.

Accelerometer The accelerometer (sometimes known as the "G" sensor) is mounted near the centre-line of the vehicle alongside the handbrake lever. It provides information to the ABS ECU regarding vehicle longitudinal acceleration.

The ABS ECU uses the input from the accelerometer to corroborate the inputs from the ABS sensors, e.g. if all four wheels are spinning, the input from the accelerometer enables the ABS ECU to determine the true speed of the vehicle.

Front Brakes

The front brakes each comprise a hub mounted, single piston caliper assembly and a vented disc. The inboard side of the disc is protected by a mudshield.

When hydraulic pressure is supplied to the caliper, the piston extends and forces the inner pad against the disc. The caliper body reacts and slides on the guide pins to bring the outer pad into contact with the disc.