Description
Description
General
Vehicle braking is provided by disc brakes on the front wheels and drum brakes on the rear wheels. The foot brakes are operated by a diagonally split, dual circuit hydraulic system with vacuum servo power assistance. A cable operated handbrake operates on the two rear brakes.
The ABS features 4-wheel electronic traction control and hill descent functions as well as anti-lock braking and electronic brake distribution.
Slip Control System
Freelander incorporates an electronic slip control, a TEVES Mk 25 ABS modulator and active ABS sensors are installed. Feature, as follows:
^ Anti-lock braking system
^ Hill descent control
^ Electronic traction control
^ Electrical brake-force distribution
^ CAN communication link
System communicate's via CAN with the Engine Control Module (ECM), the instrument pack and, on automatic derivatives, with the Electronic Automatic Transmission (EAT) ECU. The systems comprise the following components:
^ Electronic control unit
^ Modulator
^ ABS sensors
^ Mechanical brake switch
^ Brake fluid level switch
^ HDC relay and switch
^ Accelerometer
Electronic Control Unit
The Electronic Control Unit (ECU) determines the speed and acceleration of each wheel, controls appropriate hydraulic functions and monitors system operation for fault conditions and interfaces to other vehicle systems. The ECU is attached to the Modulator unit and is mounted underbonnet on the RHS valence behind the headlamp.
Under the following conditions the ECU is programmed to switch off the main software driver which will result in the illumination and disabling of the ABS, TC, HDC and EBD warning lamps:
^ If the IGN voltage drops to values, which are not sufficient to maintain a stabilized, supply voltage for the processors. This voltage is below the functional operating voltage of 8 volts. The controller will invariably switch on again when the minimum operating voltage of 10 volts is reached.
^ If the following failures or errors are detected:
^ Valve failure
^ Failure of two ABS sensors
^ Main driver failure
^ Redundancy error
^ Over voltage
The ECU will also inhibit the ABS function, traction control, hill descent control and illuminate their respective warning lamps without switching off the main driver in the following circumstances:
^ Ignition voltage supplied to the ABS ECU less than 8 volts.
^ Failure of one or more of the ABS sensors
^ Pump motor failure
If there is a CAN error message from the ECM or the EAT ECU, or if there is a brake pedal switch fault, the ABS ECU disables the ETC and HDC functions and illuminates the related warning lamps.
Hydraulic Modulator
The hydraulic unit of the modulator consists of a pump and 12 solenoid operated valves, accumulator and damper chambers. During normal braking where ABS intervention is not required, brake fluid passes straight through de-energised inlet valves (normally open). Where ABS intervention is required, pressure is maintained at a wheel by closing the appropriate inlet valve. When pressure needs to be released from a brake circuit, the appropriate outlet valve is opened (when outlet valve is opened the inlet valve must be closed) and the brake fluid is allowed to flow into the reservoir. Brake fluid is returned, via the return pump, to the Master cylinder line via the damper chamber
Brake Fluid Pressure - (inlet)
The hydraulic circuit of the ABS modulator consists of the Primary and Secondary feeds from the Brake Master cylinder. These are fed into the modulator by two 0 6 mm. brake pipes. The input pipes are easily distinguished by their size, compared to the four 0 4.76 mm. outlet pipes.
The ECU can detect electrical failure of each of the inlet valves and will generate relevant fault codes which can be accessed via TestBook/T4.
Brake Fluid Pressure - (Outlet)
The hydraulic outlet circuit of the ABS modulator consists of the four pipes leading to the front calipers and rear brake drums. The four pipes transmit the brake fluid usually at the pressure determined by the drivers brake application, but during ABS, EBD, TC and HDC intervention at the pressures modified by the ABS ECU. The pipes are attached by a series of clips into the body and terminate at the caliper/drum via a flexible hose.
The ABS ECU can detect electrical failure of each of the outlet valves and will generate relevant fault codes which can be accessed via TestBook/T4.
ABS Sensors
An active ABS sensor is installed in each of the four wheel hub carriers to provide the ABS ECU with a speed signal from each road wheel. Each of the ABS sensors is positioned in close proximity to the inboard seal of the related wheel bearing and secured with a bolt. The seals, which rotate with the wheels, each contain a magnetic element incorporating 48 pole pairs.
The ABS sensors operate using the Hall effect principle. A permanent magnet inside the sensor applies a magnetic flux to a semiconductor, which receives a power supply from the ABS ECU. When the wheels rotate, the pole pairs in the seals induce voltage fluctuations in the ABS sensors that are converted into square wave signals and output to the ABS ECU. The frequency of the signal is proportional to wheel speed.
Since the sensors are active devices, an output is available when the road wheels are not turning, which enables the ABS ECU to check the sensor while the vehicle is stationary.
All ABS Sensors
Failures or malfunctions relating to the ABS sensors and connections are detected by the ABS ECU. In the event of failure of two or more of the ABS sensors the ABS ECU switches off the system and illuminates the ABS, TC, EBD, and HDC warning lamps.
If a single sensor fails the ABS ECU maintains the minimum functions to provide safe operation and illuminates the ABS, TC, and HDC warning lamps.
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 center-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.
Front Brake Components
Rear Brakes
The rear brakes each comprise a hub mounted backplate and drum containing leading and trailing brake shoes when operated by the foot brake. An adjuster rod is incorporated to automatically adjust the brake shoes to compensate for wear of the brake linings. Adjustment occurs during operation of the foot brake.
When hydraulic pressure is supplied to the wheel cylinder, the cylinder extends and forces the brake shoes against the drum.
When a force is supplied to the shoes via the hand brake lever and cables, both brake shoes become leading.
Rear Brake Components
Hydraulic System
The hydraulic unit of the modulator consists of a pump, 12 solenoid operated valves and accumulator and damper chambers. Each brake unit has its own inlet and outlet valves.
The hydraulic system operates the brakes in response to brake pedal movement.
For normal brake operation, brake pedal movement is assisted by the brake servo assembly and transmitted to the master cylinder assembly. The master cylinder assembly converts brake pedal movement to hydraulic pressure. Primary and secondary brake pipe circuits supply the hydraulic pressure to the brakes: the primary circuit supplies the front left and rear right brakes; the secondary circuit supplies the front right and rear left brakes.
A red, brake warning lamp in the instrument pack illuminates if the fluid level in the hydraulic system falls to an unsafe level. The brake fluid level is checked via a level switch in the brake fluid reservoir.
On right hand drive vehicles the brake fluid reservoir is remotely located on a bracket on the bulkhead. Left hand drive vehicles have the brake fluid reserver in the usual position on top of the master cylinder.
Vacuum for the brake servo assembly is obtained from the engine inlet manifold through a vacuum line and non return valve.
To reduce operating noise, sleeving is installed on some of the brake pipes in the engine compartment and the pipes are located in sprung pipe clips.
ABS Hydraulic System Schematic
Brake Servo Assembly
The brake servo assembly provides power assistance to reduce the pedal load when braking.
The assembly is attached to the front of the pedal box and comprises a circular housing containing a diaphragm, push rods, valve and filter. A push rod at the rear of the housing is connected to the brake pedal. The vacuum line is connected to a port in the front face of the housing.
With the brake pedal released and the engine running, vacuum pressure is present on both sides of the diaphragm. When the brake pedal is pressed, the rear push rod moves forward and opens the valve to allow atmospheric pressure through the filter into the pedal side of the diaphragm. The pressure differential acting on the diaphragm increases the pressure being applied by the brake pedal, which is transmitted to the master cylinder via the forward push rod.
If the servo fails, the hydraulic system will still function but will require greater pedal effort due to the lack of vacuum assistance.
Vacuum Enhancer System (KV6 Only)
The vacuum enhancer system increases the relatively low vacuum available from the inlet manifold to increase braking assistance. The system consists of a venturi valve, a solenoid valve and associated pipes.
The venturi valve is installed in the vacuum pipe between the brake servo and the inlet manifold. An air feed pipe, from the intake duct upstream of the throttle body via the solenoid valve, is also connected to the venturi valve. Internal ducts in the venturi valve connect the brake servo and air feed ports to the inlet manifold port. The duct connecting the air feed port to the inlet manifold port incorporates the venturi. A check valve, integrated into the brake servo port, prevents the reverse flow of air and fuel vapor to the brake servo.
The solenoid valve controls the air feed to the venturi valve. Operation of the solenoid valve is controlled by the ECM.
To improve engine idle speed refinement, when the gearbox lever is in Park or Neutral, the solenoid valve is closed and the brake servo senses the vacuum in the inlet manifold via the venturi valve. When the gearbox lever is selected to positions other than Park or Neutral, the ECM energizes the solenoid valve and air from the upstream side of the throttle body is fed to the venturi valve. The air flows through the venturi valve into the inlet manifold. As the air flows through the venturi in the venturi valve, it increases in velocity and decreases in pressure, resulting in an increase in the vacuum sensed by the brake servo. Depending on ambient conditions and engine speed, the vacuum enhancer system increases inlet manifold vacuum by a maximum of approximately 40%.
Master Cylinder Assembly
The master cylinder assembly produces hydraulic pressure to operate the brakes when the brake pedal is pressed. The assembly is attached to the front of the brake servo assembly, and comprises a cylinder containing two pistons in tandem. The rear piston produces pressure for the primary circuit and the front piston produces pressure for the secondary circuit.
The brake fluid reservoir is located either on top of the master cylinder assembly (LH drive vehicles) or above the master cylinder on a bracket on the bulkhead (RH drive vehicles). The reservoir is internally divided to provide an independent supply of fluid to each brake circuit, and so prevent a single fluid leak from disabling both primary and secondary brake circuits.
Should a failure occur in one circuit, the remaining circuit will still operate effectively, although brake pedal travel and vehicle braking distances will increase. If the fluid level in the reservoir is too low, the brake fluid level switch in the reservoir filler cap breaks a contact to the ABS ECU, which sends a CAN message to illuminate the brake warning lamp in the instrument pack.
ABS Master Cylinder
When the brake pedal is pressed, the front push rod in the brake servo assembly pushes the primary piston along the cylinder bore. This produces pressure in the primary pressure chamber which, in conjunction with the primary spring, overcomes the secondary spring and simultaneously moves the secondary piston along the cylinder bore. The initial movement of the pistons, away from the piston stops, closes the primary and secondary center valves. Further movement of the pistons then pressurizes the fluid in the primary and secondary pressure chambers, and thus the brake circuits. The fluid in the chambers, behind the pistons, is unaffected by movement of the pistons and can flow unrestricted through the feed holes between the chambers and the reservoir.
When the brake pedal is released, the primary and secondary springs push the pistons back down the bore of the cylinder. As the pistons contact the piston stops, the primary and secondary center valves open, which allows fluid to circulate unrestricted between the two hydraulic circuits and the reservoir, through the center valves, the chambers behind the pistons and the feed holes.
ABS
The ABS is a full time, four channel system that gives individual speed control of all four wheels to provide the vehicle with anti-lock braking (ABS), Electronic Traction Control (ETC.) Hill Descent Control (HDC) and Electronic Brake force Distribution (EBD) functions.
ABS Modulator
The ABS modulator controls the supply of hydraulic pressure to the brakes in response to inputs from the ABS ECU. The modulator is attached by three mounting bushes to a bracket on the RH inner wing, and connected to the primary and secondary hydraulic circuits downstream of the master cylinder assembly. A multi-pin connector links the ABS modulator to the vehicle wiring.
Internal passages in the ABS modulator, separated into primary and secondary circuits, connect to the various components that control the supply of hydraulic pressure to the brakes. Separation valves and check valves control the flow through the internal circuits. Damper chambers and restrictors are included in each circuit to refine system operation. Inlet and outlet solenoid valves control the flow to the individual brakes. An accumulator is connected to each circuit to absorb flow surges. A common return pump is connected to both circuits to provide a pressure source and return fluid to the reservoir.
The ABS modulator has three operating modes:
^ Normal braking mode: When the brake pedal is pressed, pressurized fluid from the master cylinder assembly flows through the open separation valves and inlet valves to operate the brakes.
^ ABS braking mode: When in the normal braking mode, if the ABS ECU detects that a wheel is about to lock, it energizes the inlet and outlet solenoid valves of the related brake and starts the return pump. The inlet solenoid valve closes to isolate the brake from pressurized fluid; the outlet solenoid valve opens to release pressure from the brake into the return pump circuit. The brake releases and the wheel begins to accelerate. The ABS ECU then operates the inlet and outlet valves to control the supply of hydraulic pressure to the brake and apply the maximum braking effort (for the available traction) without locking the wheel.
^ Active braking mode: When ETC or HDC are enabled, and the ABS ECU determines that active braking is required, it starts the return pump. Hydraulic fluid, drawn from the reservoirs through the master cylinder, is pressurized by the return pump. The ABS ECU then operates the inlet and outlet solenoid valves to control the supply of hydraulic pressure to the individual brakes and slow the wheel(s).
ABS ECU
The ABS ECU controls the operation of the ABS modulator to provide the ABS, ETC and HDC functions. It also operates warning lamps in the instrument pack to provide the driver with status information on each function. The ABS ECU is attached to the ABS modulator. Incorporated into the ABS ECU are integrated circuits and software for system control and diagnostics. An electrical connector interfaces the unit with the vehicle wiring.
The warning lamps consist of:
^ An amber ABS graphic.
^ An amber ETC graphic.
^ Two inclined vehicle graphics for HDC, one amber (fault), which includes an exclamation mark, and one green (information).
The warning lamps are Light Emitting Diodes (LED) installed on the printed circuit board of the instrument pack and cannot be replaced separately.
When the ignition is switched on, the ABS ECU performs a 'bulb' 'check of the warning lamps as part of the power up procedure. If a warning lamp remains illuminated after the bulb check, a fault has been detected and repair action is required. On a serviceable system:
^ The ETC and HDC warning lamps are extinguished after 2 to 3 seconds.
The ABS warning lamp is extinguished briefly, after 1.3 to 2 seconds, is illuminated again for a further 0.5 second and then extinguished.
The ABS ECU continually calculates vehicle speed using the inputs from all four ABS sensors. Vehicle speed is used as a reference against which individual wheel speeds are monitored for unacceptable acceleration or deceleration. Vehicle speed is also output to the instrument pack for the speedometer.