Freelander System Description and Operation
BRAKES
DESCRIPTION AND OPERATION
70-3
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 Ø 6 mm. brake pipes. The input pipes are easily distinguished by
their size, compared to the four Ø 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
Up to 2002 MY
An ABS sensor is fitted to each of the four hub carriers. These sensors inform the ABS ECU about the speed of each
of the road wheels. This measurement is fundamental to the operation of the braking features. The harness wires that
connect the sensors to the ABS unit are twisted pairs. Since the sensors are reluctor devices (passive sensor) no
output is available when the road wheels are not turning. Thus, the ABS ECU is unable to test the sensor or the pole
wheel fully until the vehicle is moving.
The exciter rings for the ABS sensors are fitted to the outer diameter of the constant velocity joint on each drive shaft,
and shielded by the hub centre boss.
From 2002 MY
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.
