Range Rover P38
ABS
11
DESCRIPTION AND OPERATION
ETC control valves - up to 99MY - 7 & 8
The ETC control valves are optional on vehicles up to
99MY. The ETC inlet solenoid valve (7) is normally
closed and the ETC isolating solenoid valve (8) is
normally open when ETC operation is not required.
When ETC operation is required, the inlet solenoid
valve opens allowing fluid from the power circuit to
flow to the rear ABS solenoid control valves.
Simultaneously, the ETC isolating solenoid valve
closes, isolating the connection to the PCRV. The
ABS ECU energises the applicable ABS solenoid
valve to pressurise the applicable rear wheel brake.
ETC control valves - from 99MY - 7 & 8
The ETC control valves are standard on vehicles from
99MY. The ETC inlet solenoid valve (7) is normally
closed and the ETC isolating solenoid valve (8) is
normally open when ETC operation is not required.
When ETC operation is required, the inlet solenoid
valve opens allowing fluid from the power circuit to
flow to the power valve. The power valve is actuated
by the pressure and opens to allow fluid from the
power circuit to flow directly to the front ABS solenoid
control valves and to the rear solenoid control valves
via the PCRV. The ETC isolating solenoid valve
closes isolating the return flow from each ABS
solenoid control valve. The ABS ECU energises the
applicable ABS solenoid valve(s) to pressurise the
applicable front and/or rear wheel brake.
ABS power unit
The ABS power unit consists of an electrically driven
pump, a pressure switch and an accumulator.
The pressure switch incorporates three
electro-mechanical switches: one for the pump,
another, at a different pressure setting, to illuminate
the pressure warning lamp. The latter switch plus a
third switch inform the ECU of low pressure and that
ABS function should cease while pressure remains
low.
The pump also incorporates a non-return valve and a
pressure relief valve to protect the system.
NOTE: The pump and pressure switch are
not serviceable, if failure occurs a new unit
must be fitted.
The diaphragm type accumulator is fitted to the power
unit. The accumulator is precharged with nitrogen at
up to 80 bar. Its function is to store hydraulic energy
ready for the next brake application.
NOTE: Accumulator replacement is
possible if failure occurs. Correct disposal
of old accumulators is essential.
See
Repair.
Pressure Conscious Reducing Valve (PCRV)
The PCRV is located adjacent to the brake
booster/ABS modulator unit. It is connected between
the power valve and ABS solenoid valves for the rear
axle. Its function is to limit brake pressure to rear axle.
NOTE: The PCRV is not a serviceable item,
if failure occurs a new unit must be fitted.
Brake calipers - front and rear
NOTE: To identify separate hydraulic
circuits, they are referred to as
HYDROSTATIC and POWER circuits.
Power circuit - Consists of rear calipers and servo
cylinders, supplied by direct hydraulic power from the
power valve.
Hydrostatic circuit - Consists of servo cylinders
supplied by master cylinder pressure. The front
calipers are supplied with direct hydraulic energy from
the servo cylinders, comprising a combination of
master cylinder pressure and direct hydraulic power.