Freelander Brakes: Operation and Diagnostics
BRAKES
DESCRIPTION AND OPERATION
70-17
Hill Descent Control
This feature allows the vehicle speed to be controlled during a hill descent using the vehicle Brakes. This feature has
to be selected using the Hill descent switch with the selected gear being 'first' or 'reverse' and the brakes below 350
°
C.
When HDC is selected by operating the latching HDC switch, the HDC information warning lamp is illuminated
continuously to indicate HDC is available. If conditions are not met to enable HDC operation, after the switch is
operated, the warning lamp flashes. When going downhill and HDC is selected the vehicle will maintain a target speed
by applying the brakes. The target speed will be relative to the throttle pedal position. If the slope is not steep enough
and the speed is less than the target speed, the vehicle will not accelerate to reach the target speed. The HDC function
is brakes intervention only.
Minimum target speeds with the throttle closed are 6 mph (9.6 km/h) in first gear and 4 mph (6.5 km/h) in reverse
gear. The first gear target speed is decreased to 4.4 mph (7 km/h) if rough terrain or sharp bends (detected from ABS
sensor inputs) are encountered while already travelling at the minimum target speed. Minimum target speeds are
increased at cold idle to prevent conflict between the brakes and the engine caused by HDC trying to impose a lower
vehicle speed than is normal for the increased engine speeds at cold idle. Minimum target speeds at cold idle are 7.5
mph (12 km/h) in first gear and 4.4 mph (7 km/h) in reverse gear.
During active braking, the brakes are operated in axle pairs on one or both axles. The braking effort is distributed
between the front and rear axles as necessary to maintain vehicle stability. Distribution of the braking effort is
dependant on direction of travel and braking effort being applied. To prevent wheel lock, anti-lock braking is also
enabled during active braking.
The ABS ECU incorporates a fade out strategy that, if a fault occurs or HDC is deselected during active braking,
provides a safe transition from active braking to brakes off. The fade out strategy increases the target speed at a low
constant acceleration rate, independent of actual throttle position. If active braking is in operation, this causes the
braking effort to be gradually reduced and then discontinued. The HDC information warning lamp flashes while fade
out is in progress.
If the clutch is disengaged during active braking, the HDC information warning lamp flashes after a delay of 3 seconds.
After 60 seconds, if the clutch is still disengaged, the HDC fault warning lamp flashes and active braking operation
fades out.
To prevent the brakes overheating, the ABS ECU monitors the amount of active braking employed and, from this,
calculates brake temperature. If the ABS ECU determines brake temperature has exceeded a preset limit, it
extinguishes the HDC information warning lamp and flashes the HDC fault warning lamp to indicate that HDC should
be deselected. If active braking continues and the ABS ECU determines that brake temperature has increased a
further 50 C, it fades out active braking and disables HDC. After fade out, the HDC fault warning lamp continues to
flash, while HDC is selected, until the ABS ECU calculates brake temperature to be at an acceptable level. This
calculation continues even if the ignition is turned off, so turning the ignition off and back on will not reduce the
disabled time. When the ABS ECU calculates the brake temperature to be acceptable, it extinguishes the HDC fault
warning lamp and illuminates the HDC information warning lamp to indicate HDC is available again. The disabled time
is dependant on vehicle speed.
Diagnostics
While the ignition is on, the diagnostics function of the ABS ECU monitors the system for faults. In addition, the return
pump is tested by pulsing it briefly immediately after the engine starts provided vehicle speed exceeded 4.4 mph (7
km/h) during the previous ignition cycle. If a fault is detected at any time, the ABS ECU stores a related fault code in
memory and illuminates the appropriate warning lamps in the instrument pack. If a fault exists in a warning lamp
circuit, the lamp will not illuminate during the lamp check at ignition on, but, provided there are no other faults, the
related function will otherwise be fully operational.