BRAKES: DESCRIPTION AND OPERATION (page 3 of 5)

BRAKES

70-22

DESCRIPTION AND OPERATION

Minimum target speed
The minimum target speed depends on which gear is engaged. Reduced minimum target speeds are employed for 
some gears if rough terrain or sharp bends are encountered while already travelling at the normal minimum target 
speed. If loss of traction makes it impossible to maintain the minimum target speed, the SLABS ECU temporarily 
increases the minimum target speed to maintain stability, then restores the normal minimum target speed when 
traction improves.

HDC minimum target speeds

Fade out
To provide a safe transition from active braking to brakes off, the SLABS ECU invokes a fade out strategy if it detects 
any of the following during active braking:

l

A system fault.

l

The conditions for HDC are no longer being met.

l

Possible brake overheat.

The fade out strategy increases the target speed at a low constant acceleration rate, independent of actual throttle 
position. This results in the braking effort being gradually reduced and then discontinued. The SLABS ECU operates 
warning indications during fade out that are dependent on the cause.

Fade out warning indications

Clutch disengagement/neutral selection
During active braking, if the SLABS ECU detects the clutch is disengaged or neutral is selected, it flashes the HDC 
information warning lamp and sounds the audible warning continuously to indicate that conditions for HDC are no 
longer being met. Initially, the SLABS ECU also fixes the target speed to the applicable minimum target speed, but if 
the condition continues for approximately 60 seconds the SLABS ECU invokes fade out.

Brake overheat prevention
To prevent the brakes overheating, the SLABS ECU monitors the amount of active braking employed and, from this, 
estimates brake temperature. If the SLABS ECU estimates the brake temperature has exceeded a preset limit, it 
flashes the HDC fault warning lamp and sounds the audible warning continuously, to indicate that HDC should be 
deselected to allow the brakes to cool. If active braking continues and the SLABS ECU estimates that brake 
temperature has increased to an unacceptable level, fade out is employed and HDC is disabled. After fade out, the 
audible warning is discontinued but the HDC fault warning lamp continues to flash, while HDC is selected, until the 
SLABS ECU estimates 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 SLABS ECU estimates 
the brake temperature to be acceptable, it extinguishes the HDC fault warning lamp and illuminates the HDC 
information warning lamp to indicate that HDC is re-enabled. The disabled time is dependent on vehicle speed; typical 
times at constant vehicle speeds are as follows:

Gear

Speed, mph (km/h)

Manual gearbox

Automatic gearbox

Normal

Reduced

Normal

Reduced

1

4.4 (7.0)

4.4 (7.0)

4.4 (7.0)

4.4 (7.0)

2

5.2 (8.3)

4.4 (7.0)

4.4 (7.0)

4.4 (7.0)

3

6.0 (9.6)

4.4 (7.0)

7.5 (12.0)

6.0 (9.6)

4

7.5 (12.0)

6.0 (9.6)

7.5 (12.0)

6.0 (9.6)

5

8.8 (14.0)

7.0 (11.2)

-

-

Reverse

3.5 (5.6)

3.5 (5.6)

3.5 (5.6)

3.5 (5.6)

Neutral or clutch 
disengaged

8.8 (14.0)

Last off road speed

4.4 (7.0)

4.4 (7.0)

Cause

Warning indication

HDC fault warning lamp

HDC information 

warning lamp

Audible warning

Fault detected

On

Flashes

Continuous

HDC conditions not met

Off

Flashes

Continuous

Brake overheat prevention

Flashes

Off

Continuous