BRAKES: DESCRIPTION AND OPERATION (page 2 of 5)

BRAKES

DESCRIPTION AND OPERATION

70-21

ETC
The ETC function uses brake intervention to prevent wheel spin and maintain even torque distribution to the wheels. 
ETC is automatically enabled while the brakes are off at speeds up to 62.5 mph (100 km/h), and operates the brakes 
either individually or in axle pairs:

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At speeds up to 31.3 mph (50 km/h), ETC uses individual brake intervention to maintain even torque distribution 
between wheels on the same axle.

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Vehicles up to 03 model year  – At speeds between 0 and 62.5 mph (0 and 100 km/h), ETC also uses brake 
intervention in axle pairs to maintain even torque distribution between the front and rear axles. In effect, this mode 
of operation replaces the centre differential lock of the transfer box which, although still incorporated, is non 
operational under normal driving conditions. 
 

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 TRANSFER BOX - LT230SE, DESCRIPTION AND OPERATION, Description. If the centre differential 

lock is in the locked condition, the SLABS ECU illuminates the ABS and ETC warning lamps and inhibits the ETC 
function (the ABS, EBD and HDC functions are retained, but at degraded performance levels).

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Vehicles from 03 model year (with differential lock fitted) – At speeds between 0 and 62.5 mph (0 and 100 
km/h), ETC uses brake intervention in axle pairs to maintain even torque distribution between the front and rear 
axles. If the centre differential lock is in the locked condition, the differential lock warning lamp in the instrument 
pack is illuminated. The ABS, EBD, ETC and HDC functions are retained, but with revised parameters to suit the 
locked differential.

While the ETC function is enabled, if the SLABS ECU detects a wheel accelerating faster than the average, indicating 
loss of traction, it operates the ABS modulator in the active braking mode. Depending on the vehicle speed, active 
braking is employed for either the brake of the affected wheel or for both brakes on the affected axle, until all four 
wheels are driven at approximately the same speed again. During active braking the SLABS ECU also illuminates the 
ETC warning lamp, for a minimum of 2 seconds or for the duration that ETC is active. ETC operation is desensitised 
during 'hard' cornering.

HDC
HDC uses brake intervention to provide a controlled descent ability in off road conditions when engine braking is 
insufficient to maintain a comfortable speed. This allows the driver to leave HDC selected and to control the vehicle's 
descent speed, down to the system's minimum target speed, using only the accelerator pedal. The HDC function is 
selected on/off by a switch on the fascia. When selected on, HDC is enabled in all forward gears and reverse provided:

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Vehicle speed is below 31.3 mph (50 km/h).

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The transfer box is in low range.

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On manual gearbox vehicles, the clutch is engaged.

When HDC is enabled, the HDC information warning lamp illuminates. If HDC is selected outside the above 
conditions, the HDC information warning lamp flashes and the audible warning sounds continuously.

When HDC is enabled, the SLABS ECU calculates a target speed from the throttle position element of the engine data 
input, and compares this with actual speed. If the actual speed is higher than the target speed, the SLABS ECU 
operates the ABS modulator in the active braking mode to slow the vehicle down to the target speed. While the braking 
force is being applied, the SLABS ECU also energizes the brake lamp relay to put the brake lamps on. Active braking 
is discontinued while vehicle speed is below the target speed or if the foot brakes are applied. Applying the foot brakes 
during active braking may result in a pulse through the brake pedal, which is normal.

During active braking, the brakes are operated predominantly on the wheels of the leading axle, but if that is not 
sufficient to achieve the required deceleration the brakes of the trailing axle are also applied. The deceleration rate is 
dependent on the speed differential between initial vehicle speed and the target speed. The deceleration rates are 
relatively low at higher speed differentials, then progressively increase as vehicle speed approaches the target speed. 
Anti-lock braking is also enabled during active braking, but at very low speeds some wheel lock can occur.

The target speed increases as the accelerator pedal is pressed, from a programmed minimum with the accelerator 
pedal released, up to a maximum of 31.3 mph (50 km/h). For any given accelerator pedal position, while travelling 
uphill or on level ground the target speed is always greater than the corresponding vehicle speed, which allows the 
vehicle to be driven normally without HDC intervention. However, when travelling downhill, the gravitational effect on 
the vehicle means that for any given accelerator pedal position the target speed is less than the corresponding vehicle 
speed, and HDC intervenes to limit vehicle speed to the target speed.