Operation

When the ignition is switched on, the SLABS ECU performs a check of the brake related warning lamps as part of the power up procedure. The warning lamps are illuminated for approximately 3 seconds and then extinguished. If a fault warning lamp remains illuminated after the lamp check, a fault has been detected and repair action is required.

ABS
The ABS function prevents the road wheels locking during brake application, thus maintaining vehicle stability even under emergency conditions.

WARNING: ADS is an aid to retaining steering control and stability while braking:
^ ABS cannot defy the natural laws of physics acting on the vehicle.
^ ABS will not prevent accidents resulting from expressive cornering speeds, following another vehicle too closely, aquaplaning, etc.
^ The additional control provider by ABS must never be exploited in a dangerous or reckless manner which could jeopardize the safely of driver or other road users.
^ The fitting of ABS does not imply that the vehicle will always stop in a shorter distance.

NOTE: During normal braking the feel of the brake pedal on vehicles equipped with ABS will be the same as that on non ABS vehicles. During anti-lock braking operation the driver will experience feedback in the form of a pulsating brake pedal and solenoid/pump motor noise from the ABS modulator.

The anti-lock braking function is automatically enabled whenever the ABS modulator is in the normal braking mode. While the anti-lock braking function is enabled, if the SLABS ECU detects a wheel decelerating faster than the average and at the calibrated wheel slip limit for ABS operation, it operates the ABS modulator in the ABS braking mode for the affected wheel.

EBD
The EBD function optimizes the distribution of hydraulic pressure between the front and rear axles, under all vehicle load configurations and road conditions, to maintain vehicle stability during braking. EBD operates in forward and reverse and is automatically enabled whenever the ABS modulator is in the normal braking mode at vehicle deceleration rates of 0.3 g and above (i.e. medium to high brake pedal loads). EBD operation is similar to that of ABS, but is calibrated to intervene at lower wheel slip limits and operates the brakes in axle pairs instead of individually.
During braking, if the SLABS ECU detects the wheels of one axle going slower than those of the other axle, i.e. a potential wheel slip situation, it signals the ABS modulator to close the inlet solenoid valve for the brakes of the slower wheels. This prevents any further increase in hydraulic pressure to those brakes, while allowing the hydraulic pressure to the brakes on the other axle to increase and so maximize the overall braking effort. If the wheel speeds of the axle being subjected to EBD control return within the calibrated wheel slip limits, the SLABS ECU signals a stepped opening of the inlet solenoid valves, which allows a progressive increase of hydraulic pressure to the related brakes.
Operation of EBD is detectable from a stiffening of brake pedal movement as the inlet solenoid valves close and a slight pulsing of the brake pedal as the inlet solenoid valves open. EBD operation ceases immediately the brake pedal is released.
The wheel slip limit for EBD operation varies with vehicle speed. During normal operation, the inlet solenoid valves always operate in axle pairs, with only one axle pair closed at any one time. Since the most lightly loaded wheel during a braking maneuver will usually be the first to reach the slip limit, under most vehicle load configurations and road conditions EBD control occurs on the trailing axle. However, EBD control can occur on the leading axle or switch between axles during the braking maneuver.

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:
^ 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.
^ 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 center differential lock of the transfer box, which although still incorporated, is non operational under normal driving conditions.
TRANSFER BOX - LT230SE, DESCRIPTION AND OPERATION, Description. If the center 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).
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 desensitized during 'herd' 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:
^ Vehicle speed is below 31.3 mph (50 km/h).
^ The transfer box is in low range.
^ 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 travailing 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 traveling 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.

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 traveling 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:
^ A system fault.
^ The conditions for HDC are no longer being met.
^ 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:





Typical disabled times

Diagnostics
While the ignition is on, the diagnostics function of the SLABS 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 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. If a fault is detected during the power up, the SLABS ECU stores a related fault code in memory and illuminates the appropriate fault warning lamps. If a fault is detected later in the drive cycle, the SLABS ECU also sounds the audible warning three times.
Fault codes and diagnostic routines can be accessed by connecting Test book to the vehicle's diagnostic connector in the driver's footwell.





WARNING lamp fault operation

After detecting a fault, the SLABS ECU selects an appropriate default strategy which, where possible, retains some operational capability. A shuttle valve switch fault and throttle position signal fault are classified as permanent faults. If a permanent fault is detected, the related warning lamp illumination and default strategies are automatically employed in subsequent ignition cycles, even if the fault is intermittent, until the fault has been rectified and cleared from memory. If a non permanent fault is detected, the related warning lamp illumination and default strategies will only be employed in subsequent ignition cycles if the fault is still present.
After rectification of an ABS sensor fault, the ABS and ETC functions are disabled, and the ABS warning lamp remains illuminated after the lamp check, until vehicle speed exceeds 9.4 mph (15 km/h) (to allow additional checks to be performed).





Default strategies

Electrical data





Nominal resistance values for applicable brake control components are as detailed below: