Description

General
The Self Leveling Suspension (SLS) system is an optional fitment and comprises an Electronic Control Unit (ECU), air supply unit, two air springs and two height sensors. The SLS system only operates on the rear suspension and is designed to keep the vehicle level to compensate for uneven loads or when towing. The system controls the gap between the chassis and the rear axle to a tolerance of 0.5 mm (0.02 inch). The ride height of the rear of the vehicle can be controlled in three modes of operation; normal ride height, Off-Road Mode (ORM) and extended mode. A transportation mode, initiated using Test Book, is also available for moving the vehicle on a trailer.
The system is controlled electronically by an ECU, which is shared with the ABS system and known as the Self Leveling and Anti-Lock Braking System (SLABS) ECU. The system operates by using an air supply unit to inflate or deflate the air springs to maintain a constant ride height.
An accessory remote handset is available to remotely operate the SLS system to allow easier connection and disconnection of trailers.
Two SLS system warning lamps are located in the instrument pack. The warning lamp in the bottom left corner of the instrument pack is the SLS warning lamp. If a fault is detected in the system, the warning lamp will illuminate continuously in an amber color. The warning lamp also flashes in an amber color when the remote handset is being used. The second warning lamp, located in the top right of the instrument pack is the ORM warning lamp. When ORM is selected the warning lamp is continuously illuminated in an amber color. When the SLS is between standard ride height and ORM or in extended mode, the warning lamp will flash. Standard ride height, measured between the tip of the axle bump stop rubber and the axle, is 61.5 mm (2.42 inch). ORM ride height, measured between the tip of axle bump stop and the axle, is 100 mm (3.93 inch).
A switch is located in the group of six switches on the fascia and is used to select the ORM. The switch is non-latching and must be depressed for a minimum of 0.5 seconds to signal the ECU that ORM has been requested.

Air supply unit





The air supply unit is located in a central position on the outside of the left hand chassis longitudinal. The unit is contained in a plastic housing attached to the chassis. The housing has a removable lid, which is secured with Dzus fasteners for access to the unit.
The air supply unit comprises a 12 V electric motor, a compressor and air dryer unit, a pressure limiting valve, an exhaust valve and two air supply control valves. The exhaust and control valves are solenoid operated responding to signals from the SLABS ECU. The electric motor, compressor, air dryer and pressure limiting and exhaust valve are mounted on flexible rubber mountings to reduce operating noise.
The electric motor drives a crank with an eccentric pin to which a connecting rod is attached. The connecting rod has a piston, which fits in the bore of the compressor. Operation of the motor rotates the crank, moving the piston in the bore of the compressor.
The compressor is attached with Allen bolts to the motor housing and sealed with an O-ring. Attached to the compressor is the air dryer, which contains a silicate box for removing moisture from the compressed air. Air supplied to inflate the air springs passes through the air dryer. When the air springs are deflated, the exhaust air also passes through the air dryer, removing the moisture from the unit and expelling it to atmosphere.
Attached to end of the air dryer unit is the pressure limiting valve. The valve protects the air springs from over inflation. The pressure limiting valve also operates when the exhaust valve is opened. The valve is pneumatically operated, responding to air pressure applied to it.
The exhaust valve is also located with the pressure limiting valve. The exhaust valve is solenoid operated by the ECU and directs air from the air springs and control valves to atmosphere when required.
The two air control valves for the LH and RH air springs are located at the forward end of the housing. Each valve is connected to the compressor/air dryer unit through a shared single pipe, which directs air to and from the air springs. Each control valve is individually operated by the ECU.
All air connections to and from the air supply unit are made through the SLS air harness, which is located along the left hand chassis longitudinal.
If faults occur with the air supply unit, fault codes are stored in the SLABS ECU. These fault codes cover the compressor power supply and the LH and RH air control valves and the exhaust valve. The current and past fault codes can be retrieved with Test Book. Test Book can also be used to operate the compressor and the valves for diagnostic purposes.

Air intake filter
The air intake filter is located in the left hand 'E' post, behind the tail lamp assembly. The filter comprises a plastic molded housing which contains two filters of differing density to remove particulate matter from the air drawn in by the compressor. The air intake filter has a pipe, which is connected to the SLS air harness by a quick release connector. The air intake filter must be replaced as an assembly.

Air intake/Exhaust silencer
The air intake/exhaust silencer is located on the outside of the left hand chassis longitudinal, behind the air supply unit. The silencer is an integral part of the SLS air harness and cannot be serviced individually.
The intake and exhaust air to and from the compressor passes through the silencer. The plastic molded silencer has two chambers. One chamber reduces system exhaust noise and the other dampens intake air pulses produced by the compressor.

Height sensors





Two height sensors are located on the outside of each chassis longitudinal forward of the rear axle. Each sensor body is attached to a fabricated bracket and secured with two screws. The sensor is attached to the top of each radius arm by an arm, a link and two link ends. The link ends allow articulation of the arm to allow for suspension travel. The lower link arm is attached to a lug of the top of the radius arm and is secured with a bolt and locknut.
The sensor body and arm are manufactured from molded nylon. The two link ends are made from natural rubber and the link is made from mild steel. The rubber link ends allow flexibility of the arm and resistance to damage.
Each sensor is connected to the main chassis harness by a multiplug. The three pin multiplug provides an earth, a 5 V supply voltage and an output signal voltage to the SLABS ECU.
Each sensor operates on the Hall effect principle. A magnet is attached to the shaft and rotates with movement of the arm. The magnetic flux generated acts on a Hall effect sensor and depending on its position varies the current across the sensor. This current is measured and amplified and passed to the SLABS ECU as a linear output voltage signal, which varies depending on the angular position of the sensor. The signal information is processed and the ECU can determine the vehicle height.
When the sensors are replaced or removed for any reason, a calibration procedure is required to recalibrate the sensors and the SLABS ECU. The calibration procedure requires the use of Test Book and calibration blocks to set the axle to chassis height to a known value.
If faults occur with the height sensors, fault codes are stored in the SLABS ECU. The current and past fault codes can be retrieved with Test Book.

Air springs





On vehicles without SLS, coil springs are used. On vehicles with SLS two air springs are fitted between the chassis and the rear axle to replace the coil springs. Each air spring is located at its base on a fabricated platform on the rear axle. The top of the spring locates in a fabricated bracket attached to the outside of each chassis longitudinal.
The plastic base piston is recessed with a boss with two lugs molded in the center for attachment to the axle. The piston is secured by locating the lugs in a slotted hole in the axle platform and rotating the spring through 90°, locating the lugs in the slot. The plastic top plate has two grooved pins, which locate through holes in the chassis bracket. Two spring clips locate on the grooved pins and retain the top of the spring in position.
Each air spring comprises a top plate, an air bag and a base piston. The air bag is attached to the top plate and the piston with crimped rings.
The air bag is made from a fiber reinforced flexible rubber material, which allows the spring to expand with air pressure and deform under load. The top plate comprises the two bonded grooved pins and a female Voss connector in the center. The Voss connector allows for the attachment of the air supply pipe from the air supply unit. The piston is also made from plastic and is shaped to optimize the springs characteristics.

SLABS ECU





The SLABS ECU is mounted on a bracket behind the passenger glove box and is identified from the other ECU's by its five connectors. The five connectors are located on the lower face of the ECU and mate with five connectors from the main harness. The twelve, six and eighteen pin connectors are used to supply inputs and outputs to and from the ECU. The remaining connectors are used for the ABS operation.
The SLABS ECU receives a continuous battery supply from fuse 11 in the engine compartment fuse box. An ignition 'ON' signal is supplied from the ignition switch via fuse 28 in the passenger compartment fuse box. The ECU has the ability to control when it requires power and is not reliant on the ignition signal for it to power up.
The ECU incorporates a counter, which times the operation of the SLS system and prevents the compressor exceeding its duty cycle. The ECU can remain powered for up to 1.5 hours after ignition off is sensed to allow the counter to continue running to avoid an ignition cycle resetting the counter.
Opening any of the doors will power up the ECU, irrespective of ignition switch position. The door open signal is sensed by the door switch completing an earth path which is sensed by the ECU. The ECU cannot differentiate between any of the doors. The door open signal powers the ECU for up to 30 minutes to allow the vehicle to re-level when a load is removed or passengers leave the vehicle.
The ECU supplies a 5 V current to each of the height sensors. Each height sensor uses the current to supply an analog input to the ECU. The ECU can calculate from the input received from each height sensor the height of the vehicle and can then power the air supply unit as necessary to raise or lower one or both air springs to level the vehicle.
When SLS compressor operation is required, the ECU provides a battery supply to energize the SLS relay located in the engine compartment fuse box. When the relay contacts close, a 12 V supply passes through fusible link 9 in the engine compartment fuse box, through the relay contacts and operates the air supply unit compressor. The ECU will then supply power to operate one or both air control valve solenoids and/or the exhaust valve solenoid to inflate or deflate the air springs as required. The compressor does not need to be powered to deflate the air springs.
The ECU also controls the operation of the SLS audible warning, the SLS warning lamp and the ORM warning lamp. When the ignition is switched to position 11, the ECU performs a three second bulb check and illuminates the SLS and ORM warning lamps in the instrument pack to check for operation. When the system is operating or a fault is sensed by the ECU, the ECU will operate the appropriate warning lamp and audible warning as required. The audible warning is operated by the Body Control Unit (BCU) when it receives a signal from the SLABS ECU. The audible warning is emitted from a speaker at the rear of the instrument pack.
Depressing the ORM switch for a minimum of 0.5 seconds, completes an earth which the ECU uses as a signal to initiate the ORM if conditions allow. When the ECU starts ORM, the same earth that was completed by the ORM switch is pulled to earth by the ECU to activate the ORM warning lamp. The ECU checks for a further operation of the ORM switch by continuously and very quickly removing the earth for the ORM warning lamp. If the ORM switch is operated for more than 0.5 seconds, the ECU will detect this and de-activate the ORM.
The SLS part of the SLABS ECU also uses the road speed data generated within the SLABS ECU by the ABS system. Operation of ORM and extended mode are road speed sensitive and use the ABS signal to monitor the vehicle speed.
When the accessory remote handset is used for the SLS lower and raise functions, the handset transmits RF signals which are received by the same RF receiver used for the alarm/remote door locking system. The RF receiver passes this data as a 25 Hz PWM signal to the BCU. The BCU then transmits this data to the SLABS ECU as raise or lower data. Test Book is required to program the BCU for remote handset operation.





SLABS ECU connector pin details

Failure modes
Failures are indicated by the SLS warning lamp in the bottom left corner of the instrument pack illuminating continuously in an amber color. The following tables show the type of system failures and their effects on the system operation.





Height sensors





Door Switch Inputs





SLS off-road mode switch





Air supply unit air control valves





Air supply unit compressor