This circuit allows you to manually adjust the height of your land rover discovery series 2's rear air suspension. It 'emulates' the signal sent by the optional SLS remote control.

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Update: Kits available - email [email protected] for details!

SLS (Self levelling suspension) controller for LandRover Discover Series 2 models.

The Land Rover Discovery Series 2 has optional SLS (self-levelling) rear air suspension. Its' height can be lowered by an remote controller 'plip' available from Land Rover, to make it easy to attach trailers etc to the towball.

This circuit emulates the signal generated by the plip, and allows you to fit a switch to let you adjust the height of the suspension at your leisure.

You could fit the switch into the dashboard binnacle on a blank switch cover, or extend the wires, and have a switch mounted on the rear bumper next to the tow ball etc. to allow you to adjust the suspension height from your location of choice.

How it works

The BCU (Body control unit) communicates with the SLABS ECU (Self-levelling and ABS ECU) to tell it to adjust the rear suspension height as follows:

A 25Hz square wave is sent to pin X on the SLABS ECU whenever the ignition is on position 2, with a duty cycle of 50%, where 12v is interpreted as logic 1, and <3v is logic 0. When an UP signal is received from the SLS remote, the duty cycle is changed to 75%. When a DOWN signal is received from the SLS remote, the cycle is changed to 25%.

This circuitry uses the PIC to generate the appropriate PWM (pulse width modulation) signal and feeds it directly to the SLABS ECU, thus bypassing the BCU altogether. The circuit is powered from an ignition switched live feed so a signal is generated when the ignition switch is at position 2. When the switch is placed into either up or down, the appropriate PWM signal is generated.

Limitations

The SLS will only change height with the engine running, and with ALL doors (including tailgate) closed. You can lower the suspension to the bump stops, and raise it up to the normal ride height. Once your speed exceeds 5mph, the SLS will return to normal ride height. These limitations come from the SLABS ECU itself, not this project.

Parts needed:

Switch: You need a centre-off, single pole double throw (SPDT) switch, ideally with both poles non-latching, to control the board.

The PCB above.

A 5V Arduino Pro Mini, programmed with the attached sketch.

A suitable NPN transistor (almost any will do), I used a BC501. For the SMD version, I used a BC848b, in SOT-23 packaging.

A voltage regulator capable of 5V output from 13.8VDC input. A 100mA one is fine.

Resistors:
R1 : 3k (or near) R2 : 1k (or near)

An inline fuse holder and a 100mA or similar quickblow fuse.

A 'Scotchlock' style automotive splice connector (or you could splice and solder the wire into the existing loom.

A ring-style crimp terminal.

That's it!

Assembly

Etch up your PCB using your preferred method.

There are two .fzz files - one containing a PCB layout using SMD, and one using traditional 'through-hole' components. I went for the SMD one for myself as I'm happy soldering small components, and it saves drilling holes in the PCB and generating loads of fibreglass dust!

Assemble the circuit. I mounted the arduino on pin headers so I could remove it in case I wanted to reprogram/repurpose it in future, but you could solder it straight to the board if you wished.

The 100mA quickblow fuse should go inline with the main power feed to the board for safety.

Testing

Apply 13.8VDC (or 12, it's near enough). Ideally do this BEFORE installing the arduino. Check that 5v appears from the regulator output to the Arduino 5v input pin. This will save you from frying it if something else went wrong.

Install the arduino, and then power up the circuit. If you have an oscilloscope, check that on the output pin, you see a 25Hz square wave with a 50% duty cycle. You should notice that pulling the UP, and then DOWN pins (one at a time) that the duty cycle changes to 75%, and 25% respectively.

Installation

Have a look at the attached PDF file explaining (with photos) how to install it.