Introduction

The air suspension's Extended Profile mode, which raises the vehicle another 1.2 inches above High Profile, cannot normally be selected under driver control; it can only be selected automatically by the suspension ECU when it senses the vehicle is grounded. It can also be selected by the dealer's "Testbook" to make working under the vehicle more convenient. To enable manual selection of this extra high setting, I devised a way of raising the suspension by an extra 1.2 inches (or any other desired amount) at the flick of a switch.

Principle of Operation:

The air suspension is controlled by its own computer (ECU) which senses vehicle height using the voltages it receives from sensors at each wheel. These sensors are nothing more than potentiometers. The trick is to subtract a constant voltage from each sensor reading so the ECU thinks the vehicle is lower than it really is. The ECU then automatically increases suspension height to compensate. The necessary voltage subtraction can be achieved by injecting a negative voltage into the common return line from the sensors to the ECU.

The circuit I used to do this is depicted in blue in the diagram at right. The double-pole double throw (DPDT) switch is normally in the position shown, which disconnects the battery and grounds the common sensor return wire back to the ECU so the added circuit has no effect. When the switch contacts are in their upper positions, the 1.5 volt battery is connected to the 25 ohm potentiometer to form a low impedance voltage source which is interposed between the sensors and the ECU return line. Adjustment of the potentiometer varies the amount of voltage injected (I found that 1.0 volts is just about perfect to achieve the required 1.2 inches of extra height).

Procedure:

1. The circuit shown was built and housed in a small enclosure with a hole in it to access the potentiometer control with a screwdriver for adjustment. The other components used were a double pole switch from Radio Shack, a regular alkaline "D" cell, and a low resistance potentiometer (I used an old 25 ohm one but the value should not be critical from 5 to 100 ohms).

2. I disabled the suspension ECU by removing the timer controller (actually a complex circuit in its own right but looks like a large relay) also located under the left front seat. I was being cautious here -- I have since fiddled around with this circuit without removing the relay and have experienced no ill effects as yet -- knock on wood!!!).

3. I located and cut the black/pink line going to pin 24 on connector #117, which plugs into the air suspension ECU (located under the left front seat). To be sure I got the right wire I removed the connector, took its cover off and traced the wire from pin 24. Standard automotive spade connecors were added to the cut ends so I could reconnect them if needed.
 

View under left front seat from outside the vehicle, showing position of the EAS timer relay. It can be lifted off to disable the air suspension.	View under left front seat from the front footwell, showing position of the EAS connector 117. The EAS ECU is the lower of the two under the seat.

4. The control box was connected to the black/pink wire using similar connectors added to the jumper wires coming out of the circuit box. Care was taken to get the polarity of the connectors right -- if the connections were reversed the suspension would go lower instead of higher.

5. Reassembling the ECU connector, plucgging it back in, replacing the EAS timer control and putting the trim piece back under the seat, I mounted the control box on the trim under the seat front where it is easily accessible.
 

Completed Extended Profile Control Box mounted on trim under front of driver's seat. The red switch toggles between standard and extended settings. A hole in the top of the enclosure allows adjustment of height setting with a screwdriver.

With the bogus Extended Profile selected, ground clearance is awesome (Photos at top of this page and here. When I first compelted this project in 1999, I parked the vehicle next to a Discovery at a British Car meet. The 4.0 in Extended Profile made the Discovery look like a Low Rider!

Cautionary Note:

There is no assurance that this circuit will do no damage to the ECU or other electronics -- use at your own risk. Also, because you are fooling the ECU, if the car actually does become grounded you could trigger an even higher height increase, possibly damaging the springs. So use it with care.