Cross-Linking the P38 Air Suspension
Design and Safety Considerations
Hardy Neale's Installation
Introduction: Basic Principle
The new Mk III (L322) Range Rover introduced in 2002 features an innovation that greatly enhances off road traction and ride -- namely the ability to let air flow freely from left to right air springs during axle articulation on uneven terrain. This effectively softens the spring rate to zero. (For an explanation of the traction advantages of softer spring rates, see the Range Rover Suspension page, and for more information about the Mk III suspension see the Mk III RR Suspension Operation Page). When one wheel is pushed up, it will help push the other wheel down, also aiding traction. The vehicle still stays upright because of the action of the anti-sway bars, which are still connected.
Even before the advent of the Mk III model, I had often thought of implementing this feature on my 4.0SE, but have not got around to it yet. However, Hardy Neale of Australia has pioneered this technique for us on his 4.6, and shares the details with us on this page. As of this writing, testing is at a preliminary stage and some caveats apply (see caveats).
The object of the exercise is to be able to link the right and left front air springs together, and the right and left rears together (separate from the front circuit) when off road in uneven terrain and more traction or a softer ride is desired.
For maximum response speed, the cross linking should ideally be done with larger gauge tubing than the 6mm stock air lines, but that would involve modifying the spring tops themselves and installing larger fittings. However as any owner can testify, the air can flow fairly quickly out through the stock tubing (for example when you lower the vehicle to the access position).
Cross-Coupling Activation: Safety Considerations
The cross coupling air connections should (as on the new Mk III) be made switchable by solenoid air valves so that it is only activated in off road situations, and in normal highway driving there is no connection between right and left springs. (If activated on the highway it would cause scary handling problems). Also, if you try implementing any kind of cross linking, you should leave the sway bar connected!!! Otherwise there is nothing to prevent the vehicle "falling on its side".
On the Mk III, the responsibility of deciding when to activate cross-coupling is given to the air suspension ECU, which monitors wheel articulation and switches on the cross-linking when it thinks the vehicle is in uneven off road terrain. On the 4.0/4.6, one option would be to wire the system so it is only activated in low range. Another would be to have manual control switches (this is the solution chosen by Hardy Neale -- see below).
Lazy Man's Design
On the 4.0/4.6 it would be possible to implement either front or rear cross-linking (but not both) by merely tapping into the wiring to the existing valve block and switching on the left and right air spring solenoid valves so that air could flow between them via the valve block. A possible problem with this approach is that the solenoid valves may not be rated for continuous duty (see below). However they may be OK for operation for, say, a minute at a time, which may be enough to help the vehicle through a tricky patch of terrain.
Installing Separate Solenoid Valves
A better approach which allows both front and rear cross linking simultaneously is to install separate additional solenoid valves specifically for the cross linking function, as is done on the Mk III. On the 4.0/4.6, the easiest place to access the air lines for solenoid placement is at the existing valve block. Of course, this is not the most efficient location, as it involves long runs of air line for the cross coupling, particularly for the rear wheels. (In the Mk III Range Rover this problem is solved by installing the cross-coupling solenoids at separate locations near the front and rear axles).
Choice of Solenoid Valves:
It should be noted that on the Mk III special solenoid air valves are used that are rated for continuous operation -- i.e. they can be kept in the "open" condition indefinitely. This is not the case on the solenoid valves used in the valve block on the Mk III, which can only be operated on about a 33% duty cycle lest they overheat (see Mk III Range Rover Air Suspension Fault Diagnosis Page). This may also be the case on the solenoid valves in the 4.0/4.6 valve block. If this were not the case, on the 4.0/4.6 it would be possible to implement either front or rear cross-linking (but not both) by merely switching on the left and right air spring solenoid valves so that air could flow between them directly through the valve block.
Hardy Neale's Installation
Hardy decided to mount his cross-linking solenoids beside the valve block where the air lines are easily accessible. As shown in the photo at the top of this page, he used a pack of four valves -- i.e. twin solenoid valves per circuit, closed without power. (The valves only hold pressure one way, so 2 were required back to back for the front crosslinking and another two for the rear).
Hardy fitted simple push fit 'T' pieces near the EAS valve block, then mounted the block of 4 solenoids on a bracket nearby, routing new 6mm hose as required. He then installed two illuminated rockers switches on the driver's side dash to operate the valves.
Some parts of the cross-linking
system can be seen in these photos by Hardy Neale.
control switches are shown above.
All switches activate up instead of down (i.e. they use the American
convention). Switch functions are as follows:
Hardy reports: "All seems to be holding pressure, but it is easy to revert back to factory standard if there are any problems. I think 6mm hose will be okay - we're talking 150psi so I'm sure 1 wheel up = other wheel down. Next weekend is a Rover Club trip - I'll test it all out then, driving the same line with and without, see it really does make a difference. I won't use it on medium/fast terrain.
"The cross linking of air springs has not had any detrimental effect as yet, but its benefits are hard to assess. In reality it is quite dangerous, especially if accidentally activated whilst cornering on bitumen or if both axles opened on any terrain! The theory will have to suffice for now until I find a proper way to prove it to be an enhancement".
Four "normally closed"
one way air solenoid valves rated for continuous duty at over 150 psi
One foot of 6mm nylon air tubing rated over 150psi
Automotive grade control switches that are not likely to short out and accidentally activate the cross linking