hi,
"Does Unity claim their module will fool the ASM so it continues to operate the rears?"
I don't think so and not even expect it to do that. I just keep the rear original shocks and visually check 'm until time to replace (if needed). so far they're looking good.
the compressor treat each shock separately (as I understand) so the fact that the front air shocks are out, does not affect on the rear at all. when I'll connect the module defuser (soon, one day) I hope to eliminate the error messages, and hope for a smooth ride. I drove today over 300 miles with the new springs shocks, north Miami Beach to Sebring round trip. it felt great. "nothing to worry about the front."

 

The compressor does not pressurize each air spring separately. The compressor pressurizes the air reservoir in the trunk/boot, under the spare tire. The valve body/manifold then distributes air from the reservoir to the individual air springs under the control of the ASM.

With your front air springs disconnected, you may find that your compressor is no longer operating at all. When certain faults are detected by the ASM, it disables the compressor so the compressor does not run excessively and overheat and wear out the piston ring/seal prematurely. You may find that when the air currently stored in your rear air springs eventually leaks out, they will not repressurize.

For more information on the air suspension, see this post:

Air Suspension & ECATS - System Summary: Components & Operation

Cheers,

Don

 

My take on that would be that the level on the coils in front would never be "out," so to speak, so the ASM module would never be trying to do anything with the front. That is, unless the car reaches the highway speed at which the ASM wants to lower the car, and it sees the front not come down and maybe throws a fault.

The rears have to be maintained by the system or the car will eventually just sit on the bottom, like an old Lincoln Town car that's lost its rear air:


The ASM has no way to know the fronts aren't air springs unless it tries and fails to adjust them, at which time it throws a code and shuts down any further attempt until the car is started. Depowering the ASM and/or installing a module to disable the CATS warning would result in the rears eventually sagging to the bottom rests.

To @shartov once the ASM throws a code and puts a message on the dash like "Air Suspension Fault," it shuts down to protect the compressor from constant running and thus overheating. It will not resume air suspension control until the car is shut off and restarted. Replacing one end of the car only will not be a permanent solution! Coils at one end and air at the other is a Bad Idea...

 

Not THAT bad.. or it would not be so commonly done.

Roll back to the basics. In the extreme case, the struts do not know or care if they are provided air from 'decisions' made by a no-longer-active electronics module.. or by an owner with an analog guage, "calibrated eyeball" or a Locke Level, and an ignorant pushbutton commanding anything-BUT a WABCO compressor.

Same as Monroe or Gabriel air shocks I began use of, dawn of the 1970's. Nothing "dynamic" about them when charged with a service-station's air-hose. Only went "partially" dynamic AKA adjustable on (or OFF) the road - when I tucked an onboard compressor into the '79 Wagoneer Quadra-whatsis for hauling the six-bunk RV rig.

That "extreme case" is seldom needed, but it IS a road VERY well-traveled for basic load-leveling. Which doesn't often NEED to be "dynamic" anyway.

With that as backstop/fallback, anything that DOES retain and make even partial use of the OEM system can still be a welcome improvement.

 

hey guys, thank god you are here and thank you for sharing.
I'm here to enjoy the company of wise and experience Jag owners. and most important I'm here to learn. the more I learn, the more I'll enjoy the car. this is my second one. my first was in New York City a 2004 XJ8 which my son took once for a ride and returned it with rear-end accident. total lose....
now it's me only around with this beautiful 2006 XJ8. today she got a wash, see photo. soon the hood emblem will arrive.
now back to the thread. I'm listening and I will follow the advise I get here. it's great. I'm sure I'll need to change the back struts as well. right now I'm in the "wait and see" state,
again, thank you all.

at the car wash

 

X350's are relatively light in weight. Though nothing much changes from the front seats, forward, as there is always a "driver", so it is only one passenger's added mass - or not. NO change in mass under the bonnet.

Center of vehicle rearward is more variable, what with sensitivity to fuel level, luggage, and rear-seat passenger loads, if any.
Active suspension quite aside, self leveling is more than just a "convenience". Do try to retain that self-leveling for the rear if you can.

Not to forget that just as coil-overs can be installed, so, too can the full original system be restored, ECATS as well as air..

That option has always been more about money than black-magic.

 

You can see Jaguar's thinking in that the later X351 XJ dropped the front air stuff and only use air in the back end where there is much more variation as Bill posts above. It must have worked because we see very little air suspension problems on the X351 XJ's.
.
.
.

 

Rears usually outlast fronts on the X350's as well. That might be because "Jaguar people" do not often fill the rear seats, nor carry cargo?

The "obvious" option.. and ISTR it has been explored, already, but wasn't practical?

- Swapping in an X351's gear, all around - and WITH its brain-box substituted ... ???

...or maybe from a Kia or Hyundai? They look like the same car as an X351.
..and "Pinwheel" Lyons does another round of rotations in his grave...

 

So you're manually controlling the rear height? Running the compressor is not enough by itself to do that if the ASM is unpowered; something has to control the valve body solenoids and the compressor's vent valve,

I still think it's a Bad Idea, and I would never consider it on my own car. Your example of air shocks is totally invalid, as those were not the entire support of the vehicle the way the Jaguar's air suspension is. Air shocks give supplemental support to the steel springs that are also in place. I had an Aurora that had automatic rear leveling through air shocks and an onboard compressor, but again, the air was not the entire suspension, just a bit of lift for when the car was loaded.

 

Only to the extent of calling for more pressure if/as/when the system goes awry.

A pernicous but very slow, leak of a front shock seal, cold weather, high airflow whilst moving is the more usual case.

Compressor ALMOST keeps up, but then timer shuts if off, and "sensor plausibility error" forces it to remain off - at speed, already - while the vehicle settles down onto the bump stops.... miles from a safe place to stop, snowstorm, freezing weather.

Take control of the back-bleed valve away from the Artificial Idiot as didn't NEED to drop the remaining pressure. That's an easy remedial step, regardless. 'Fail Safe" should be max inflation, not minimum. Even if one corner goes low, the whole chassis isn't dragging on the ground whilst beating the suspension to death.

Kick a second compressor with 100% duty cycle capability onto the tasking, manually? Now it is equipped to get to a better place. Slow leak and all.

Not fussed if it is a 'popular' approach. It were MY fossilizing **** as was freezing.

Few tweaks can't make it into a DS-19.. but nor is an X350 as butt-ugly on the outside, so I otherwise like the results the system CAN deliver..