After compiling some of this information for another thread, it occurred to me that a summary of the air suspension components and operation might be a handy reference for the forum. At this point my eyes are crossed so please let me know of any errors or glaring omissions.

The most thorough system description I have found is in the '04 New Model Technical Introduction Dealer Training Manual, a copy of which I was fortunate to add to my library. I scanned the Air Suspension & ECATS section to pdf format and Graham uploaded it to the forum download area so you can have your own copy:

X350 Air Suspension Manual

What follows is a summary of the information contained in the above manual, plus the Electrical Guide, the Workshop Manual, Technical Service Bulletins, research by some of our fellow members, and a few other sources. The statements are either quoted directly from the references or paraphrased for brevity and/or clarity. For additional detail and illustrations refer to the manuals.

Air Suspension System:

Except for the air springs/dampers (shock absorbers), which were made by Bilstein, the system was made by WABCO, founded in the U.S. in 1869 as the Westinghouse Air Brake Company and now headquartered in Brussels, Belgium and Hanover, Germany. It is now owned by ZF, the global OEM that also manufactures transmissions, steering racks, shock absorbers, hood lift support struts, and other components for Jaguar. In addition to Jaguar, WABCO supplies Audi, Bentley, BMW, Ford, Land Rover, Mercedes-Benz, Porsche, Rolls-Royce, Volkswagen, Volvo, and many other makers of autos, busses and trucks.

Jaguar cleverly calls the adaptive damping system ECATS, or "Enhanced Computer Active Technology Suspension," but it is actually WABCO's ECAS system, or "Electronically Controlled Air Suspension." You can download the WABCO ECAS brochure at the link below:

WABCO ECAS Brochure


Air Suspension Control Module (ASM or ASCM):

The ASM is located behind the right hand rear seat back. The ASM is an Electronic Control Unit (ECU) that controls all aspects of air suspension operation based on input signals from the following components:

Ride Height Sensors
Valve Block Pressure Sensor
Front Vertical Accelerometer
Rear Vertical Accelerometer
Road Speed Signal from the Instrument Cluster (via the CAN bus)
Lateral Acceleration Signal from the ABS/Dynamic Stability Control Module (via the CAN bus)
Brake Switch (via the CAN bus)
Brake Pressure Switch (via the CAN bus)
Engine Torque Rate (Longitudinal Acceleration) Signal from the ECM (via the CAN bus)

The ASM controls air suspension operation via outputs to the following components:

Air Spring Solenoid Valves in the Valve Block
Reservoir Solenoid Valve in the Valve Block
Damper ECATS adaptive damping actuators/solenoid valves (one at the top of each air spring/damper)
Air Compressor Motor
Air Compressor Exhaust/Vent Valve (attached to the air compressor)
Air Compressor High Pressure Relief Valve (attached to the air compressor)
Air Dryer (a cylinder filled with molecular sieve desiccant beads, part of the air compressor assembly)
Headlamp leveling circuits

Air Compressor:

The air supply compressor is mounted behind the front bumper cover above the left fog lamp. It is a single-piston design and incorporates an inline air filter, an air dryer bed filled with desiccant beads, a pressure relief safety valve, and an exhaust vent solenoid valve. The Nominal pressure developed by the compressor is 15 bar / 218 psi. The safety pressure-relief valve is set to relieve pressure above 17.5 bar / 254 psi. The air exhaust vent solenoid valve provides for relief of pressure when lowering the vehicle and to backflow dried air through the air dryer to partially dry and reactivate the desiccant beads. You will sometimes see the air dryer described as the "water trap," but a water trap is a different type of device and there is no water trap in the stock air suspension system.

In the Jaguar implementation, there is no temperature sensor to aid in protecting the compressor from overheating. Instead, an algorithm programmed into the ASM limits the compressor motor to no more than 120 seconds of continuous operation before imposing a mandatory cool-down period of 45 seconds.

The primary job of the air compressor is to maintain adequate pressure in the air reservoir. When an increase in air spring pressure is required, the pressure is supplied from the air reservoir. If the reservoir pressure is insufficient, raising of the vehicle will be inhibited unless the road speed exceeds 25 mph or the transmission is in Park, at which times the compressor will run to raise the vehicle and continue to run until the reservoir pressure is restored. The compressor is operated only as needed, and typically only when the vehicle road speed exceeds 25 mph or when the transmission is in Park. This is part of the overall NVH management strategy (Noise, Vibration and Harshness).

When pressure in one or more air springs must be reduced, the only exit path is via the exhaust valve in the compressor. The appropriate solenoid valve in the valve body opens to allow pressure from the air spring to escape, and the exhaust valve in the compressor opens to allow the pressure coming through the valve body to escape to the atmosphere. When the pressure declines to the desired level the valves close.

The exhaust vent valve is also opened periodically to backflow dried air from the suspension system to partially dry and reactivate the desiccant beads in the air dryer.

You can download the Wabco passenger car air compressor brochure here:

WABCO Air Compressors Brochure

One of the most common failure points in the air suspension is a worn compressor piston ring/seal, which prevents the compressor from pressurizing the system in the time allotted by the ASM before it triggers diagnostic trouble code C2303 for Reservoir Plausibility Error. Replacing the piston ring is easy and far less expensive than replacing the compressor. You can order an inexpensive new piston ring kit personally manufactured by precision machinist and forum member bagpipingandy:

Bagpipingandy's Compressor Piston Ring Kit

Below are links to photos showing how I changed the piston ring using Andy's kit. I also replaced the molecular sieve desiccant beads in the air dryer, but most owners don't bother:

Air Compressor Rebuild Part 1 of 7
Air Compressor Rebuild Part 2 of 7
Air Compressor Rebuild Part 3 of 7
Air Compressor Rebuild Part 4 of 7
Air Compressor Rebuild Part 5 of 7
Air Compressor Rebuild Part 6 of 7
Air Compressor Rebuild Part 7 of 7

Air Reservoir:

The air reservoir is mounted in the trunk/boot beneath the spare wheel and a sound-deadening foam cover. The reservoir has a capacity of 4.5 liters with a maximum design pressure of 15 bar / 217 psi. When fully charged the reservoir capacity is sufficient for at least one full lift of the vehicle at gross vehicle weight (GVW). The air suspension system does not deplete the reservoir contents below 9 bar / 145 psi under normal operating conditions.

From pdf page 8 of the Air Suspension section of the Dealer Training manual: "This means that the system is operating within a pressure range. This is done to prevent the air pressure held in the air springs from being transferred into the reservoir."

Valve Block:

The valve block is mounted in the trunk/boot beneath the spare wheel and a sound-deadening foam cover. It shares the same mounting bracket as the air reservoir. The valve block contains five individual solenoid valves, one for each of the four air springs and one for the reservoir. A pressure sensor (made by Denso) is mounted to the valve block manifold. The sensor monitors system pressure and communicates this information to the ASM. Six air hoses or pipes are mounted to the valve block manifold, one for each of the four air springs, one for the reservoir, and one for the air compressor. The air hoses for the front air springs are 6mm and the hoses for the rear air springs are 4mm. The air hoses are color-coded as follows:

Left Rear air spring: Blue
Left Front air spring: Brown
Right Rear air spring: Red
Right Front air spring: Yellow

Under the control of the ASM, the solenoid valves in the valve block perform the following operations:

Increase or decrease the pressure in the front air springs as a pair (or individually on early cars with 4 height sensors)
Increase or decrease the pressure in each individual rear air spring
Direct air from the compressor to repressurize the reservoir
Allow air from one or more air spring(s) to be released through the compressor exhaust vent solenoid valve

Ride Height Sensors:

The ride height sensors are mounted on the suspension subframe and connected to a control arm via a “Poke Yoke” arm. On most cars there are three sensors, one at the front left corner and one at each rear corner. On very early cars there is a fourth sensor at the front right corner, but from VIN G26872 the right front sensor was eliminated. The sensors are of the single channel Hall effect type using a ring magnet design, and their electrical connection is via three wires: supply, ground, and feedback. The ASM monitors the height sensor signals via software filters for various operating conditions and adjusts the air pressure in the air springs accordingly.

The ASM revises pressure in the air springs to level the vehicle, but the ASM will not attempt level while braking, cornering or accelerating.

Ride Height:

The ride height is measured from the center of the wheel to the lower edge of the wheel arch in the fender/wing. The factory specification for this measurement is 386mm front and 373mm rear, +/-15mm. In the thread at the link below, Cambo shows how to calibrate or change the ride height:

How to Lower Your X350

Air Springs/Dampers (Shock Absorbers):

The air springs/dampers were manufactured by Bilstein. They combine into one unit a traditional gas/oil damper with an air spring bladder that replaces the coil spring in a conventional “coil over” design. Normal operating air pressure in the air spring bladders is 7 - 9 bar / 101.5 - 130.5 psi. Maximum "full bump" spring pressure at GVW is in the region of 20 bar / 290 psi. The air spring burst pressure (when new) is around 40 bar / 580 psi [note that the factor of safety or yield stress/working stress ratio is 2:1].

There are two derivatives of the air spring/damper units:

Comfort: higher air volume and softer ride, identified by a green dot approximately 1 inch / 25 mm in diameter.
Sport: lower air volume and stiffer ride, identified by a red dot approximately 1 inch / 25 mm in diameter.

Pressure-retaining valves (PRVs) in each air spring ensure that a minimum of 3 bar / 43.5 psi remain in the air bladder to prevent folding or creasing of the rubber bladder that could cause damage. These are not "one-way" valves. They allow air to move in both directions so the ASM can increase or decrease the pressure in the bladder to level the car and maintain proper ride height under all conditions. They are only "one-way" in the sense that they will only allow the pressure in the air spring to be reduced to 3 bar / 43.5 psi and no lower. They will allow the pressure to be reduced from the typical maximum operating pressure of 9 bar / 130.5 psi down to 3 bar / 43.5 psi.

A few examples of when air pressure is adjusted:

1. When the trunk/boot is filled with heavy luggage, the ASM increases the pressure in the rear air springs to re-level the car. When that luggage is removed, the ASM reduces the pressure in the rear air springs to lower the car to standard ride height.

2. When the vehicle exceeds 105 mph, the ASM reduces pressure in all four air springs to lower the ride height by 15 mm to improve aerodynamics and fuel economy. When the speed slows to below 80 mph, the ASM will increase the pressure in the air springs to return to standard ride height.

3. When the car is parked on an inclined surface, the ASM will relieve pressure in the air springs on the higher end of the vehicle to make it more level with the lower end.

For cutaway views of an air spring/damper unit and the inner components, see the photos in the thread below with thanks to our member reyesl:

Air Spring Cutaway Photos by reyesl

Leaks in the system are a common issue, particularly in the air springs. Leaks at air hose fittings can often be detected by spritzing the fitting with soapy water and watching for bubbles that indicate a leak. Take care not to wet the ECATS electrical connectors. Our member TusconDave came up with a means of testing for leaks in an air spring using an inexpensive 1/8 inch brass pipe cap:

Testing For Air Leaks with 1/8 inch Brass Caps

To aid in locating less obvious leaks, our member lcmjaguar has assembled a set of test gauges which he is graciously willing to lend to fellow members:

Air Suspension Leak Test Gauges for Loan from lcmjaguar

ECATS Adaptive Damping System:

ECATS is the new name for the updated Jaguar CATS adaptive damping system introduced with the 2003 S-Type. ECATS stands for Enhanced Computer Active Technology Suspension, Jaguar’s marketing name for what is actually Wabco’s ECAS system, or "Electronically-Controlled Air Suspension," optimized for the S-Type and X350.

Adaptive damping refers to the capacity of the system to change the damping rates of the shock absorbers between “firm” and “soft” depending on driving conditions and vehicle attitude.

The ECATS adaptive damping ride control is managed by the ASM. System inputs include signals from two Texas Instruments vertical accelerometers, and a lateral acceleration signal from the ABS/Dynamic Stability Control Module. The front sensor is mounted behind the front right wheel arch liner, and the rear sensor is mounted on the left side of the trunk/boot ahead of the Telematic stack/CD changer.

The ABS/Dynamic Stability Control Module is a Teves mk25 system manufactured by ATE. It receives inputs from the Steering Angle Sensor and Yaw Rate and Lateral Acceleration Sensors cluster and passes a resulting lateral acceleration signal to the ASM via the CAN bus.

Adaptive damping is controlled by the ASM via solenoid valves in each damper. The solenoids coils have an impedance of 5.4 ohms. The solenoids operate in two stages, open or closed, and are operated by the ASM. The default setting of the dampers with no signal is “firm.” To change the damping to “soft” the ASM sends a 5 volt 400Hz PWM (Pulse Width Modulation) signal to the ECATS solenoid valve, which opens and changes the damping rate. Click on the link below for a photo of a scope showing the PWM signal, thanks to Cambo:

Scope Display of ECATS PWM Signal by Cambo

ECATS Operation:

Control of the damper solenoid switching can be one of the following:

All Firm
All Soft
Front Soft / Rear Firm
Front Firm / Rear Soft

At system startup all dampers are set to firm. Firm is also the default condition should an electrical fault occur in the system. Below a predefined lower threshold speed, the setting is firm. Above this preset threshold the dampers are switched to soft for vehicle ride comfort. Above the predetermined upper threshold speed the system is switched back to firm to provide better stability and reduce wallowing. The lower and upper threshold speeds are not specified in the manual. My current assumption is that the lower threshold is 25 mph, the speed below which the compressor will not run, and the upper threshold is 105 mph, the speed at which the suspension lowers 15mm. I will correct this guess if I find the specifics in Jaguar documentation.

Longitudinal damper switching assists in resisting dive and squat due to braking and acceleration. The ASM implements anti-dive and anti-squat corrections in response to inputs from the brake switch, brake pressure switch and engine torque rate (longitudinal acceleration) signals via the CAN bus.

When undulations are sensed on a fast straight road, the dampers are switched to firm to counteract vertical body oscillations.

When cornering, the system controls the front wheels and rear wheels in axle pairs to improve vehicle stability. At low speeds the rear dampers are switched to firm slightly ahead of the fronts to reduce transient understeer. At high speeds the front dampers are switched to firm slightly ahead of the rears to increase transient understeer.

Cheers,

Don

 

Thank you, Don, that's very useful.
Especially because upon aging the air suspensions become an item requiring careful follow-up and maintenance.

 

That's good stuff! Thanks!

 

Nice writeup Don. Thanks for taking the time to put the info all together !

 

Thank you Sir!

This write up is perfect timing for me my system just went out.

Griff

 

Excellent write up, well done. Thanks for taking the time.

 

Make it a 'sticky' Don ?

 

Terrific post Don. Like others I am experiencing intermittent problems with my suspension at this time So this has really helped.

 

Kinda new to these posts and all, but I have a question.
My 2005 XJ8 is leaning toward the passengers side. Looks like more in the rear than in the front.
Dealer said both shock units on that side were leaking.
I would like to stay with the air shocks and don't mind paying a realistic price for them.
My car has 75,000 miles on it and seems to be in good shape otherwise.
So, my question. Is there some way to test these short of pulling them out and putting new ones in?
Also, who knows of a good supplier of the shocks. I plan on replacing all of them at the same time.
Thanks

 

Arnotts makes good replacement shocks, watch the prices, they are pricey when at full price, i got two new fronts for $830 delivered. They fit & work well, the rears are still stock and hear they last a lot longer as the weight is thrown onto the fronts, & off the rears when you stop. Much like the brakes, & tires on most cars, the fronts wear more quickly.

 

Hi - I posted here a simple way to cap off an air strut line to confirm which shock is leaking:

https://www.jaguarforums.com/forum/x...ostics-182440/

I'm still a fan of RMT rebuilt OEM air shocks - $350.

 

Hi TusconDave,

Great idea! I took the liberty of adding a link to your thread to my original summary post in the section about leak detection.

Thanks!

Don

 

comfort vs sport ... where is the green or red dot location?

also some suggestions on where to purchase ... dont want to spend much since i doubt i will be keeping this car much longer

thanks in advance for the help

 

[left]Looking at this thread and recently having suspension issues, I was able to troubleshoot and found a leaky front strut, drivers side. May look into replacing with remanufactured unit from RMT. My main concern would be the sport vs comfort struts. Seems like everyone sells comfort only. I know the passenger side was replaced some time ago (looks like the Arnott replacement) and does not look like the OEM one having a cone sticking up where the electrical connector goes. It's possible the passenger side is comfort but I don't know.

 

Hi
I went with the RMT remanufactured because they are OEM rebuilds, so they have sport/comfort. I've been fine with my replacement, others said they failed or squeaked and they weren't happy. Previous owner of my XJ8 had replaced a new shock with a new shock, which failed in 15K miles, RMT has lifetime warranty, so that made me go to RMT.

 

Thanks for the reply. I took a second look at the passenger side (one that was replaced) and I'm pretty sure it is an Arnott strut.

My question is, would it matter if the front struts don't match. One Arnott and one OEM remanufactured?

Come to think about it, that is what I'm running right now because the drivers side is original.

I always noticed the left front side always a little lower than the Arnott side (right).

 

Don, Thanks for the great post. My 2006 XJ VDP (65,000 miles) in in my garage (just came back from dealer $8000 check out.) dealer could not find cold weather air suspension fix. When temp is below freezing, she squats. No apparent leak in "bladders", ect. Where would you start to fix this problem. Thanks.

 

Hi JHB,

So far, the consensus is that the cold air leak occurs at the top seal of one or more air springs, typically one or both of the fronts. The top seal is a large rubber molding that is permanently compressed between inner and outer components in the air spring/shock absorber and is not replaceable. You can see part of the seal as a rubber ring around the circumference of the recess of your air springs, in the center of which is the ECATS solenoid electrical connector.

You can check for leaks at the top seal on a cold day by running the engine to charge the air suspension, then spritz some soapy water around the recess in the air spring, taking care not to wet the ECATS connector. Bubbles indicate a leak.

A few owners have attempted repairs by packing this recess with epoxy resin to seal the area where the seal is visible. Only time will tell if these repairs will last. I have considered attempting to apply a high-pressure sealant to that area while pulling a slight vacuum on the air spring to help draw the sealant into the leak(s), but the only suitable high-pressure sealant I've found is expensive, so I keep putting off the attempt.

The most expedient solution is to replace one or both air springs/shock absorbers.

Cheers,

Don

 

Thank you for this write up.

I must say ECATS are the most cumbersome dampers I've ever had. It'll now be the 3rd time I'm replacing the front ones and I've only done 260,000KM!

My Mazda using normal springs and shocks has only had 1 pair of shocks fitted and that was mainly because I lowered the front suspension. It has done over 310,000KM.

 

Thanks for the good write up.
And I have a question about mit X350 year 2003:
It was standing half a year at the dealer and with almost no pressure on the dampers and i guess long time without battery. After lifting one air strut manually by direct voltage on one relay, alle 4 dampers came up again and the fault message went away. Only the yellow and red lights are still blinking. The suspension is very firm on all 4 wheels and it doesn't change. Looks like the air strut which I moved up manually is ab bid deeper than the others.
No faults in ASU module after erasing.