I've decided to make an attempt to use stock coil overs from a 2009 XFR to replace the front air shocks on my 2004 XJ8. After years of maintaining the stock air suspension with used parts I've finally given up after yet another front air shock failure and endless noise and clunking from the front end.

SexyBeast took a similar approach using coil over shocks from an S-Type but he did not retain CATS and his rear ride height changed.

https://www.jaguarforums.com/forum/x...version-79499/

Here are a few thoughts on my approach:

- I believe the 2009 XFR upper strut mount is the same as the S-Type so I am anticipating that I will need to modify the chassis bolt holes as SexyBeast did during his modification. This will be confirmed when the units arrive at the end of the week.

- 2009 XFR is ~120lb heavier than the 2004 XJ8 base so the spring rate should be approximately correct if not slightly stiffer (read sportier.) I haven't checked the weight distribution to see how the front axle weight compares on each vehicle but I'm assuming it isn't all that different.

- 2009 XFR uses a CATS damper system similar to the X350 series. I'm taking a risk in assuming that the 2009 XFR CATS damper will respond properly to the signal output of the 2004 Wabco controller. At the very least I am hoping it will satisfy the controller that a proper CATS damper is present and prevent an error code from flagging.

- If the front ride height is slightly different and the Wabco module tries unsuccessfully to adjust I will determine the proper ride height sensor resistance and jump the wires with a fixed resistor to simulate the proper ride height. This should keep the Wabco controller happy.

- I will attempt to retain functional air suspension on the rear axle for several reasons:

- Rear air suspension accomplishes load leveling and proper suspension geometry under all driving conditions with a variety of passenger and cargo loads. (I travel with this car so I sometimes have a trunk full of luggage.)

- Proper rear suspension ride height maintains good balance during weight transfer maneuvers in hard cornering and braking. My goal is to retain as much of the stock road and handling feel as possible.

- In the event of a rear air shock failure the car can be driven a short distance to limp home for service while total failure of the front air suspension renders the car almost totally disabled. One of my fenders has already suffered damage due to scupping from a flat air spring.

- I can source good used rear air shocks much more easily than I can find good used front struts. I also happen to have a spare rear shock ready to install.

- I will attempt to retain CATS damping for several reasons:

- CATS provides excellent handling response during aggressive maneuvers and provides a smooth ride under light load.

- X350 couples CATS and air suspension systems with a single Wabco control module. Disabling air suspension also eliminates CATS and vice versa. I want to retain air suspension on the rear axle for load leveling and ride height.

- My hope is that retaining CATS on all four corners will yield a better ride in normal driving mode under light loading conditions.

Q&A:

Why am I not trying to use S-Type R CATS dampers?

1. I'm not using STR CATS front struts because I want to source used parts and the reality is that used STR parts are old. If the XFR struts work well I can source much newer used parts.

2. I believe the stock ride height of the XFR is closer to the XJ8 than the STR.

3. In the event that the X350 Wabco controller doesn't play well with the 2009 XFR CATS dampers I might try to purchse a used CATS controller and Air Suspension module for an X351 XJ 10+ and integrate them into the X350 vehicle network.

Why not just purchase an aftermarket kit with replacement coil overs or replacement air shocks?

1. All of the aftermarket kits whether they are coil over struts without air or air struts without CATS depend on disabling the Air/CATS module. I want to retain air on the rear axle and CATS on all four corners.

2. My goal isn't to just make the car more reliable, my goal is to develop a better solution than what the aftermarket currently offers.

Why not just suck it up and replace the popped air strut with a used part off eBay?

1. Both front struts are making noise and most used front dampers I've purchased will hold air but ride poorly on rough roads. In other words the car is aging out and so are the used parts available from salvage yards. My guess is that Arnott stopped offering rebuilt units for the same reason... nothing left to rebuild.

2. I'd like to keep the car running for a couple years as a daily driver and I hate having to fear getting stuck by the side of the road by a bad front air spring. They tend to fail without much warning and it could easily get me stranded far from home as I have a lengthy commute.

I'll update my post as the project moves forward... parts are on the way.

 

As a followup I should mention that I'll also be rebuilding the entire front end during this process as well. I'm going to replace all of the control arm bushings, ball joints, upper arms, inner and outer tie rods, stabilizer bar links, bushings. Everything is on order.

I own a hydraulic press and I'm very well equipped so I'm not too worried about the other suspension work but I hope I can avoid replacing lower control arms. I'd like to save money by just pressing new bushings. Ball joints bother me as well... the design is unique to the XJ8 / S-Type as near as I can tell. Ford pressed the lower ball joints through from the top and they mate up with a one-piece lower arm. I like the Ford ball joint design better because it can't work its way out of the spindle (unlikely scenario but I hope the new ones press tightly.)

 

Does anyone know for sure whether the S-Type inner tie rod is the same as the X350? I've ordered a pair in hopes that it's a match.

 

This will be a very interesting thread. Good luck to you, sir!

You can get replacement balljoints from Rockauto. They are a perfect match and will press in perfectly.

I don't like the low speed bumpiness of the coilover conversion kit but I can't live with the unreliability of the air shocks. If this works, this might be an interesting alternative.

 

What an enthusiasm!)) I just wish you good luck and hope you reach the destination of this journey.. I stepped away at some point with S-Type R all around as could not sort CATS dilemma.. theoretically height sensors can be foolished but all the rest maintenance procedures of wabco module can not and you face ASF (air susp fault) and stuck in hard shock until you stop and restart the car. But the effect is absolutely worth it to swap to S-Type R susp for examp from airs, its like to jump into audi from river boat especially with 400HP car. Some of your goals are totally optimistic as to friendship some electronic modules from different platforms but who knows, Arduino may help)

Today i'm back on airs again for the winter.. but slowly cooking the idea to swap cats shocks from s-type r coils for non-cat sport bilsteins.. but again dilemma with wabco.. my generation feeds height sensors info for headlamps leveling.. I cant just unplug it, I guess.. Good luck, man once again! Will follow your investigations

PS All suspension arms are totally identical between revisioned s-type, x350, x351, xf..

 

The only maintenance mode I'm concerned about is high speed lowering... rear leveling should be retained because I am keeping rear air shocks. Standard drive mode should be retained because I am either locking the ride height at an acceptable level or fooling the sensors to maintain that level. In high speed mode it might trip a code if it tries to lower the car and fails. Arduino could clean that up by just detecting the solenoid valve output and sweeping the ride sensor signal up or down depending on what the controller is trying to do. I hope I don't have to go to that length.

 

Ride height sensors on the X350 are hall effect solid state, non contact devices. A magnet rotates with the link arm inside the sensor and alters the magnetic flux through a semiconductor element which is converted to a signal voltage ranging from 0V to 5V given an input voltage of 5V. The 2004 MY XJ Electrical Guide tells me exactly which wire returns the signal voltage to connector CR90 on the Wabco controller. All I need to do is allow the suspension to pump up while I probe between the two front ride height sensor return wires and ground to determine the steady state return voltage signal that satisfies the controller.

In a pinch I could actually try just splicing the rear ride height sensors into the front sensor inputs so that the computer sees the same signal from the front axle as the rear axle... this would probably resolve the issue of mode detection as the controller would still be able to alter the rear ride height. I'll probably try this approach before I lock the value artificially. I'm assuming the ride height sensors are calibrated to approximately the same voltage range when the ride height is correct.

I may also need to plumb the two valve block front strut ports into the rear strut air lines if I splice the sensor signals.

 

I always wondered if there's a way to swap the CATS suspension module from an S-Type/XF/XK in order to completely remove the air suspension system. Basically you'd just duplicate what's in the other models, so the system wouldn't be looking for input for the air system at all. Not sure if that would require reprogramming with IDS/SDD, & the high-speed lowering would be gone. Gotta get the schematics for the XJ and S-Type to compare the two

 

I think it's going to be easier to fake the signal for the front ride height sensors. I actually want to keep air on the rear axle because I like to have load leveling. If a simple fake out like a fixed resistor or duplicate of the rear signals isn't good enough to keep the controller happy I'll probably just program an arduino to simulate the front air system. I can just input the solenoid signal to the arduino and have it output a pwm signal to a voltage ramp circuit that would sweep 0 to 5v. When the chip sees a command to air up it would sweep the signal to simulate a rising suspension. When the chip sees a vent command it would sweep the signal to simulate a lowering suspension. Arduino can output a pwm signal to level the headlights as well. Essentially I'll just use an arduino chip to clean up the side effects of deleting the front air shocks

 

Just a quick update on progress... Nearly two weeks into the project and I'm just now finished with my press work. I took extra time to polish and clean all of the control arms and both knuckles to a like new appearance. I also replaced both inner and outer tie rods which took some creative handy work in fabricating a socket adapter to fit the inner tie hex as it is slightly bigger than 1-7/16 readily available. Pressing bushings and new ball joints involved a lot of frustrating runs to home depot to find various "round" metal objects of the proper size to press each component as the standard press kit available from Autozone was useless. That said my last remaining step before bolting everything on is to modify the strut bolt holes to match the XFR pattern. I'm hoping to get both struts mounted tonight and begin the lengthy process of bolting everything back together. I also need to rivet the brake shields back onto the knuckles as I had to remove them to press the ball joints.

 

Have you thought about using RMT?
Remanufactured Front Suspension Air Spring Bag Struts - Pair | Jaguar XJ-Series (X350/X358) 2004-2009

 

Mike, yes. The whole point of this exercise is to eliminate air suspension on the front axle. Rebuilt struts are worthless as the vehicle platform ages because the factory damper cores from Bilstein are getting old. You might get a rebuilt unit that holds air but it's increasingly likely that the damper will be noisy or weak.

 

Just to post a project update... I'm taking my time building a drill jig / pattern to properly locate and drill out the new holes for the XFR strut mount. The challenge is that the holes are just slightly off from the OE X350 pattern which means its very hard to keep a drill bit on center and properly located. I've purchased and dismantled a moog upper strut mount for the XFR and I am working on fabricating my jig from the top plate of the mount because it already has the proper hole pattern for the studs. I'll post pictures of the finished product if it works well. I'm going to use the center hole in the strut tower to center the jig and bolt pattern. You've really only got one shot to drill the holes properly, I don't mind taking my time here.

 

Weekend progress update... I have 95% of the mechanical installation complete. Only have to tighten the lower control arm mount bolts and the sway bar link bolt with weight on the suspension. I dropped the vehicle down last night on it's wheels for the first time and initial results are promising. The front ride height looks about right, maybe a little lower than it was with air shocks. Even though I have CATS disconnected and two open air lines the Wabco controller still activated the compressor and raised the rear axle.

Here's the rub... after reading more carefully in all of my literature it appears as though Jag updated the CATS system in 2010 on the XFR and went to a lower resistance damper solenoid with a broader range of PWM variation. Unfortunately I didn't fully realize this difference before buying my used struts on eBay and of course I ended up with one 2009 and one 2010 unit. This means the internal resistance of the 2010 unit measures ~3 ohms while the 2009 unit measures identical to my 2004 at ~5.8 ohms. I don't think it will hurt the Wabco module to try driving the lower resistance coil but I'm still trying to decide if I want to risk attempting plugging it in. I've made an offer on eBay for a 2009 unit to correct my mistake.

Additionally it should be noted that the XFR mount is clocked significantly with respect to the axis of the lower control arm bolt. This means that in order to install the shocks I needed to rotate the spring assembly with respect to the lower shock tube.

 

I'm pretty sure you won't be able to use the newer version CATS shocks on the older CATS system. IIRC there was a forum member on the XK X150-side that accidentally got newer version shocks on his car & had to return them because the ride was unbearable

 

2009 XFR CATS system is a close match to the CATS system used on the X350. 2010+ is the new design. I took the car out for a ride last night for the first time and I can definitely feel the stiff 2010 CATS strut on the driver's side, I have a 2009 unit on the way. Passenger side is running the 2009 unit and it seems to ride better. My main problem right now is that the strut mounts are making a lot of noise. I may need to drop them down, clean the surfaces and reinstall new nuts with hardened washers. The upper strut mount on the XFR has much less surface area and it might need an isolation pad like a rubber/cork pad used many OE mounts in order to seat well on the X350 tower. I'm picking up a lot of air suspension faults but strangely no CATS fault with the 2010 low resistance damper coil. Unfortunately the 2010 damper is too stiff so I need to swap it out anyway but it doesn't look like it's causing the system to go to firm mode. Tonight I'm going to try to address the noisy strut mounts and use my British Diagnostics setup to calibrate the air suspension to basically tell the Wabco module that the new front ride height is "normal."

 

Project Update: My first ride with the new front suspension was a little discouraging because I was picking up all sorts of clatter from the upper strut mounts but after a quick check yesterday evening I found that the strut center nut was very loose on the passenger side and the driver side needed to be snugged up as well. The four mount nuts on both sides needed a final touch. After tightening the strut mounts I took it for another ride and found that the clatter/clunking noises are gone. Unfortunately I'm left with a very stiff ride. The alignment is horrendous so it's in the shop today getting aligned. Once the alignment is complete I can start drilling down on the root cause of the stiff ride. I believe it's probably related to the 2010 damper on the driver side. I'll swap it out for a 5.6 ohm 2009 unit when it arrives next Tuesday. If the 2009 units continue to ride stiff then I need to break out my Arduino and breadboard a PWM driver to ramp up the power going to the damper solenoids to see if that loosens things up and makes the ride more compliant. If the springs are too stiff I can start looking for a lighter spring to fit the factory coil-over. I need to measure the duty cycle, amplitude and frequency of the PWM signal generated by the Wabco module. That should give me a good sense of the total coil power in "soft" mode. Then I can build a circuit that drives more power to the coil which should open the ports in the damper and give me control over the soft ride feel.

 

I don't think the ride stiffness is caused by the springs because they seem to compress quite a bit as the vehicle weight is loaded onto them which implies that the spring rate is within reason. Stiff springs, i.e. high K constant will bear the weight of the car but compress very little. Light springs will travel too much and the ride height will suffer given the same overall package. The springs seem to be in a good working range. I'm guessing the damper solenoid valve is designed to be driven to a higher duty cycle in the 2009 iteration and probably an even higher duty cycle in the 2010 iteration. That means our X350 Wabco unit is probably not pumping out enough power to the damper coils to soften them up. It's relatively simple to create a variable PWM signal to test that theory using Arduino.

 

You have to back all your air components to stock to be able to set new height to wabco. All civilian methods go through self-test procedure that you cannt skip. You need literally to hack calibration file otherwise. Springs from STR are very soft, I beleive from XFR as well - all stiffness goes from shocks. Start to make CATS contoller, don't waste time)))

 

Car is back from the alignment shop and feeling fantastic. The ride quality when the Wabco module is in "soft" mode before it trips an error code is superb, in fact softer and smoother than the original air struts and I haven't even replaced the low resistance 2010 damper yet. Absolutely zero front end noise and it handles like a brand new car! I couldn't be more happy... now I just need to fake out the air suspension controller with a clever Arduino hack. I'm also going to use the Arduino board to give me control over the rear ride height. Sochi is correct, I could just put the air shocks back on the car and recalibrate the ride height to match what it is with the fixed springs installed but if the computer ever wants to make an adjustment for speed, jacking or otherwise it will trip an error code. What I want to do instead is to use an Arduino to act like the front air suspension is actually installed. The Wabco module will "see" the front axle rising or falling when it opens the solenoid valves.