My XJ has an air leak somewhere since the front drops overnight. The car always goes up again so I'm taking my time to see what's the problem.

I'm using the SDD to get to learn the behavior of the air suspension and it's very interesting to see what happens. If the system is filling what's happening is this.

First the compressor kicks in, then the reservoir solenoid opens and pressure is building up in the reservoir. Usually it takes a few cycles to get up to 13/14 bar. After every cycle first the reservoir solenoid closes. Then the vent solenoid opens and then the compressor stops and at the same time the vent valve closes again.

In my case what is very noticeable is that as soon as the compressor stops the measured pressure in the reservoir (it's one pressure sensor) is dropping considerably. All the way from 10-13 bar (compressor running) to 4-5 bar (compressor off). I know the suspension only operates on 3-4 bar but I would have expected more pressure in the reservoir itself. Has anybody else done these kind of measurements? Would be curious to know.

Yesterday I used the SDD to deflate the front suspension because I wanted to do some testing. Then I used the inflate option to get it to rise again but it didn't. The compressor kept on cycling but the car hardly rose. I forced the SDD to quit and started it again to get to the datalogger. In the meantime the compressor keeps cycling but since I've seen that before I left it as is.

Started the datalogger and measured the pressure and state of the different solenoids. Pressure was building up to no more than 4,1 bar and what was very strange was that the vent solenoid was fully open! No wonder there's no pressure building up.

Car was low and also the suspension to low warning message was on. I've had this problem once before and strange enough what fixes it is to drive the car slowly. After a while the vent solenoid closes and the car rises again. Now everything is back to normal. Could there be a relation with the height sensors?

Anymore X350 owners out there who like to tinker with the SDD and want to know exactly how the air suspension works? Would be very interested in you findings, especially also from a car that has no suspension issues to establish what is "normal". Maybe this might help to get a more structured diagnose of air suspension problems.

Guus

 

At the end of the day if you are loosing pressure you have a leak to find.

In simple terms the compressor adds the air
The pressure sensors feed info the ASU. If the pressure is low it turns on the compressor and if it it high it turns on the vent solenoid

The airbags are controlled by individual solenoids that in turn are triggered by the ASU. The ASU will add or remove air to the bags using the signal from the height sensor. If the value stored in the ASU is less than the value from the height sensor air will be added (increasing the height). The reverse will happen if the sensor value is high

With that being said . With the car stationary you should get the compressor to start, pressurise the tank and cutoff. Assuming your not moving there is no reason for the compressor to run unless you have a leak. IE the tank pressure has dropped

Start with leak testing the system at each fitting. If you cant find a leak start isolating parts of the system and testing those as you may have an issue with s leaking line, check or vent valve

Cheers
34by151

 

Thanks 34by151.

Yes, I know I have an air leak somewhere but since I couldn't find it straight away and I don't want to trough $$ at parts without knowing what to look for I'm experimenting a bit. The XJ is my hobby car so it doesn't matter when it's not always drivable.

Normally my compressor kicks in, the car rises and the compressor does't come on again when stationary. The situation (SDD testing) I described is of course not normal. I was just surprised why the vent solenoid was open in this situation which prevented the compressor from building up pressure in the reservoir when it's needed.

My problem is in the front suspension and the right one is the one going lowest overnight. Haven't been able to find a leak with soapy water yet. I would like to be able to shut the lines and/or springs individually but haven't been able to find the thread used in the connectors. Seems to be something special. I do have access to 4 and 6 mm air lines and could check individual parts if I had the right connectors. It's strange that there no simple unit available to pressurize an individual airbag and see if it holds air. That way you can rule out lines, controle valves and the sort. Most people start replacing parts at great cost without knowing the real problem.

I also like fiddling with the SDD and I'm trying to find out a bit more about the XJ system. Not many people seem to know (or are interested) how it actually works.

Guus

 

All you need to do is go to a hose fitting supplier and get some push on fittings

I purchased
1 Box, threaded pushon fittings
1 Box, pushon joiners
1 Box Threaded schrader valves(tyre valves)
1 Box thread blocking fittings in brass
1 box of brass olives (never used)
2m Hose

From memory the air hose is 6mm and the threads are 1/4bspt but you can check that at the hose shop

What you need to do is make up
8 * Pushon hose fittings with schrader valves. These are for connecting to each airbag. Once attached you can pump up the bag with a tyre pump and let the car sit. Assuming no leaks the height will not change

4 * Pushon fittings with blocking plugs. This is to block off the airline

You have 2 options when testing.
1. Cut the line and join with a pushon joiner latter.
2. Remove the line and fit a short hose using a new olive. You will probably find though the the old oldave will need to be cut off anyway but can get around this with a few brass fittings

If you are testing the lines to the bags. You are going to generate errors if the bags are not at normal height. This is going to be from the feedback from the height sensors. You can either inflate the bags to the correct height or ignore as your testing leaks anyway

Assuming the bags hold pressure you will be testing the lines next. so we will work our way back though the system.

Remove the lines from the valve block and attach pushon fitting with a schrader valve. You had previously connected a blocking one to the other end. Set the regulator to a specific pressure on your shop compressor. Lets make it easy and say 100psi. Pump up the line and leave it a few hours. Come back and measure the line pressure with a pressure gauge. You should have 100psi if no line leaks. If your tyre guage does not go to 100psi lower that to the max pressure of your guage

Obviously if you have a line leak you need to replace some lines and if not you need test the line to the tank, and the compressor line in the same way then the valve block. Do this in the same way with one end blocked and a schrader valve on the other end

If all thats all OK you have now tested all the lines and the bags hte only thing left is a leaking compressor, vent or valve block

You can test each in the same way using a line and schrader valve

Hope that all makes sense
If it does you should easily be able to find any leak in a very short time

PS I have 4 pushon Joiners, 4 pushon schrader valves and 4 bpush on blocking fittings in the car all the time. I keep them in a zip-lock bag with the jack just in case of toal failure on the road. the only thing that does not cover is a bag failure on the raod but thats a tow anyway

Cheers
34by151

 

For testing the air shocks themselves for leaks, here is a great article by Brutal that shows how to make a pressor test device.

https://www.jaguarforums.com/forum/x...agnosis-42703/

 

Reading with interest. My 2005 XJR is at dealer now for air suspension fault. Dealer says front air shocks need to be replaced at cost over $3000. Only have 52k miles. I know XJR reported to have issues with air suspension but usually compressor or leak somewhere. Thanks for link to shocks. Good luck OP finding your problem.

 

34by151, thanks again for your great description. I've looked at the fittings on the reservoir and there made by Voss so I'm going to look for those.

On the compressor line my olive broke and I wasn't able to find a new one locally. I've drilled the hole in the compressor a bit bigger 1/8" and fitted a Festo push on fitting. Bagpipeandy send me an original one later, these seem to have special thread but hopefully the lines/bags don't.

lcmjaguar, I've read Brutal's piece once but had a hard time finding it again. Thanks for sharing! The front connectors seem to be 1/8" then.

tbsjaguar, hope your car gets fixed soon without breaking the bank. I was quoted €710 euro / piece for new OEM Bilstein front airbags. Not a bad price but still a lot of money so I want to be sure what's my problem first.

Time to get some fittings and stuff!

Guus

 

Out of curiosity, did you consider just going with the Arnott replacements and ditching the air suspension system entirely?

 

Would you do me a favor and not start a thread on the conversion to mechanical suspension again. There are already numerous threads on that subject and I would like to grow the knowledge on the air suspension in this threat. Thanks!

Guus

 

If you suspect a suspension leak but cannot find it, please try to change the o-rings first. They are cheap and the OEM types often suffer sporadic failure.

 

LeoJagger, thanks! Those are on the shopping list.

Guus

 

Found a cheap valve block so I decided to exchange mine just to see what would happen. I wasn’t expecting it to fix the problem but was interested if there was some kind of internal valve leak as well. Replacing the valve block is easy, remove the electric connectors, unscrew the air lines and undo the two bolts of the valve block to the frame. I let all the air escape from both suspension and reservoir so I had to fill/pressurize the complete system again.

That gave me some interesting insights. Because I wanted to flush out any debris/water in the valve block I decided to refit the air lines back to the block one by one (or two). First I ran the compressor to vent the line to the block, then fitted the line. Then I expected the valve to the reservoir would open but it didn’t. First the two valve going to the rear suspension open en the car is lifted from the rear. When the car is raised enough these valves close and both the front ones open to lift the car at the front. The way of filling is also described in the workshop manual.

After (unexpectedly) leveling the car first before filling the reservoir I then expected the filling of the reservoir would then be the final step but the reservoir valve didn’t open at all!

At this point I connected the SDD and did some testing just to check if the valve was good and if it switch alright. An interesting observation I made when measuring the pressure with the SDD is that you see the pressure going up (apron 12-13 bar) when the compressor is running and going down again to a remaining pressure of approximately 1,6 bar. All this time the reservoir wasn’t connected at all. So the pressure you measure through the SDD is the pressure in the valve block and not necessary the pressure that is in the reservoir.

The car was at riding height so I decided to connect the reservoir line and go for a drive. I had a gut feeling the the different modes of the suspension (Sleep Mode, Preliminary Mode, Post Mode, Stance Mode, Drive Mode) might have something to do with the reservoir not filling. Decided to connect the SDD and log the data during a 10 minute interval from the start.

The chart below shows the values I logged with the SDD, pressure, compressor on/off, reservoir solenoid open/closed, exhaust solenoid open/closed and the 4 solenoids of the front and rear airbags. I omitted the rear ones from the Excel graphic because they didn’t switch at all during my testing. I’ve done similar tests before an until now only saw the front ones switch occasionally and the rear never. Maybe on full acceleration they do.

So the starting pressure of 1,6 is what is measured in the valve block. The pressure in the airbags will probably be higher at this point (but I don’t know how high) and the pressure in the reservoir is still zero.

The first 140 seconds nothing happens and then the compressor kicks in. Pressure builds up to 3,8 bar and then drops off sharp as the valve of the reservoir opens and the reservoir starts to fill. Then slowly the compressor raises the pressure up to 3 bar at which point the reservoir valve closes again and you see the pressure spiking to 4,4 bar because now it’s just compressor pressure again. But not for long because at this point the compressor stops and at the same time the exhaust valve opens to „vent” the system. This is supposed to be done to remove moisture from the system. The pressure drops again to 3 bar and almost immediately the compressor kicks in again. Normally the compressor has a cooling down time but my theory is that if the total pressure in the system drops below 3 bar the compressor kicks in no matter what to keep the car at height. In this case it has to because there is almost no pressure in the reservoir, normally that pressure could be used to allow for cooling time.

At that point I must have either braked or more likely drove over a speed bump. You see the front airbag valve open and the pressure shoots up to almost 7,5 bar. The opening of the airbag valves is probably triggered by the hight sensors. Then a few things happen, the compressor stops and the exhaust valve opens probably to get rit of the high pressure. Again almost immediately the compressor kicks back in and the reservoir valve opens again and after the compressor stops again the exhaust valve opens again. This happens another time probably to even out the pressure fluctuations. At 181 seconds the normal cycle resumes, compressor starts, reservoir valve opens and now steadily the pressure runs up to 11,3 bar. What is very noticeable is that the measured pressure drops really quick once the compressor is off and the exhaust valve opens. But since this pressure is measured in the valve block it doesn’t tell anything about the pressure in the airbags or reservoir at that moment.

At the end you see the same pressure peak as before, probably another speed bump again.

Well the valve block didn’t fix the problem but I believe the car stays upright longer. Got some fittings today and some 6mm air line, still need to buy the pressure gauges before I can pressure test the air bags themselves. That's for next time.


Guus
PS A bit of Dutch translation ;-)

Schakelgedrag luchtvering = switch behavior air suspension
Druk = Pressure
Compressor = Yep
Reservoirsolenoid = Reservoir solenoid/valve
Uitlaatsolenoid = Exhaust solenoid/valve
Vering voor = Front airbag solenoids

 

wow, lots of info, thanks!

 

Guus,

Many thanks for your methodical examination of the air suspension system behavior! I'm convinced that collectively we can come to understand all or most of the various faults and malfunctions and eventually save owners thousands of dollars that might otherwise be spent replacing parts that may not have been faulty.

Please keep us informed of your further progress!

In appreciation,

Don

 

Great posts on this subject. Here is my update. Dealer first indicated replace front shocks. While waiting for brake parts dealer performed more tests on air system. Drop is in spec but out of spec after 48 hours. I decide alright I'll live with it. Dealer washes XJR for pick up and throws code. Diagnosis electrical connector pins are corroded. Dealer repairs connector problem solved. Bottom line check the electrical connector also as you troubleshoot. Good luck!

 

Hi Tom,

Do you know specifically which electrical connector was corroded and where it is located in the car?

Thanks,

Don

 

Thanks.

I see a lot of messages on air suspension and lots of people giving well intended advice based on their own experience. What is missing I thinks is more knowledge how the systems works so you could use the onboard diagnostics and SDD to at least rule out certain parts of the system.

What would be very interesting is to also measure these values from a XJ with know good suspension to get some of baseline.

Been measuring some more today but haven't had time for a write up yet.

Guus

 

Guus,


I have SDD and would be happy to try to do some measuring on our '04. What function in SDD do you use in order to observe the operation of the air suspension?


Don

 

Don,

If you have the SDD software and something like a Mangoose you can do it. I don't know how wel you know the software but start up a session first and choose diagnostics. It's not really important what you find but you need to select a symptom to get the menu at the top with Recommendations. In that menu you can choose the datalogger. For this it's important that the mongoose is connected or it won't start.

In the datalogger go into suspension and choose pressure sensor, compressor status, and the solenoids of the FL, FR, RR, RL suspension, the reservoir and the exhaust or vent solenoid. If you choose these from the list you go into the bottom second icon from the left and change it from measure value to trend (a little chart). Do this for all sensors separately. Then go into the third icon from the left and choose the 10 minute measuring interval.

The go to the fifth (I think) icon which is the datalogger itself. You see all the sensors and a button on the right like the play button on a CD or old tapedeck. If you click that it will ask you to turn the ignition on and if all is well you probably already get a pressure reading.

Start the car, press the recording icon just above the play button and it will start recording. Take the car for a drive. Please turn the laptop to the side you won't be tempted to watch the screen. I'd like you to drive safely!

The recording automatically stops after 10 minutes. You can disconnect the Mangoose from the car but leave it connected to the laptop. You can play back the recorded session or if you close the session it will be save so you can work with it again next time.

Hope this description helps, otherwise just let me know where you need help.

Good luck!

Guus
(Going to bed now!)

 

I'll try to add some screenshots tomorrow.

Guus