Hi,
my XJ never showed any signs of wear on air suspension.
Yesterday, my front wheels where sunken to the ground.
Rear wheels where still up perfectly.

I took my SDD and checked magnet valves and pressure sensor.
With compressor running, I get 4.5bar and the valves of the sunken wheels are opened. Somehow the car does not rise...

I emptied the rear wheels with SDD, which worked.
But filling them doesn't work.

On the dashboard I get a message when the compressor runs too long.
SDD doesn't get a fault code.. my OBD2-reader only gets P1111.. like usual.


How much pressure is necessary to rise the car ?

Is it common for the compressor to die that sudden ?
What about a defective valve ?

 

Hi nebelfuerst,

If I understand your signature correctly, and your car was made in October of 2003, I assume it is a 2004 model year. If the VIN is G26872 or later, you have one front height sensor (on the left side) and both front shocks are pressurized by the same air supply from the valve block in the trunk/boot. A leak in either front shock or either air hose will cause the entire front end to drop to the stops, while the rear end remains at normal ride height. If your VIN is G26871 or earlier, you have two front height sensors and each front shock receives its own air supply from the valve block, so your symptoms would be more difficult to diagnose (perhaps an internal failure in the valve block?).


According to the '04 New Model Introduction Dealer Training Manual, the nominal pressure developed by the compressor is 15 bar (218 psi), but I believe it will take up to 60 seconds for that much pressure to develop in the air suspension system since all the unpressurized air in the lines must be recompressed and the reservoir may also need to be replenished.

The reservoir pressure has probably been depleted, and if there is a leak in your system, the compressor may not be able to build pressure at a rate faster than it is lost via the leak. According to the training manual, the system will not deplete the reservoir below 9 bar (145 psi) under normal operating conditions. The reservoir has a volume of 4.5 liters with a maximum design pressure of 15 bar (217 psi), and the manual states that when fully charged the reservoir is capable of at least one full vehicle lift at Gross Vehicle Weight. You can downloade the Air Supension section of the manual I'm referring to here:

http://www.mediafire.com/download/7r...on_Section.pdf


You might review all the potential air suspension fault codes in the Workshop Manual (available for download in six sections in the X350 'HOW TO' quick links thread on the main page of the X350 forum). I can't remember if the system will trigger a DTC due to an air leak at a front shock, for example.


Since the most common compressor failure mode is piston ring wear, I assume that its ability to charge the system decreases gradually over time and not suddenly. A sudden leak in a shock air bladder has been reported several times, so you may want to see if there is any obvious damage to the corrugated gaiters on the front shocks which could indicate failure of the air bladder inside. You could also use some soapy water to check for air leaks around the air hose fittings on the tops of each front shock.

Hopefully others will have some additional thoughts.

Cheers,

Don

 

A little addition to Don's excellent post.

Even with an empty reservoir the car will rise. Even as little as 4 bar will get the car to it's correct height.

From what you describe (suddenly both front wheels to the ground) my guess would be either a defect height sensor (assuming your car has one, not two) or otherwise a leak to the front suspension. Two failed struts at once seems highly unlikely to me.

This situation could very well be the cause of the rear not rising anymore after deflating them. During my own testing after deflating and trying to lift the car again, I've had several times that the compressor keeps running ( also triggered from the SDD) but the car does not rise. It seems to get in a mode where the valves stay shut. Are the rears open like the front ones?

Driving the car a little bit probably would see the rears inflate again but in your case with an unfixed problem at the front you might want to fix that first. You can drive with an drooping front and at slow speed but not to far otherwise you might damage the airbags of the suspension.

Don't know for sure but are there tests in the SDD Data logger for the height sensors? Voltage and measured height if I recal.

Guus

 

Guus,

Thanks for the additional ideas. We both agree that if nebelfuerst's car has only one front height sensor, a leak in a front shock or air hose could cause his symptoms. I had briefly wondered about the height sensor but for some reason had ruled it out in my own mind. But thinking about your mention of a defective height sensor prompted me to try to think it through again. It could make sense that if the sensor's signal leads the ASM to believe the front end is too high, the ASM would exhaust air from the front end to attempt to bring it back down to what it believes is the correct level. So I agree that nebelfuerst should check the front height sensor for damage, contamination of the electrical connector, etc. Thanks for prompting me to think this through! If I've made an error in logic, I'd welcome correction.

Cheers,

Don

 

Rereading it you would think that the open valves from the front shocks would at least let car rise unless there's a leak to the front suspension. One problem I've seen using the SDD manipulating the suspension and the car not rising again is that the exhaust valve stays open, effectively letting all the air out as fast as the compressor delivers it.

The valves are not in the suspension airbags themselves but in the valve block in the trunk with two separate lines going to the front. If an air leak is in one of the shocks and it's big enough to empty both shocks it has to be a pretty big leak. I would think you'd hear it.

I would go for a short drive/move to see if the car will rise again and if yes only the rear or the front as well. Just to make sure the system is in working mode and search further from there.

Guus

 

If I read the manuals correctly, the exhaust valve should never be open while the compressor is running, so if that happens, something is wrong. I can only think of a few things that could cause that to happen: the exhaust valve is stuck open due to corrosion; there is a fault in the ASM; or SDD, for some reason, is prompting this behavior whether intentionally or unintentionally.

 

Thanks for your help. (@Don B: excelent paper to read, @Xag: your SDD-suspension research-thread is a good reference..)

My VIN ends with G09825, so there are two hight sensors for the front wheels.
I checked them in SDD ( all 4 sensors) and they show plausible values.
My rear ones changed from -2cm to somewhat like -7cm after releasing pressure.

I also used datalogger in SDD to see what's going on when my car starts:
- Compressor starts, pressure peaks from 4 bar to 6bar for 1second.
- Valves for front wheels open. pressure drops to 4 bar.
(other valves stay closed.)
- no change on height or pressure for 1 minute. compressor stops.

I never read a value below 4bar. Is this the lower limit of the sensor or is it a hint what is going wrong ?

I also tried to inflate the rear wheels after deflating with SDD. Unfortunately SDD is blocked with the inflate-command, so I cannot run datalogger to see whats going on.
The same applies for filling the air reservoir with SDD... I cannot check the status of valves during this operation.


As my car has become a real "low rider" 5cm above ground, I cannot steer. Is it a reasonable approach to disconnect some line of the valve block in the trunk and replace it with my air compressor ?
How much pressure would be harmless ?

 

Hallo Nebelfuerst (wieso? :-)

If your car is really that low really driving it might not be an option but can you move it a bit in a straight line at all?

You could connect a compressor to the individual lines but the problem is how to detach it again without the air escaping again? What you could do is disconnect the line coming from the compressor and use the valve to control things. The question is if the valves are being controlled correct at this time. If you use 4-5 bar it should be safe and enough. But I would try something else first, see below.

I've had SDD getting stuck in the function to rise the pressure. What I did was to force quit the SDD and start again. The only problem I had was that the compressor kept on running and I was afraid it might get damaged. In my case driving the car a bit got it out of the SDD filling loop and back in the normal situation.

Just to get some things clear, what's the current situation?

I assume that if the car is off, no compressor is running? Quit SDD and start a new session and start the datalogger. Log: pressure, compressor on/off, vent valve, 4 wheel valves and reservoir valve.

Start car, compressor starting to run or not? (by ear and on SDD?)
Which valves are open, which are closed? What's the pressure?

If the compressor doesn't start running, I would first use the SDD and try to fill the reservoir. This SDD function usually works without the SDD getting stuck. Hear if the compressor starts and let the program run. This way you can trigger and check the compressor.

If this works, go to the datalogger again and check all valves and pressure.

Let us know what you find.

Guus

 

Don, I don't think it should do either but it did! I think SDD error.

Guus

 

I drove my car for 5m forward and backward a few time, but it didn't change height.
So I did a datarecording with SDD.
You can seen the compressor start ( I can feel/hear it running).
Pressure drops from 8 bar to 4.5 bar when compressor is running... I cannot understand that.
At the stop of compressor, I get DTC C2302..

The exhaust valve never openes.. the steering of the valves seems quite reasonable to me...
See attachment ( it german, but I think you can guess the variables..)

 

Deutsch ist kein Problem aber wir machen's hier besser auf English ;-)

It's logical the pressure drops. What you measure is the pressure in the valve block. This depends on how much pressure is left in the this part of the system, in this case 8 bar. What happens when the compressor kicks in, is that at the same time the valves to the front suspension open and because the pressure there is 0 you see the resulting average, in this case 4,5 bar. The compressor may output more pressure but it's a BIG empty system!

Looking at the position of all valves I would definitely expect the car to rise.

Have you tried to fill the reservoir with the SDD?

Guus

 

After a short break, I got back to my "low rider jag".
I cleaned all the electrical contacts of the valve block and tried again with SDD.
When I joined my car after the break, the pressure was measured to 9bar.
When starting the car, the pressure dropped to 4.5bar and slowly increased to 6 bar before stop of compressor.

Setting the commands to empty front / rear / reservoir allways gives a short noise of air, for less than 500ms. But pressure didn't drop below 4.5 bar.... is that called empty ?

I really wonder where that high pressure values come from, as the compressors fails to pump higher than 6bar...

BTW: Xag: Good to see, you're german capable. My brain thinks german, so some grammar might be shocking to a native speaker. Unintentional, of course

 

So, pressure in the valve block, no other valves (suspension/reservoir etc) open?

Which valves opened when starting the car? Front, rear, reservoir, vent?

What you measure is the pressure in the valve block. Only when the valves to the front, rear or reservoir are open this equals the pressure in those systems. So the pressure you measure doesn't has to be that value. From what I've read (not measured) there seems to be a minimum of 3 bar left in the shocks themselves to avoid the airbags to fully deflate and get damaged.

If the pressure is really not getting higher then 6 bar you compressor might be faulty. But it can take several runs of the compressor to get the pressure higher. Especially if you have a leak. On my car (leaking front shock) when the air is out it can take 3 runs of the compressor to get up to 12-13 bar.

On my car every time I start it again, after a minute or so the compressor starts. Does yours too? If that's the case, try the following.

Connect the SDD and measure the pressure. Start the car and let it run till the compressor stops then turn of the car. Wait for 2 minutes (should be enough to cool the compressor) and start the car again. See if the pressure rises more.

Guus

 

After another intense SDD session, I finally found my problem.

To check whether the pressure sensor is correct, I removed it.
It told 0 bar, which is correct.
Now I emptied the reservoir and lots of air came out.
I don't understand, why that air didn't come out when the pressure sensor was in place.
Then I emptied front and back shocks and put the sensor back in.
Now the car showed an steady increasing pressure when compressor is running.
My back lifted well, but my front took long time to rise.. but it finally did.

As I also watched the 4 hight sensors on SDD datalogger, I could see the car sink mostly on the front left, as soon as the compressor stopped.
( I actually sat in the car, but I'm not that overweight, to feel guilty for the car to sink. )
Now I even can hear the air escape from the front left shock.. somewhere internal of the shock.
Somehow the experiment with 0bar seems to sightly have patched the leak...so the air steering unit escaped from it deadlock..

 

Great! Gratuliere!

Just to understand correct: do you mean in place or out?

The fronts need a lot more air to rise because of the weight of the engine.

So you know this shock needs replacing. I'm having mine replace next month. I'm buying from a dutch specialist. Where are you located?

Not sure what you mean by patched the leak. Maybe the control circuit got caught in a deadlock where making the system pressureless got it out.

Guus

 

Well done, nebelfuerst!


According to page 7-15 of the Air Supsension section I uploaded to the forum from the X350 dealer training manual:

"The air suspension does not deplete the reservoir contents below 9 bar (145 psi) under normal operating conditions.

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."

I assume the above may explain why your reservoir retained so much pressure, and I also assume that the pressure in the reservoir is controlled by the ASM and a solenoid in the valve block.

Cheers,

Don

 

Another turn of events today
I wanted to record my compressor pumping and my car rising and sinking due to the faulty shock.... but it didn't sink !
So I made a longterm SSD recording with motor stopped... no loss in height.
Now I removed fuse 52 and my car stays in the same height as before for hours.

Somehow the shock seems to be healed over night.
I drove a litte round trip and I could see the compressor just working for filling the reservoir.

Confidence to do a long trip is not reached yet. I must understand what has happened.

Story in short:

1) My car didn't show any signs of suspension probs.
2) I drove home on highway (autobahn) at heavy rain.
3) After 3 hours my car hat its front shocks emptied.
4) Rising the car didn't work, compressor timed out without lifting the front.
5) I removed the pressure sensor and now emptying the reservoir and shocks seemed to work... it didn't work when the sensor was in place.
As the removed sensor now reported 0bar, the SDD doesn't seem to care about low pressure.
6) I noticed condensed water on the pressure sensor, which I didn't care about, as my big compressor also created condensed water.
7) I put all together and the car started rising, but the front left shock made the noise of escaping air and the car sunk within minutes.
8) I just waited one more day and everything seems to be fixed.


Is a temporary failure of a shock known to happen ?

My car has a few days warranty left, which raises the question whether i should claim some change or just hope for the car to be auto-healed ..

 

Hi nebelfuerst,

I can't think of any law of physics that would allow for a temporary air leak to heal permanently on its own, especially if the leak is in the air bladder in the front left shock.

On the other hand, I'm wondering if at least part of your problem could be in the valve block. Could it be malfunctioning due to an accumulation of water? Condensed water in your big air compressor may not be a problem (unless you're using it to spray paint, or the water accumulates inside your air-powered tools), but water in the air suspension can definitely lead to problems. Our systems are only protected by the desiccant in the air dryer that is part of the compressor assembly, but most after-market air suspension systems include a true water trap (which is different from a dryer) to help protect sensitive components from water accumulation.

It might be worth disconnecting all the air hoses from the valve block and checking for the presence of moisture.

If I had even one day of warranty left, I'd be on my way to report the problems right now!!!

Cheers,

Don

 

Just to be sure: both shocks were empty? Both equally down?

If yes, then it's not a leaking shock in my opinion. Two failing at once is highly unlikely.

Somehow I don't see the connection between water in the system and the two shocks going flat. If a car is level there's no reason to vent or add large quantities of air. And IF there was some kind of water block on the sensor giving false information to the system why only empty the front ones?

Could it be the driving in the rain that caused a fault with the front height sensor?

Guus
PS Did the noise really come from the front left shock? Don't forget the compressor vent is also front left....

 

A very good point, Guus!