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  #1  
Old 10-01-2014, 09:48 AM
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Rear Air Suspension programming incorrect?

Hi all, do you suppose the design/ programming of the rear air suspension could be the cause of Air pump failures.

The reason I question this is that after doing some faultfinding on my own system (which needs new bellows/ air-bags), this is what I found.

The system continually tries to fill the air-bags whenever the rear doors are opened or when the car is in motion. I also believe that it does it when the car is stationary after measuring (and recording) the height over two consecutive nights. I also removed the Air Compressor fuse, and within 90min the vehicle had lowered.

If there is a leak (larger than pinhole, smaller than bullet hole - we live in Africa remember ) and if the compressor spends most of the day trying to inflate the air-bags, would that not then burn out the Air Compressor leading to extra cost and maintenance.

And you would be none the wiser, because only when the leak gets to a point where the Air Compressor cannot keep up, would you see a visible difference - by that time it is too late. (the compressor does eventually time-out, but that does not stop it from trying again, and again, and again.)

Would be interested if anyone has experienced differently and what the BMW techs on this forum have to say.

thanks
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  #2  
Old 10-01-2014, 10:17 AM
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When the electronics go to "sleep mode",which is usually 2-5 minutes after there is no activity in/around the car,the pump will not work anymore.It will re-start after a door has been opened and the car wakes up.A loving owner will notice that the car is sagging in the morning.
If your pump works overnight,that means your car is not going to sleep and that is a completely different problem.
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Old 10-01-2014, 10:57 AM
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thanks, yes that could be the other issue, is that the vehicle never goes into sleep mode and you might experience "high battery discharge". And then during the day it also cycles the air compressor numerous times.

I don't believe BMW designers would miss this??
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Old 10-01-2014, 12:06 PM
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Engineering practices usually design based on 'single fault conditions': what will happen if X fails, or Y fails.

The system is not designed to accommodate failed airbags in a car that ALSO has some other electronic failure preventing sleep mode

If the airbag fails, the owner will note a sag. I will bet there is something, somehwere in the owners manual that says 'see your dealer if _______'.

In product design there are a handful of 'controls' that one can apply, which vary based on the risk of the failure. Life threatening (or car control) issues tend to have much stronger controls than others...but at the end of the day the owners manual is the place where all that ties to the driver.

Hazard analysis, risk management, Design failure mode analysis, Use analysis, etc...all try to capture failures then risks. Some of these may even look at certain cascading faults (dual fault) based on risk (ie fire, crash). These documents will run to thousands of pages and are among the most confidential of company documents.

And no, I dont think BMW engineers missed this.
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Old 10-02-2014, 10:20 AM
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Quote:
Originally Posted by ard View Post
Engineering practices usually design based on 'single fault conditions': what will happen if X fails, or Y fails.

The system is not designed to accommodate failed airbags in a car that ALSO has some other electronic failure preventing sleep mode

If the airbag fails, the owner will note a sag. I will bet there is something, somehwere in the owners manual that says 'see your dealer if _______'.

In product design there are a handful of 'controls' that one can apply, which vary based on the risk of the failure. Life threatening (or car control) issues tend to have much stronger controls than others...but at the end of the day the owners manual is the place where all that ties to the driver.

Hazard analysis, risk management, Design failure mode analysis, Use analysis, etc...all try to capture failures then risks. Some of these may even look at certain cascading faults (dual fault) based on risk (ie fire, crash). These documents will run to thousands of pages and are among the most confidential of company documents.

And no, I dont think BMW engineers missed this.
Thanks Ard for flying the BMW flag. I was looking for something a bit more conclusive than just design concepts and brand reputation.
So let me be more spcific;

Over two consecutive nights I checked mm deviation using a scan tool as well as measuring height with a tape measure. On both mornings no visible lowering of the car and mm deviation. This led me to believe that the air springs are good or the compressor is keeping them inflated throughout the night AND therefore prevented the vehicle from going into "sleep" mode.

Then I removed the fuse for the compressor, within 90mins the car was visibly at its lowest point. This lead me to beleive that either the bags were deliberatly deflated due to error code or they leaked out.

Therefore if you did not know that your airsprings were leaking, you would not know about it until the leak was big enough to overcome the compressor filling rate. By this time the compressor may have passed its normal duty cycle and failed.

What I was looking for was a fail-safe shutdown of the compressor after say three failed attempts to fill the air springs.
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Old 10-02-2014, 01:17 PM
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are you looking for an actual shutdown in your car as designed, and wondering why it doesnt do it? or were you wondering if the BMW engineers should have done this but didnt?



You need to consider safety and risk to BMW. Would they like to reduce your repair cost by saving the compressor, or pay one lawsuit when a soccer mom loses control when the car doesnt attempt to pump the suspension up because it is saving the compressor?

Safety trumps cost every time.

im not flying a bmw flag. You want to say your analysis proves BMW engineers blew it, be my guest....you just dont have enough info to make that call.
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Old 10-02-2014, 02:02 PM
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keep your shirt on, makes sense. Safety probably is the overriding factor.
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Old 10-02-2014, 04:47 PM
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Hey MO, sorry so late chiming in. Boer I had the same fault. Exactly the same reaction. Exactly the same response you are displaying...namely utter disbelief at the potential cost of replacing BOTH air springs when every fibre in your body is telling you something else is at play...there isn't. Trust me, your air springs are BOTH damaged. The reason the vehicle remains inflated is simple. It was explained to me by a qualified person that due to the way the air springs work ( they literally roll up and down over themselves on a conical shaft) they tend to fail at the point where the fold-over originates. I duly did as instructed by said qualified person and waited until my rear air springs had been fully inflated, climbed under the vehicle and sprayed the air springs with a soapy solution. Sure enough a froth began to form over multiple seams which were not immediately apparant to me. As to why the vehicle doesn't always sag overnight or within a few hours? Believe it or not I found an explanation for that too. In my case the leaks were due to cracks in the fabric right at the point where the rubber rolled over the conical plastic portion of the spring and this would often result in the leaks being sealed by the resultant pressure.

In fairness to you and anyone else who reads this, this is obviously only my experience and I don't doubt that it is possible for there to be other plausible explanations but in my experience nothing that I've been through with my vehicle is specific to my vehicle...there is nothing to suggest that my X5 is special is what I'm trying to say. It therefore stands to reason that the most likely of causes is the most frequently experienced and seeing as how Arnott bother to manufacture an aftermarket air spring for the X5 it stands to reason that they do it because there is profit in it.
Raise your hands all those who've read this and have had your rear air springs replaced at some point.
I wish you were a little closer to my neck of the woods because I would gladly help out.
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Old 10-03-2014, 03:04 AM
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Howzit bru, no you are right both air bags are definitely leaking (and probably a faulty height sensor). I will change out next week.
Was just wondering why the compressor kept on trying, Ard is probably right it is a safety factor.
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Old 10-03-2014, 04:53 AM
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Quote:
Originally Posted by Mujahid View Post
Howzit bru, no you are right both air bags are definitely leaking (and probably a faulty height sensor). I will change out next week.
Was just wondering why the compressor kept on trying, Ard is probably right it is a safety factor.
What safety factor? Everyone's car goes down after after it is shutdown and locked.Only your stays up.See the pattern?
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