depressurise rear airbags

[EJenner]

So I had a problem with the rear airbags on my W212 2010 350 CDI estate.

Apparently from reading the internet there is a common problem with the ball-joints on the suspension sensor rusting. Then the plastic link arm between the suspension and the sensor can't move because of the rust and then it snaps.

I've repaired the arm and fitted it back to the car.

A secondary issue of the system depressurising occurred at the same time. The air supply line popped off one of the airbags.

I thought as I'd repaired both faults the system would be capable of levelling itself but it seems that's not the case. As it's all the same hardware I would've thought once all reassembled the sensor values would be the same as they were before so the ride height would be the same.

So the problem I have is that one side is a bit higher than the other... enough so you can see the difference with the naked eye.

I thought one potential way of fixing this would be to depressurise both airbags in the rear and allow it to reinflate from scratch.

 

[EJenner]

Anybody think that could work and any guidance on how to depressurise the airbags?

 

[Uncle Benz]

On a w212 if you remove the plastic cover under the bumper, rear left corner you will find a plastic manifold that splits the air supply to each air spring. Loosen the brass fitting slightly with a 10mm spanner and air will escape. I don’t know if it will fix your problem. Air pipes don’t “pop off” on these. The fittings are a one way barb. Push fit, but only the once.
Are you sure your repaired arm is the same length as before? The length of the arm dictates the ride height of that corner

 

[EJenner]

Thanks, that's useful info on where to look to deliberately depressurise. I have to make sure I can get to that place once the suspension has fully sunk to seal it back up.

As for the barbs.. I found actually you can remove the nut from the airbag and reinsert the hose very securely. This is where the hose popped off mine. It depressurised the whole system and both sides collapsed.

 

[EJenner]

The air pipe popped off after the arm snapped on the level sensor. When the arm snapped the system responded by jacking both sides up as high as possible. It seemed as though it was continuing to pump even though both bags were full. It must have been the case that the broken sensor had told the control unit that the car was riding low and it should try to raise it.

So with this problem occurring some 60-odd miles away from home I was attempting to get home without having to repair the car at the side of the road. Especially as I didn't know what was wrong with it at that stage.

With the rear suspension fully extended and the pump continuing to pressurise the system it must have been bumps in the road which caused the hose to pop off... too much pressure and that's how it happened. I heard a 'pop' and a noise of air rapidly escaping. I had hoped that was the system actuating a pressure release valve and the ride height was returning to normal. I pulled into a layby to have a look and found in fact it was sitting on it's arse and the pipe had popped off the nearside airbag.

 

[EJenner]

RE: length of the arm... It should be identical. However, I did take off the bracket holding the sensor and refitted later on. The whole thing could be a 1 or 2 mm different from before... but I wouldn't have thought that could result in a ride height so different you can actually see it.

 

[Uncle Benz]

I would replace the air fitting if I were you. I think that will be a weak point if you don’t, and may let go again at an inopertune moment. They are a couple of quid from Mercedes.
A few mm on the arm may be enough to make the difference. The difference is magnified by the time its at the wheel/body end of the equation

 

[EJenner]

I'm pretty good with stuff like this TBH. I doubt it'll be popping out. The only thing which can wear out on the fitting is the outside of the nylon hose where the compression fitting grips it. But every time you remove and refit the hose the compression fitting is going to clamp it in a different place. That being said... there would come a time if repeatedly removing and refitting that the hose would be too chewed up at the end for it to stay attached and the part you'd have to replace would be the hose rather than the fitting. There is also an o-ring on the fitting which could be replaced, you can see that in the photo.

 

[Uncle Benz]

If reusing the pipe it’s recommended to cut about 6mm off the end to remove the crushed section

 

[EJenner]

I successfully depressurised the system and allowed it to reinflate. It seemed a little more level after that. But still not right... perhaps an inch higher on the passenger side where the link had broken. Incidentally... my car has a metal arm on the drivers side so that was obviously replaced already. The metal arm supersedes the plastic one.

As I still wasn't happy with it I took off both rear wheels and had a closer look at the sensors. There's two on the 2010 model year.

I realised the sensor I'd removed and refitted was leaning downward away from the hub where it's mounted. There was a gap of around a millimetre compared to the other side where there was no gap. When I bent the bracket back so it was flat against the hub it made quite a big movement in the arm on the sensor. And once again I made the mistake of assuming since all was all 'back to normal now' that the car would be fixed.

Not so...

Now I have 'Malfunction' on the dashboard and the passenger side where all the issues are has raised up higher than ever.

So I think what I've learned today is that small adjustments to the sensor do make a difference and that what I did made the problem worse so I've got to undo that and move further the other way to hopefully fix it.

In the meantime the wife is saying she won't drive it like that and that it's ok... "I can drive the BMW"... so it's pretty urgent to get it fixed. I could buy one of those adjustable sensor arms and play around with that maybe?

 

[EJenner]

CORRECTION - I'd forgotten to plug in the valve at the back which I'd disconnected when bleeding the rear bags. So the 'Malfunction' has gone from the display and the car is now level.

So basically, it was a bit of messing around but I fixed it for free. The best kind of fix.

This is the broken arm:

I fixed it by supergluing the plastic back together and then I fitted the whole thing inside an aluminium box tube. It's held into the tube with araldite resin which is very strong.

The ball joint at the top attached to the sensor had rusted quite a lot. I broke the bracket while I was trying to remove the rusty nut off the ball-joint. So I had to weld that back together. Where the ball had rusted it was now too small to hold the link properly so I put a bit of heat-shrink tubing over the ball and that was enough replacement material to make up the difference. The whole bracket and ball joint situation has been very well greased to stop it rusting in the future.

Have to say, I don't rate this build quality too highly. My car has only 70k on the clock so it's not a battered old thing, why is such a vital component made from such poorly protected and corrosion prone material? I also think the way they leave the nylon tubes exposed and mounted just behind the rear bumper is well dodgy. If you had a significant rear-end bump there's a good chance it's going to take out the air-suspension and the car will be instantly undrivable. With traditional springs at least you'd be able to scrape up the bits off the road and drive home.

 

[Uncle Benz]

Haha! That’s some proper backyard engineering! Glad you have resolved it!

 

[EJenner]

I'm used to working on classic cars where the option of going out and buying a new bit isn't on the table!

 

[LostKiwi]

Nice fix. I'd probably have printed a new part

 

[EJenner]

the other option is the aftermarket adjustable arms. Also, I think Mercedes sell a metal replacement arm as that's what I have on the other side and people have mentioned it elsewhere on other forums. Think it was £37 where it was previously mentioned... that's not too bad.

 

[Sindret]

Hi. Sorry for hi jacking your thread, but I'm in need of a replacement arm myself. Did a homemade solution well that's probably gonna last, but would ease my mind to actually get a proper one.
Anyone know where to get one? I'm out of ideas at least, been searching Google in and out for it. Maybe I'm using some wrong keywords or something..
Maybe 3d printing one could be a solution, but unsure of the strength 3d printed items.

Thanks in advance

 

[EJenner]

You can buy an arm from Mercedes. Give them your vehicle details and they'll get you exactally the right part. Alternatively buy some of the adjustable arms off eBay.

Mercedes E Class air suspension lowering kit, height adjustment link,linkages. | eBay


Zinc plate sockets, stainless steel connectors and lock nuts, pre greased.

www.ebay.co.uk