atxs500

Hi all,

I bought my first Mercedes two months ago, a 2001 S500. I'd always wanted an S-class as I enjoy the waft of the big barges, and my wife has an SLK55 so if I feel like hot-rodding I can always take that out. Anyway, I'd heard a lot about the various expensive issues these cars can have, including Airmatic, but figured I'd take my chances. I prefer to drive something fun and don't mind spending a bit more than Honda money for maintenance in order to do so.

Anyway I found a 1 owner car with 69,000 on the clock at a local dealer. It wasn't perfect but straight enough and original. Took it for a drive and was hooked! Perfect blend of power and silky smooth ride, just what I was looking for.

Fast forward two months and I get the dreaded "Stop - car too low" warning - only infrequently at first, but after a few weeks the front of the car was in the weeds and undriveable. A search around this site and the web suggested the struts were gone sure enough, upon inspection the tops of the front struts had hairline cracks, and the outer dust covers were completely destroyed.

So I limped the car to a local Euro car specialist - I had already done my homework, asked them if they could diagnose the problem and if it were the struts, whether they would fit Arnott refurbished struts. They were OK with that so off I went to work in the bus the next day awaiting a call.

When I took the call from the shop it was not good news - as expected, they said the struts needed replacing and the job would be $2,000. The struts would be $1,350 for the pair of Arnotts. Knowing the actual price of the struts from Arnott is more like $350 a piece, I went back to the shop after work, said "thanks very much" and drove off. But not before the guy at the counter tried to offer me $1,500 for my car "because they can fix it cheap and use it as a loaner". No thanks.

I resolved to fix the car myself. But Airmatic seemed like a hugely complex system beyond the skills of the average backyard mechanic. So I read every article and forum post I could find describing my problem - both front struts collapsed. Causes ranged from the struts, compressor, compressor relay and valve block. As mentioned earlier I inspected the struts which were clearly part their use-by date, so I ordered a pair of remanufactured Arnotts from BuyAutoParts.com - the price was $535 shipped, which I thought was an excellent deal. I also ordered a new Hella replacement compressor relay.

However, I wasn't convinced the struts were the whole problem - I suspected the compressor or valve block and I figured I really needed to get an accurate diagnosis - after my indy shop experience I wasn't prepared to go back to another shop. Fortunately I was able to obtain access to a Star Diagnostic system and with the help of a friend who's a Mercedes tech, I ran the Airmatic diagnostics. I started with the compressor - sure enough, it struggled to push 7 bar, when the system requires 14.


So I figured I'd start with replacing the compressor - I located a new P-2192 Arnott compressor for $200 shipped - it's the OEM Wabco unit, actually for a later model E-class but the W220 S-class unit is superseded by the E-class one. It's essentially the same but the air filter mounts on the opposite end. I also ordered a new compressor air filter.


Saturday morning the tools came out. I jacked the car on both front jacking points with two 2 ton jacks, and put jack stands under the sills as well. I removed both the left and right hand side engine covers, then removed the three nuts holding the compressor and compressor shield in place. Unfortunately they're spring loaded so when you remove the nuts, the springs and washers fly everywhere and it's impossible to tell how they go back together. I spent quite a bit of time studying diagrams, videos and matching wear marks on the parts to figure it out. The shield hides the mountings so you can't take a photo of the detail.


Once I got the old compressor out I could see the wires for the pressure relief valve were stripped bare, so not a good start. I swapped the mounting rubbers on to the new compressor, also the filter hose. Getting the new compressor in place for mounting in the car was extremely difficult with only one pair of hands - get a helper if you try it. The air filter hose being on the wrong end of the unit wasn't an issue, I just manoeuvred the hose so the air filter intake would point upwards and cable-tied it in place. After getting the electric cables back in place the last piece of the puzzle was the pressure line. Arnott documentation specifies removing the pressure fitting from the line to the car and pushing the bare hose in to the new fitting on the compressor. I was dubious that a push fitting would handle 14 bar, and I was right. I ran the DAS compressor test again and the pressure hose blew off immediately. So I re-fitted the old pressure coupling and removed the Arnott one - screwed the line in to the compressor and re-ran the test. The compressor made 14 bar immediately! Result! I lowered the car gradually, the same amount each side, while inflating the struts with DAS. Eventually the car was resting at the correct height with no support.


The next day the car was still at the correct height unassisted, so clearly the bulk of the problem was the weak compressor, but that of course could have been hastened by leaky struts, so they get done next weekend.


BTW I do have a few pics of some of the above details, but haven't figured out how to insert them in the right places. I'll keep trying.


I'll do another post once I've done the struts, also hopefully with pics after the job.


The lesson I've taken away from all this is: don't be daunted by seemingly complex repairs, just do as much research as you can (this board is fantastic!) and access to the right diagnostic tools is crucial. Most of all don't blindly accept a quote from a shop. Do your homework. If I had asked the shop to replace the struts, the problem would still not have been solved, and they would have charged me another $700 on top of the $2,000.


An aside: when I first started the car after replacing the compressor, virtually every warning light on the dash came on - ABS, Check Engine, ESP. The transmission wouldn't shift out of first, the steering was heavy and the suspension was rock hard. I pulled diagnostics on the ESP unit which showed it wasn't getting any power. A quick call to my Mercedes tech buddy revealed fuses 34, 16 and 6 feed the ESP unit, so I checked them all .Sure enough, fuse 34 had been knocked out of place while I was changing the compressor relay - plugged it back in and all the warnings disappeared. A test drive showed the car back to its old self - riding on air!

tusabes

Awesome detailed post !
Glad the issue was just a $200 compressor

I'm betting you can get tons more miles out of your current struts
If it were me I wouldn't change the struts until you notice a problem
Just get some epoxy and seal your strut tops for $5

The dust covers are meaningless

atxs500

Thanks tusabes!


I have thought about just returning the struts and buying them later if and when I need them. But I know they can leak from the bellows inside the unit as well as the strut top so figure I may as well just replace them to avoid problems later with the struts leaking and causing another compressor failure. And I'd probably incur a return handling fee too, which is reasonable under the circumstances.


Anyway, I've got a few days to mull the pros and cons before the weekend so will give it some more thought.

Jack_88

You'll notice the loss in ride height sitting well before the struts have gotten bad enough to damage the compressor. Also, these struts behave comparable to new all the way until failure, so if one goes bad, you don't have to replace the set.

kajtek1

FYI you can buy compressor rebuild kit on ebay for $16
I am strong believer in adding pressure gauge to the system, not only to be able see troubles before they become disasters, but also to avoid $2000 "proposals"