I'm creating this thread for people who want to understand their airmatic struts better. I'm no expert, and I've researched this topic dreading that I need to replace mine sooner. I found the Arnott vs. Bilstein debate to be ongoing, and mostly subjective: based on what people say, and I wanted to know what is objectively the best thing for the car that's also cost effective. I could be wong about a lot of these things, so please feel free to correct me. Note: This explanation is for the 2010 Facelift S550 4matic, so some details might be different for what you have.

Background
1) From what I understand, Bilstein originally made the w221 airmatic struts.
2) The strut has two components: the "Air spring" and the hydraulic shock absorber. Since they come together, they're known as the "air strut assembly".
3) The air spring is responsible for keeping your car level, raising it, and lowering it. This happens (i) at certain speeds, (ii) when the user clicks into sports-transmission mode, but (iii) doesn't happen when you switch between comfort & sport.
4) The shock dampener has "active control" (called "ADS": active dampened suspension).
5) Overall, the air strut assembly is Bilstein's "B4" model, but it was specifically modified for Mercedes to include the ADS. It is my conclusion that these shock absorbers are responsible for the ride quality.

What is the air spring, really?
Here's my understanding of this: The S-class means luxury, and for the Mercedes engineers wanted control over (1) Raising/lowering the car, and (2) Stiffening and loosing the shock absorbers, so that given the situation you're in the car adapts to you & gives you the best possible. This sounds perfectly reasonable. In fact, if you don't want your driver to worry about anything, then this system should be computerized & auto-adjust, based on speed, load, turning, highway driving, vs other things.

To this, they needed light weight adjustable springs. I think that the only way, or the best way to do this, was to fill a durable "balloon" with air, or what they called "the air bag". In other series, e.g. the E-series, I think this bag was separate from the shock absorber & required removing the subframe to be removed (picture below). In the W124 s-class, this air bag was a separate thing & was considered very robust. In the w220, engineers put the air-bag with the shock absorber--together as an assembly, and there were downsides to this. This is why the w220 suspension is expensive (because this was the first time, I think, the mercedes did this. I could be wrong) & they hadn't worked out all the kinks (the materials, durability, edge cases, maintainence etc). In the w221, they fixed most of these mistakes, which is why the structure of this assembly (strut + airbag together) has been the same till the 2024 W223.

The airbag is circled as yellow. The "Extra" modification Bilstein made for its B4 shock absorbers, is circled in red. That is the ADS sensor. You can see now. why the the airmatic strut looks different. The black "covering" on it is just a dust protector that hides a lot of details.


How it comes together. Source: Bilstein's website.

Let's talk about the ADS. It's ingeniously simple.
Basically, look, here's what's happening. How do we "absorb" bumpy-road shocks? Think of the shock absorbed like a syringe! If it's filled with air, then you can press on it a bit, but it won't go all the way. If you fill it will liquid (like water or oil) then you won't have the springy feeling, when you press on the syringe. It'll be harder. Stiffer. That's what a stiff suspension is. But there is another way to make it stuff or not stiff. Imagine if you could make the closed syringe some how bigger: the you would have more space for the air/fluid to flow and then the syringe, when pressing on it, will feel softer, as if there is less total stuff in it.

So, the "ADS", is basically a compartment: some extra space that has a valve in it. Normally, this valve is closed -- which mean a stiffer suspension. But if you pass current through it, this valve opens up and gives you "Air ride". Et Voila!
This should make perfect sense now, why people say "my dampener is shot" and their ride feels rough as helll! Read forums where people have mentioned this, and now youll see why that's happening.

More importantly of note: imagine if this valve is faulty and the shock absorber is not, your car will feel you're driving a cart. A rikhshaw. Tha-thud tha-thud.
This valve is called the "solenoid valve" (which is the technical term) or the "dampening valve), and if your ADS is faulty you'll get error code mentioning this. Another way to check this is using a multimeter, (scroll forwards to where he uses it). And, now you understand why this works or doesn't.

The Air Spring: Why is my car sagging?
To understand this, understand the system: your car has
(1) An air compressor (located in front of the front right tire) that pumps air.
(2) The air goes through "air-lines", so there is no leakage.
(3) Air first goes to a "valve block" and gets sent to each of the air springs.
(4) Each of the front tires have their own "level sensors", to check how raised the car is. The rear two wheels have only one sensor.
(5) The sensors tell the ECU, or whatever the airmatic control module/computer, the level of the car, which tells the valve block to fill appropriate air into whichever air spring needs it most.

When a car is sagging, you need to look at this whole flow and not just one component.
(1) People have reported "microtears" in the air line that have caused it to leak.
(2) Of course, air spring -- which is like a balloon, tears sometimes, with air leaking from it.
(3) The valve body can be faulty but in my readings, this was often not the case.
(4) Slight sagging may not mean the system is bad. If you're on an uneven pavement, by design the car will want to stay level (duh), so don't mistake that as a problem.

How do you isolate this?
(1) Does the car level properly after you start it, and sags over night? Or is it the case that one tire or multiple don't get filled with air at all?
(2) Is the sensor faulty or is the air spring?
(3) Is it happening to both the rear tyres? That's inconveneint that both struts would fail at the same time, so maybe they're both not getting filled (because they have 1 sensor, combined) in the first place.
(4) Plug in a cheap diagnostic tool (or an expensive one), it'll show you the voltage of each of the sensors, as well as the car's height.
--- If you do have 0V on a sagging side, or if the car is completely sagged even after it has been started, I'd recommend that you that you raise the car by that tyre's nearest jacking point incrementally & check your readings on the scanner. Jacking the car should simulate it as "rising" & I would expect the readings to change if the sensor is working properly. If it doesn't then maybe it's a faulty sensor.

Arnott vs. Bilstein.
There is no doubt in my mind that Bilstein is OEM, meaning new air springs from them should feel like the car just rolled off the factory floor (assuming everything else is fine on the car), but these are expensive! So, then I looked at other options. One thing to note in mind is that not every replacement comes with the ADS (your ability to have a comfort ride vs. a stiff ride at different speeds & other settings). This will mean that a "comfort" ride on the highway will feel as if the car is floaty. A stiff ride on the highway will feel terrible on bumpy city roads.

In fact, one replacement Arnott sells for the w221 (the expensive one) does not have these ADS modules. The remanufactored ones do, and they're relatively inexpensive. This should sound like the best option. Except, look at posts from people: they got the remanufactored part, and it felt like crap. Why's that? From different places I have pieced together, but not yet confirmed, that duringh the remanufactoring process, Arnott mostly deals with the airbag and replaces it with "continental" materials (arguably more durable). But what about the shock absorber, and the rest? I think they _do not_ address this.

Arnott itself says this: "Arnott remanufactured struts are inspected, steam cleaned, and then spin blasted to bare metal before the shock portion is recharged with new high performance shock oil. Worn shock components such as the piston and rod guides, as well as the seal pack, are replaced before being repainted with corrosion inhibiting paint and re-built with a new bump stop, name-brand air bladder, and other tier one components."

However, interesting enough, Bilstein says something different. I think they're not calling Arnott out by name. basically that things may look good on the outside because they're painted and made to look nice but from the inside they're not properly done.

If Bilstein is right, then what customers have said makes sense. Others will point out that Bilstein wants to make money and buy their expensive part, but I'll disagree: I think what Bilstein wants is quality and given their german origins, they have the same mindset as the engineers of Mercedes & specifically the w221: screw money, make the best thing possible. But I can see that I could be wrong.

Next steps:
(1) Find out what is Arnott actually doing to remanufactor these parts.
(2) Find out if Bilstein will sell a kit to change all the seals, valves & refurbish the dampener. Is there a DIY?
(3) When a dampener/strut "wears out" after let's say working well over 80,000 miles (140,000km), what exactly is worn out? Why can't it be changed? I'm pretty sure what Arnott is saying in their "remanufacturing process" can be done for under a $100 -- I know it. The question is how.

Please let me know what you think. Again, as mentioned, I could be wrong about things, but to be productive, if you'll say things, consider mentioning your source so we can get to the bottom of it!

 

Just saw your post and have had experience with Arnott's. I have a 2011 S550 4matic and replaced the struts, valve body and compressor at 150k miles. I was not having any problems but did so as a preventive measure. The first problem was that one of the rear Arnott units was defective out of the box, a 25% failure rate. They replaced it but I was out extra mechanic fees and without transportation. The first thing I noticed was that the ride quality was not as good as the originals and I would get a bang sound when going over large drops in pavement, which eventually stopped. 8 months later I started having suspension issues and took it to a different Indy shop who diagnosed that the compressor and valve body, both Arnott units, were going bad. They felt that the valve body went bad causing the compressor to overheat and fail. They had a valve body in stock and luckily I had kept the OEM compressor and they reinstalled it. Arnott replaced both units but I was out another $800 in mechanic fees. This Indy shop also said that they had stopped using Arnott parts and said that the rubber seals were not as heavy as OEM and would only last about 2 years before failure. I probably ended up spending as much as if I had I gone with rebuilt OEM or Bilstein units. As far as ride quality I can tell a difference although passengers still think it rides great.
Hope that helps.

 

Arnott has gone down the drain for a while now. Yes they are prompt on replacing their junk with more junk but the people that have to pay for labor, will not enjoy that experience. Now if you did the work yourself that might make a difference. I would not go the Arnott way and I do my own work as well.