OK, so I found the driver's rear sagging and since my 2012 GL350 has 73,000 miles on it, figured that I must need new air springs in the rear. I had just replaced a failed compressor about a month ago so wasn't really surprised. I ordered parts (Arnott) and got them installed yesterday; easy job, maybe an hour and a half with getting the car up on the lift, etc. When I originally turned on the car there was a very loud 'pop' from one of the springs which did not seem normal, but I can't find any evidence that something went wrong. I assumed that it was just the spring unfolding from the compressed position.

Since I already had the car in the air, and it's warming up I figured I would also swap the summer tires back on, which I did. When I initially had the car back down on the ground it was normal height, but when I came back an hour later it was sagging again. I started it again but it stayed down on that same corner. Perplexed, but our of time, I left it for the night.

This morning I went back down and thought maybe I got a bad unit out of the box, but I had set aside the good spring from the passenger side, so I pulled the new Arnott (which was totallly flat) and installed the old but good OEM unit. Started the car, no air to the spring at all.

So now I'm trying to think of what could possibly be wrong:

1. Bad air line (seems unlikely, but I guess anything is possible)
2. Bad valve block not sending air to that one spring
3. A leak somewhere in between

I'm going to do some research today. I do have a diagnostic tool (Autel 906) but I don't know if there's anything in Airmatic that senses pressure in springs. I guess I'll find out.

Any suggestions or experience are appreciated. I'm traveling this week so have time to order parts. Guess I'll be taking an Uber to the airport....

 

Well, here's an update. I put the Arnott spring back on since it didn't appear that the spring was the problem. Then I hooked my diagnostic tool up and started messing with it. No faults detected so I started running some tests using the tool.

Clearly the computer knows the ride height is off:


So next I got into the menu where you can test individual pressures and raise each corner.



So finally I kept hitting F7 to raise the left rear corner until the numbers matched the right rear. The spring pressure increased slightly each time I hit F7:




So now the spring is full, the car is level and as I was running the test diagnostics on the airmatic system...the car battery died. So now I'm charging the battery, but it's still sitting level.

Will report back. I think this confirms that the valve block works at least sometime, but still more questions than answers.

 

So just to complete the circle on this. I got the battery charged and the vehicle is behaving normally; new Arnott air springs are working as intended. I have not been able to determine why I had to inflate the left rear with the computer rather than it just filling by itself. Either way, it's working now.

 

The Airmatic module has leak detection. If it sends too much air in one shock over certain time it would assume there is a leak and disable that part of the system.

You somehow without star/ das managed to enable the circuit back on.

 

Thanks Alx that solves the mystery then. I'm using an Autel Maxisys which is a generic equivalent to Star/das but works on multiple makes. So far from what I can tell, it has the ability to do all the coding, etc.