How My Mercedes-Benz W221 S-Class Nearly Betrayed Me on the Road Due to a Sensor Failure

One of the most common problems with the Mercedes-Benz W221 S-Class is the failure of the rear-level sensor. The level sensor rusts over time and breaks off, causing the car to lower to the ground or rise.

Background

It is my first Mercedes, and it’s been treating me well.  I ran into my first issue with it a couple of weeks ago.  I drove into work in the morning and had no issues; it was a normal drive. I drove home for lunch, and on my way down my driveway, the rear sounded and felt stiff.

I saw both sides of the rear were lowered to the top of the rear tires behind the wheel wells. Low rider!  The front was fine. I hooked up SDS and ran the quick test to see if it would throw a code.  It did not.  I also did not have the front dash warning about not driving because the car was too low.  Next, I ran the pneumatic tests.

The pump and front level control valves passed, but the rears both failed for the level control valves. I ran all the other tests, and they all passed, including pump and leak tests.  Playing around, I noticed you could manually go into each strut and add or bleed air.

To my surprise, I was able to pump the rear up. I re-ran the tests, and the rear passed this time, with a slight exception, which I only realized later.

When it runs the pneumatic tests, it attempts to level each air strut if needed.  Even when pumped up, the rear tests would stop at the test for a second and move on.  The fronts would take their time and feel it out.

I left it for 30 minutes, and it held its position.  Then I drove up the driveway and back down, a 30-second drive at best. When I returned to the garage, the rear was almost back to the floor.

What the heck? I hooked up SDS again and pumped up the rear. I left it overnight, and it didn’t move. I went up the driveway, and again, the rear end sank. I was puzzled. I did some reading, and everyone was pointing to the air struts.

It was “just one of the things you will have to do with airmatic suspension; they blow out.”  A local indy was looking at $1800-2000 to replace both rear struts and was convinced it was likely the issue.  I am not one to give in, though, and persist.

I was puzzled about both rear struts going out simultaneously and the ability to pump up the rear and then pass the tests via SDS.  It went down once I started driving, but I had never heard of a leak or conducted tests. Finally, I got a schematic of the airmatic system and started testing additional components.

I moved the car up and down the driveway again, checking the back wheels every 5-10 seconds. They were moving down. When I got back down the driveway, they were nearly down again. I hooked up SDS and found no trouble codes in the airmatic system.

I ran all the pneumatic tests, and the left and right rear failed.  It said, “Component ‘Y36/6y3 and /6y4(left and right rear level control valves)’ is not operating correctly, or the line between the valve block and the suspension strut is not leaking tight.” So, I ran the following tests, and they passed them all. The airmatic pressure release valve, leakage test between the valve block and central reservoir, and leakage between the compressor and valve block.

There didn’t seem to be a leak anywhere, so I dug deeper into SDS and found additional testing for the airmatic components. When I tested the rear level sensor, it was severely undervoltage, at 0.45V instead of the range of 2.00-3.00V. Everything else passed.

A couple of clicks later, I had the location and jacked the car onto some jack stands. Once underneath the rear, I saw the sensor. The wiring was intact.

Problem

Then, I saw the pivot arm attached to the sensor on one side and a rear sway bar on the other. The latter end was no longer connected. (See pics) I could zip-tie it in place to see if I was right.

Once rigged in place, I ran the tests in SDS again, and the rear sensor passed with the correct voltage range.  I reran the pneumatic tests, and all the corners leveled themselves. I took the car for a ride, and it was smooth and normal.

Woohoo!  It felt great to figure this one out.

This pic is of an intact rear sensor.  I put a curve on where it looks like mine broke off.  I have since swapped in a new one.

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