My GL350 airmatic suspension compressor failed and it was replaced, The cause was deemed to be a small leak in one of the plastic hoses at the block valve very near the feral end. The compressor (FCP Euro) was replaced but the relay shipped with the new compressor did not work so the old one was reinstalled. The new compressor started to run for long periods of time but would stop just before I brought the vehicle to a stop, then once forward motion started it would start up again. I took it into the Indy shop. The rear height sensors for the airmatic suspension have control arms that were seized on one side and quite stiff on the other side so they were replaced. My vehicle can be parked for several days and there is no droop at all in the suspension, there seems to be no system leak. Yet when I drive the vehicle around town the compressor comes on and stays on until just before the vehicle is brought to a stop. The block valve has been replaced and again the compressor continues to run constantly when driven around town and shuts off just as the vehicle is slowed to ~10 km/h and comes to a stop and most times resumes once forward motion above ~10km is achieved. What can cause the compressor to run if there is no system leak?

The vehicle has another symptom when it is being driven on icy roads the rear end is very unstable, the stability program is often called upon to keep it from skidding (applying brakes to one wheel etc), During testing under the slippery road conditions I shut off the stability control program thinking perhaps it was causing the instability however the instability of the back end continued so for now I ruled out the stability control as the cause. Our vehicle has cross wind assist and auto levelling. I though of mine is that something is telling the suspension compressor to operate and the the ride height is being adjusted and this is creating the instability on slippery roads.

I have asked them to try another relay - perhaps the relay can stick? I don't know

The Indy Shop is not certain what to do next- We get no codes, the vehicle does not droop at all even when parked for several days and driven with the compressor disabled (compressor relay removed) .

 

Did you ever resolve your issues JSWGL350? I have or had both with my GL550 also.

I'm still dealing with the exact same symptoms as you describe related to your airmatic suspension compressor. Replaced the valve block and then the compressor, but no change. Vehicle can sit for weeks and does not settle, so I don't believe the air springs are leaking. Less than 60,000 total kilometers (36,000 miles) so I can't imagine things are worn out? I'm worried my replacement compressor will eventually wear out, again, because it seems to run constantly when the vehicle is in motion. Like yours, it seems to pressure up just as the vehicle comes to a stop and shut off, but then starts running again once driving.

I also had the same problem you describe with driving on icy roads. Did you happen to have your GL aligned before this started? The reason I ask is that I did (at one of the local MB dealerships) and that is when my GL started feeling very unstable in the rear while driving on icy roads. It was perfectly fine before the wheel alignment but was almost undriveable afterwards. I should note that the only reason I took it in for and alignment in the first place was because the steering wheel had been slightly off center ever since new and I finally got tired of it... Anyhow, I looked at the alignment printout (which I had asked the dealer for when the work was done) and noticed they made a large adjustment of toe on the rear axle to bring it within "spec". I didn't understand this because it drove very well before hand (other than the steering wheel being slightly off center) so why such a large adjustment was needed was beyond me? I took it back to the dealer to check the alignment which they confirmed once again was "in spec" but there was no way I was going to be able to drive it the way it was, so I made them adjust the rear axle "toe in" setting back to near zero (where it was initially). After doing that, it drives perfect once again. I don't understand why the factory specifications calls for so much "toe in" on the rear axle, but it is wrong and definitely affected the icy road driving characteristics of my GL in a bad way.

 

In answer to your questions. The continuous running of the compressor seems to have been cured by completing a recalibration of the height adjustment sensors after the sensors were replaced, initially the arms on the sensor were replaced as one was seized and the other was pretty stiff. It is not clear to me if the Indy shop did not do it correctly in the first instance or if the sensors were defective or there was some bug that got cleared when the recalibrated them for the second or third time. Have you checked your ride height sensors - they have little plastic arms molded to metal socket joint that are a known problem for seizing up and you will want them working properly. I would suggest a recalibration first to see if that solves your issue alternative you can try to replace the sensors and then recalibrate. As I understand it the vehicle needs to be level when they are recalibrated.
Regarding the instability. We have not had the alignment completed but will have it checked. Since it is summer it is difficult to know if we have the same problem or if it was related to the compressor/ride height sensor issue and now resolved- it was only noted on icy roads. Thank you for sharing your solution.