The relay which turns the airmatic suspension pump on may fail causing the pump to run continuously. This can result in failure of the pump motor or a dead battery as the pump motor will continue to run after the car is turned off. Also, Worn airmatic pump mount bushings can causes noises in the front axle while driving on bumpy roads or a buzzing noise while the pump is running.
Airmatic Pump Motor May Fail Due to Sticking Relay on Mercedes-Benz E350 4MATIC
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I replaced both the air pump and air pump relay last year 04/13. Within 7 months both air pump and air pump relay failed again. Got no help from parts dealer or manufacturer regarding premature failure. Don't expect help from Mercedes either. They have a serious problem here. Like GM they are not dealing fairly with their customers. A lot of people have bought the Bosch Air Pump and have had to return them because of failure. In fact, many car parts dealers are not selling this air pump any more because of the problems. I should have realized there was a problem when it took me over a month to find the previous part. It is really annoying to have to do the same repair a little over one year after doing the previous work.
After a week in LT airport parking, I returned to a pimped out wagon. The air had released from airmatic but fortunately, it filled back up after a few min of engine idle. Still, dealer is replacing under ext warranty. Also, I gather this is a June 2012 recall item.