One of great features S-class Mercedes Benz has is door and trunk closing assistance. Once door is and trunk pushed to close electric pump pulls it in to complete shut position. Many pumps fail, unfortunately, leaving driver to slam doors and trunk to close them fully. Trunk, however, will be the hardest to slam and once closed there will be a gap between trunk and car body. All that is caused by faulty close assist pump. Once trunk is pushed to close, assist pump pulls it in. If it fails to pull the trunk in, it will latch in but will not be pulled to fully shut position.
Many people reset the pump by pulling the fuse out trunk fuse panel. After some time pump stops operating because the problem is not repaired with reset. There are few things that can go wrong: vacuum leak inside door or trunk close assist mechanism, leaking vacuum connections on pump or actuators, leaking vacuum hose, pump out the adjustment internally, pump valves leaking, shut off valve leaking, and pump motor blown if pump doesn’t run at all.
There are many used 94-up model pumps on eBay but there are no earlier ones. Newer models are not interchangeable with older models. I had successfully readjusted my earlier 1992 model pump. Recently I repaired my internal shut off valve in my 1997 S320 pump by compressing spring. Please view Youtube video below with description what was done. ( Sorry for no audio. File became corrupted at some point.)
It is preferred to have the pump connected to the car in order to adjust it correctly as every car has its own wear rate. It took me some time to get mine right. You can mail me your pump and I can adjust it on the bench and mail it back. Pump adjustment is $100 plus shipping. If you’re local, you can bring the car over and I will be able to adjust your pump while you wait. If your pump stops operating after every fuse #9 reset then you might have other pump failure and simple adjustment might not fix it. Let me know your symptoms and we can take it from there.
Just fixed a pump from local 97 S500. Video below shows pump disassembled. After making video I found tiny rapture in diaphragm. I used piece of latex glove and placed it between diaphragm and valve rear case. No silicone was used on this valve, just shortened the spring and piece of high quality latex glove to cover a rapture. Pump vanes were also worn. I polished pump internals with Dremmel and polishing paste, flipped vanes around. Now pump works. It’s slightly slower than new pump because of vane wear but before pump didn’t work at all.
Please contact me with any questions you have.
Closing assist pump located in the trunk above driver wheel well toward the interior. There is another pump found under rear seat that operates door locks.