My power locks, trunk handle and soft close, rear seat headrest, and lumbar support on my 1999 S500 aren't working. These all stopped working the same time so I suspected they are connected. I found my trunk fuse #11 was blown, and when I replaced it my locks finally worked... Once. After one click of the lock button or use of any of the other malfunctioning parts the fuse would blow. I'm ready to fix this problem because even though it doesn't effect the performance of the car in any way, it is inconvenience to have to reach and unlock all the locks everytime other people ride in my car.

I've heard many things from other forms, but none have worked for me so far. Please let me know if anyone has any ideas. I have looked at my pump under my right rear seat and heard that that can be a common culprit, but I didn't want to start pulling stuff apart without having an idea to what I was doing.

 

It is your pump—the one under the rear seat right side....likely your pump motor shorted or is shorting out....use an open end 7mm thin wrench to pry the hose connectors off hopefully without breaking the nipples themselves....either get another WORKING pump or replace the motor yourself (just do an internet search; there’s plenty of info out there on the subject) and look for a replacement motor (again do a search on whats compatible)

 

There are two electric vacuum/air pumps in a fully optioned W140. The one under the rear seat works its wonders for the power pull-in/down feature for the doors and rear deck lid and provides pressure for the lumbar adjustment. Fuse rear #9. There is a second pump located behind the left side of the rear seat back, adjacent to the fuel tank. It is accessed by removing the panel which separates the trunk from the fuel tank. I believe it provides for the central locking and rear seat headrest drop features. Please don't "shoot the messenger" if I've overlooked something. I've spent hundreds of $US trying to purchase a comprehensive service manual for this car,and all I get from MBNA is "we don't support anything other than our [expletive omitted: expensive subscription] service for on-line access. At 20+ years of age, don't feel cheated if either or both of these parts of an old car show failure. I've installed a refurbished underseat unit, which works perfectly. If the other unit fails, I also will replace it with a rebuild. The price asked by "The Official MRBZ" parts chain for these obsolete parts is totally beyond reason.

 

Actually, its the other way round The pump in the boot manages the close assist on the doors, and the one under the seat manages the central locking and supplies backup vacuum for the lumbar support and HVAC.

 

It is entirely possible that the plumbing for the W140 underwent changes during its production. I know only what is the case for my 1997 W140SWB. The pump under the rear seat works the soft door pull-in function and the trunk ("boot") cover pull-down. However, not all of it. If (rear) F11 is pulled, retraction of the trunk handle stops, even though the pull-down feature works. The interworkings of the two vacuum/pressure units are totally inscrutable, and I think most owners know MRBZ underwent a major effort to "re-engineer" the W140 after its disappointing first few years of sales fell far short of expectations.

 

You're right. Sorry to have posted incorrect info. If anyone has experienced the problem of a broken-off nipple on either pump there is a workaround. This part: MB Part # 111 078 01 81 is a 90 deg. neoprene elbow which fits over the stub of the nipple and accepts the 3mm diameter nylon vacuum line. Originally for a different application, but it works very well here.