1999 S500 trunk close assist not working
 

The trunk is ajar about a half inch or so. I found a MB TSB stating to check the pneumatic pump under the rear seat to see if it makes any noise after closing the trunk. With the rear seat bottom out, I found the pump. It does make a whirring noise when the trunk is closed. The TSB says to look for the red vacuum hose which has probably come disconnected from the pump. The problem is there is no red vacuum line off the pump, only yellow and gray lines. Does this mean a new pump is needed? I hate this car.

Not exactly thrilled to answer someone who hates their car.

When this happened on my S500, it was the vacuum element that moves the handle that sticks out when the trunk is opened. If the handle is not retracted, the close assist does not work.

You need to take the trunk lid liner out to get to it.

Thanks, I'll pull that off and see. By the way, where is the pump that operates the wholeclose assist system? It's actually NOT under the bench seat. That pump is just the door lock system etc.

Actually, the trunk doesn't close that last half-inch but the chrome trunk handle DOES pop out just above the license plate and then retract right after the trunk is closed - so I don't think we're talking about the same thing. It's that final half-inch or so closure with that quiet hum that is missing on the doors and the trunk. The doors close flush with the frame though with minimal effort of the hand, that's what's odd.

In your 99 model, the central locking pump has nothing to do with closing the trunk. Closing is performed by the closing assist pump in the trunk.

Read all about it:

http://v12uberalles.com/Closing_Assist_Pump.htm

The central locking pump operates the trunk handle, and it also is responsible for releasing the striker eye in the trunk lid so that it is ready to be pulled in again by the closing assist pump to close the trunk. I'm unclear on the the exact situation on the earlier models, but I think the trunk handle used to be operated by the closing assist pump. I believe there was either a TSB or service campaign with a kit for switching the trunk handle operation from the closing assist pump to the central locking pump. I think the trunk closing has always been performed by the closing assist pump.

Brett

WADR, because you are a known expert on this, I don't think so.

I believe that the two work together. The one vacuum element retracts the handle, and then the closer does its thing. I don't think it will close assist if the handle is not retacted. This element also operates the trunk lock.

Once I got the handle retraction working, the close assist started working as well.

Sounds like this is a different problem, purely in the close assist function.

BTW, the close assist pump is behind the liner on the left side of the trunk.

In the early 140s the trunk handle was driven off the pump under the rear seat, black hose going to port SFG. There was a service bulletin in 95 to relocate it to the close asssist pump, where it is in line with the retracting stiker (which required a new latch). A quick test would be to rotate the lower latch and see if the upper latch draws in if it does not the trouble lies in either the pump, the lines going to the latch, the latch or the handle. The next step would be to pull the fuse in location 9 in the trunk to see if this brings this back to life. The other thing to check is the screws on the lower latch loosening allowing the latch to move upward.

Good point. Let's not forget to check that. The non-closing trunk may have nothing to do with either pump.

Bostonbenz, do as Peter mentions. With the trunk open, operate the catch mechanism in the lower latch with your thumb, and observe what happens. Does the striker eye retract into the trunk lid and latch properly? If so, the closing assist pump is doing its thing just fine. The problem is elsewhere.

Brett

All I can say is how my 1997 model's system works since I have never worked on any other car in person. And, I have to qualify that statement by saying that I cannot be absolutely sure that the system is as it was when it left the factory, because it had clearly been messed with. But, having said that, the "messing with" that I fixed was simply disconnected lines that I simply reconnected, so I don't think that major changes (i.e. switching operation of functions from one to the other pump) were made.

Here is how my system is set up. The central locking pump has a supply line going to the trunk lid. That line splits and feeds the trunk handle actuator and the striker eye release actuator (There are two separate actuators for these functions). The closing assist pump has one supply line feeding only the closing assist actuator. The closing assist pump does nothing else.

The operation of the trunk handle and the closing assist are not connected pneumatically. It is possible that there is some electrical connection (i.e. a microswitch in the trunk handle mechanism that checks to see if the trunk handle is closed before allowing the closing assist function to operate), but I haven't observed behavior of the system to support this nor have I read anything about the system to indicate this. I'd have to go and watch the action closely again, but I'm pretty sure that the trunk handle and closing assist operate at the same time when closing the trunk. They aren't sequential, which is what your comments might suggest (e.g. The trunk handle has to retract, trip the microswitch, which then allows the closing assist pump to function.)

But, I have to say, I have never tested this. I have never purposefully disabled the trunk handle and checked to see if the closing assist still works. That's the definitive experiment.

I've always been concerned with Peter's comments (which I paraphrased wrongly above) about the trunk handle being moved to the closing assist pump (backwards from what I said). My car is configured exactly how he says "earlier" cars were configured. My car is a 97. I have always assumed that my car is representative of how the system is configured in later cars. Peter?

But, if the air lines are configured such that the trunk handle and the closing assist are both operated off the closing assist pump (what Peter says), and the trunk handle mechanism has failed in a manner that causes a major air leak, then yes, the closing assist function would likely also fail. Fixing the trunk handle mechanism will then restore the closing function.

Brett

In my humble opinion, my trunk lid and moving doors did not draw down on my 92 500 SEL, the dealer suggested if it was a problem pull the fuse to reset the system, after a while I spent more time in the trunk than I wanted to. They replaced the pump and it worked for a while, and then it acted up again ($900). I had purchased a few MBs so the service manager told me about a service bulletin in 94.
To remove the handle functionfrom the PSE, and relocate it to the "close assist pump" This involved a new upper latch, a shorting jumper, for a micrositch on the handle, and a "T' in the close assist line to the trunk. The lid now draws down and the handle goes at the same time.
This would go fit the pattern of repacing the PSE with a universal pump with no handle port.
I can comment on the screws loosening, and did that, some years later rather than purchasing pump #3 I followed the adjustment proceedure with sucess, I did not mention MB replaced pump #2 under good will, under a bet there were no leaks in the system. My shop manager did indicate they could never do the pressure ajustment, under MB rules.

I found the pump in the left rear corner of the trunk. I pulled it out while leaving the lines attached. I disconnected the electrical connection, then reconnected it. It now works. Bizarre. All the doors and the trunk close as they should. I'm not going to mess with it beyond that since it is now working. I guess unplugging it "reset" it if there is such a thing. Thanks Brett for your V12 write up. By the way, I sent you an email to your san.rr.com account.

So your trunk line had simply shut down because of timing out twice. Without doing anything more than rebooting the pump, it will shut down again for the same reason that it shut down this time. You need to do more in order to have a long term fix. Reducing your pump's shut off pressure (after confirming that you have no major air leaks) is probably the best thing you can do.

My system has run flawlessly for 3 years now since I performed the simple adjustment. Not one single failure to close any door or the trunk.

Brett

I finally came across mention of the change in the trunk mechanism. Alldata says this:

NOTE : As of chassis end no. 118121, the trunk lid closing assist striker is no longer controlled by the closing assist supply pump, but by the PSE control module. Therefore, the microswitch (S88/3) as well as some mechanical parts have been eliminated.

So it is the striker release mechanism that was moved over to the PSE pump from the closing assist pump, not the trunk handle. The trunk handle has always been controlled by the PSE pump.

Brett

I guess I was misinformed on how the handle was controlled especially the shorting of the microswitch, the new latch and the "T" insertion. The drawdown and handle were two operations, now it happens at the same time.

S500 trunk latch will not close
 

My 1999 s500, I opened the trunk and now it will not close. Latch stays open even when I try to close the lid it will not latch. I can close the latch with my fingers and it open's when i push the key button mechanism. will not latch closed. any ideas how to get this closed or any way to manually latch it closed?