Close assist system misinformation corrected. At least as it pertains to my car.

[Brett San Diego]

I have seen it posted more than once that the trunk closing assist mechanism runs off the trunk mounted pump, but that isn't entirely true. To function properly, the mechanism has inputs from both the trunk pump and the central locking pump under the rear seat. True, the trunk pump is responsible for driving the piston that actually does the closing assist. But, the central locking pump is responsible for unlatching the striker in the mechanism so it can extend, ready for retraction by the close assist system next time the trunk is closed. If the line from the central locking pump isn't attached, you'll get a single close assist function, then you'll have to manually close the trunk thereafter.

I found on my car (97 s600 that I just took possession of) that both lines had been disconnected. I wondered why. After studying the mechanism for a while, it occurred to me. The p.o. must have had the close assist stop working on him, so he followed the owner's manual's directions and set the striker for manual closing. But, because the striker is released by the central locking pump every time you open the trunk, you have to reset the striker manually every time you go to close the trunk. I figure the p.o. (or his mechanic) was annoyed by this and took the easy way out. Instead of being competent and solving the problem of the close assist not working, he simply pulled and plugged the line from the central locking pump that kept unlatching the striker. Problem solved... the trunk latch was permanently set for manual closing.

The central locking pump is also responsible for extending and retracting the trunk handle. There is a T-connection in the pump line right next to both mechanisms in the trunk. One goes to the handle, and one goes to the trunk latch.

So that's the real story. Both pumps are required for full operation of the trunk closing assist.

Cheers,
Brett

[Peter Guenther]

As shipped, the trunk handle was driven off the locking pump, if there was an issue there was a bulletin from MB that the handle could be driven off the same source as the closing assist on the trunk lid. It directed to put in a "tee" and plug the other line. My 92 had both the pump replaced, and then the "mod" done by the dealer in 96, If you have the "mod" the handle, and lid "pull down" will be at the same time. If not, it will appear as two seperate actions. The dealer replaced the pump again last year N/C as "good-will" since I paid dearly for the fist one!
I am under the opinion the safety and high friction in the locks could really be some of the issues.
There was a post on one of the forums where someone "tweeked the safety" and got it to work reliably. That is a real gamble, and could do damage to the actuators, which are probably very expensive and hard to get to. The pump builds vacuum (I always thought) or pressure, to "pull in" the doors but will shut down if the safety opens to prevent blowing the actuators. I think three failures, and it diables that door. As I recall the dealership used a "Mighty Vac" to check the lines for leakage before replacing the pump.