Close Assist Problem
 

We are working on a 1998 Mercedes S420 that has an intermittent close assist failure. Once you remove the relevent fuse and re-install the same fuse the close assist system will work for 5-10 times and then fail again. It primarily fails on the driver's door but sometimes the trunk as well. When the system fails it rarely fails on all the doors but occasionally may.

We would appreciate feed back. Per the dealer we have replaced the Lt Frt Door Lock Assy and the close assist switch in the left front door without any improvement.

Thank-you!
Steve C

search "closing assist"
 

The most common issue is the deteriation of the close assist pump, it provides pressure to draw in and vacuum to reset the diaphram. It halso has timing circuits that disable a circuit (door) if it does not complete the task in 2 or three tries. On top of that they have put in pressure safeties. There are three schools of thought #1 the lock mechanism gets stiff with age, so lubing it thoroughly, and or the microswitches dont give a "close" indication. #2 that there is a minor leak in the system. #3 is that the operating paramaters of the pump are marginal. My first one on my 92 SEL failed in 97 or so, they did nothing except exchange the $600 pump. My second failed in 03, I *****ed up a storm and after checking for leaks they replaced it under "goodwill" they still did not open the doors or replace anything but the pump. It is still working, these pumps are the subject of most forums just search "closing assist", some have tried to tweak them with little or no success.
I dont know about the dealer info you are getting but I would not have replaced the lock before trying a new pump and using a "Mighty Vac" to draw a vacuum in the line to see if it holds. By doing the fuse #9 pull in the trunk and sometimes #3.You are resetting the pump, most people get it to work for a while. I got tired of doing it and went back to MB (its one of those 140 things)
You probably have a few hundred invested in the lock mechanism, a pump is in the $500 area if you get a discount, and there are some available on line.
It is located on top of the drivers rear wheel, and best gotten to by removing the paneling over the back (gas tank) and fender.

Pump info...

http://v12uberalles.com/Closing_Assist_Pump.htm

Yesterday, my passenger door was not closed and so another case of the closing-assist 'gremlin'!

Unlike many that have a bad pump, my problem was the Closing Assist Actuator (CAA). I've written the details on how-to-trouble-shoot on my pages in MENU#28.

The worst case is getting the coupe's door panel removed to get to the CAA and then to R&R it. But it's a cheap part $65 retail and all-in-all, an easy fix.

Since this was the first time it happened to me, I learned alot from the other threads here, from Peter's great explanation in post#2 and finally, from Pete's link in post #3.