So I played around with my closing assist pump this evening.
I bought the car with the seller informing me that the close assist system wasn't functioning. (And it wasn't when I got it.) I pulled out the pump. Funny that it's just crammed into a block of spongy foam in a niche behind the seats. Opened and closed doors. Nothing from the pump. Clicked the trunk latch closed by hand, no sound from the pump. I've read here that pulling fuse 9 in the trunk, which is clearly labelled Vacuum Close Assist, and replacing it can sometimes reactivate the pump. I got nothing. Disconnected the electrical switch and all the hoses, and took the cover off the pump just to see how it worked. It's got two manifolds running off opposite sides of the pump, one for vacuum and one for pressure, and a bank of solenoid switches that switches each element (each door and the trunk) between vacuum and pressure. The system should be called a pressure assist closing system because the closing assistance comes through pressure not vacuum. A short vacuum is run after each application of pressure, I believe only to ensure full retraction of the pressure activated pistons that do the actual work in the locking mechanisms. But I get ahead of myself. So I put the electrical connector back on the pump, and voila, it works! Every door, but not the trunk. Well, I found someone had messed with the trunk closer lines disconnecting and plugging them, so it was no wonder it didn't work. So I reconnect the line and the mechanism worked for a bit, then stopped. Pulled the fuse again and replaced, nothing. Pulled the connector to the pump and replaced; the trunk worked again... twice. Repeated again. The trunk worked twice and stopped. This was completely reproducible 5 or 6 times in a row. By stopping I mean the pump never turned on again for the trunk after two completely normal operations. Two and out every time. Strange. The doors continued to function normally (well, almost. At one point, the driver's side rear stopped functioning but was restored after disconnecting and reconnecting the electrical.)
Anyone else noticed such regular malfunction of the pump? I know it has been said that pulling the fuse is only a temporary fix. I find it bizarre that it's temporary for EXACTLY two operations of the trunk, while the doors are continuing to function. We'll see how long they go for. Is this the electrical issue that that one guy fixes in his rebuilds (Can't remember his name now. I've got his compuserve email written somewhere.) Before I send an email, does anyone know whether he still provides the rebuilds?
Hope you learned a little something if you made it through all this rambling. Why would it work twice, then never again? This is really driving me nuts!!!
RE: So I played around with my closing assist pump this evening.
this is quite clear - the pistons ont he assis mechanism in the trunk are dead / leaking.
the pump looks at how long it takes to achieve a certain pressure - if its too long it cuts off that circuit. it does it after the second time cos it needs to be sure that its not a one off occourance. the reason the lines were plugged was to fool the pump into thinking the mech was still air tight.
replace trunk pull close mechanism
parts :-
220 800 05 75 - 'element' - i assumer this is the cylinder.. all from a 1995 S 500 (orig part number was 140 800 20 75) - 43 dollars roughly.
202 800 20 75 - pawl release (i asssume somthing to do with unlocking)
202 800 03 22 distributor (in the trunk lid - i assume some sort of valve block)
The trunk mechanism works. I manually unlatched the striker from the manual close setting and hooked up the close assist pressure line then closed the trunk latch with my thumb and watched. The pump turned on, and the piston fully extended and retracted the striker as it was supposed to do.
But, here's the catch. The rubber joiner that connects the trunk latch piston to the pressure line from the pump made a poor seal, and it leaks. The lines even blew apart under pressure from the pump. The inside of the joiner is damaged from a screw being inserted in it to try to seal it off (idiot p.o.). So, if you're right and there is a logic circuit programmed into the pump control that allows only two "time-outs" then shuts down that function, then that would make sense. And, then problem is the poor seal and not the piston. I'm going to try plugging the line cleanly to see if the trunk line will work more than twice.
Come to think of it, I think you are right. When the trunk line stopped working, I connected the driver's side rear door pump output to the trunk line, opened and closed the rear door and observed the trunk mechanism working again. I think the rear door pump output worked twice and also quit.
So, if one is observing the two-times-and-out behavior, the pump essentially never needs to be replaced. You just have to track down a leak somewhere that prevents the shut-off pressure from being achieved. Come to think of it, I noticed what appeared to be a shut-down mechanism on the pump when I took it apart. It had an adjustable electrical switch contact attached to what I now think was a pressure diaphragm. Looks like the shut-off pressure can be adjusted by turning a screw. I'm gonna have to play with this. If you can lower the shut-off pressure, maybe you can get away with minor leaks.
RE: So I played around with my closing assist pump this evening.
Hi Brett,
Thanks for your very informative information. By any chance, do you have any pics of the procedure you went through? I also have a malfunctioning pump, but I'm not exactly sure where to locate the pump and/or lines.
RE: So I played around with my closing assist pump this evening.
I have same problem with close assist that I experienced in both 500 and 600 124 series.
Always is behaved to the "2 failures and out" rule, that is by disconnecting fuse 9 in the trunk the system re-set and the door assist worked fine twice then failed.
In my case it is only at one door (on the 500 the passenger on the 600 the driver's door.
If this failure is due to detection of a fault in that particular door channel is this due to a leak on that channel or is it due to "lack of lube" or similar in the actuator. I would welcome any advice
A NU BEE
If Guy is right (and I think he is), then the failure is a result of an air leak.
If the pump doesn't reach it's max pressure within a set time period, it times out. If it times out twice, the microprocessor cuts service to that door or the trunk permanently.
I don't think there is a mechanism for the pump to stop working if the problem is that the actuator piston doesn't extend properly and do it's job. Presumably in this situation, the pressure would be fine, and so the pump would think that everything is fine.
RE: So I played around with my closing assist pump this evening.
Guy, Brett Thanks for your observations in this area. Now, to leak-test the line that is causing the problem ?? Does anyone know if MB have released a workshop Manual for 140 series in any format yet? I had a beautiful set of Official Manuals for the 126 S that made life much easier than is currently the case on the 140.
RE: So I played around with my closing assist pump this evening.
I hear the CD is in the works, but that also has been the story for the last few months. Soon was the last word!
The nice manuals went away in 92, promises ever since, I did get an Intro-to Service which shed a lot of lighr on the components that were introduced in 92 but very little detail. I also have had a complete set for every MB I have ever owned
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