Getting to know your closing assist pump: function, tips, and fixes.
Getting to know your closing assist pump: function, tips, and fixes.
I. Introduction
If you’re reading this, you must be having problems with your closing assist system. Probably it doesn’t work at all. You may have experienced the slow door by door failure that many seem to report. One door stops working. Then over the course of a week or so, all the doors and the trunk stop working one by one until eventually nothing works. Before you let a less knowledgeable person convince you that you need a new pump (about $400 right now), allow me to see if I can convince you otherwise.
Before I say more, one of the best things I did was get a subscription to www.alldatadiy.com for my car. It’s a very well-spent $25 for the first year and $14/year thereafter. Alternatively, you can purchase a W140 manual CDROM from various sources for $150-$200. I have both, and I have found that alldata has had everything that I’ve looked at on the CD so far.
I bought my W140 (1997 model S600 sedan) knowing that the closing assist system was not functioning. Some posts on this board got me started in the right direction, but I learned more than I expected as I dug into the problems with my system. I’ve been able to breathe new life into my pump and return my system to 100% reliable operation for almost a year so far. This document compiles my expertise gained from troubleshooting my own system and from helping a couple others on the board do the same over the last few months.
There are two air pumps onboard your W140, the pneumatic system equipment (PSE) pump (also referred to as the central locking pump) and the closing assist pump. The PSE pump performs several functions including locking/unlocking of doors, inflation of lumbar support bladders, operation of reverse antennae on the earlier cars, extension and retraction of the trunk handle, release of the trunk striker eye for auto-closing, and providing vacuum to the vacuum reservoir when the car is not running. This pump is located under the rear seat on the passenger side. The closing assist pump is involved only with the auto-close function of the doors and trunk lid. It is located in the trunk.
II. Accessing the Closing Assist Pump
The closing assist pump is simply crammed into a foam cube tucked between the left rear fender well and the gas tank. You must remove the trunk liner on the left side, and it helps a lot to remove the liner piece at the forward wall of the trunk. It is possible to just bend the corner down to get the pump out, but access is much easier if you remove the whole liner. It only takes removal of a few more of the plastic push rivets. If you have a CD player, you must remove this first in order to remove the left side liner. It’s easy: 2 screws and one electrical connector. If you don’t see the pump at first, follow the bundle of red plastic air lines.
Just pull the pump out. The lines are labelled with initials which, I believe, are short for German. VR = vor rechts = front right door. HR = hinter rechts = rear right door. VL = vor links = front left door. HL = hinter links = rear left door. HD = not sure = trunk. SK = not sure of this one either = pump outlet line, hooked up to nothing.
III. Pump Function
Understanding how the pump functions is central to identifying any problems with your system. The heart of the pump is a sizeable electric motor that directly drives the air pump. What is neat about the design is that the pump can both pull a vacuum or develop pressure on any of the door or trunk lines independently and at the same time. That way only the door or doors that need it get pulled closed. This is accomplished by electric solenoids that switch each line between pressure and vacuum. The solenoids are controlled by the pump electronics which receive signals from microswitches in the door latches and trunk latch telling the pump that a door has partially closed and needs to be pulled shut.
The pump does the closing of the doors and trunk with pressure. It pressurizes the lines causing the extension of pistons in actuators, which in turn move levers in the various latches, which pull the doors or trunk closed. The pump runs until one of two things happens: 1. It reaches a pre-set shut-off pressure (at which point the pump "thinks" it has properly done its job of closing the door or trunk), or 2. It never reaches its pre-set shut-off pressure and then runs for a set amount of time and turns off (that is, it "times out"). After pressurizing the line and doing the auto-closing, the pump then pulls a vacuum on the line, presumably to ensure retraction of the actuator piston. The pistons are spring loaded, but I suppose occasionally, they may not retract fully when the pump releases pressure.
Case 2 is where a lot of problems arise. If the pump times out twice on the same door or trunk line, the pump electronics permanently turn off function for that line. That’s just the way the system is programmed, and it’s usually why just one door stops working. The pump specifically turned off that line because it timed out twice. Back to why the pump timed out… because it couldn’t reach the pre-set shut-off pressure. Why couldn’t it reach its pre-set shut-off pressure? Two reasons: 1. Because an air leak has prevented build up of enough pressure or 2. Because the shut-off pressure is set too high. So the course of action is clear. If you don’t have an obvious air leak, you’ll want to address the shut-off pressure. The pump has a pressure sensing shut-off switch which is adjustable. The following troubleshooting section will show you how to reduce the shut-off pressure on your pump thereby preventing the pump from timing out and shutting down on its own.
This very simple adjustment completely restored reliable function to my system. Hopefully, it can for yours, too, but there are many other failure possibilities, which I’ll discuss in the next section.
IV. Troubleshooting Your System
Note: Be careful when removing the air lines from the pump. I used a screwdriver to pry them off and had no problems, but I know that others have broken off the nozzles, which may force you to buy a new pump. I don’t have any hints for easy removal of these lines.
a. Bringing Your Pump Back to Life
There are two reasons why a door may fail to close. Either your pump is not working, or your pump is not getting a signal telling it that a door has partially closed, i.e. the microswitch is bad or out of adjustment, or the wiring from the switch to the pump is bad. Most likely, it’s the pump that has shut down. First thing to do is to try pulling fuse 9 in the trunk fuse box. Pull it out, wait several seconds and replace it. This fuse is labelled for the closing assist pump. Try a door or the trunk to see if the pump turns on. Some have reported this works to bring their pumps back to life, but this never worked for me. At this point, pull out the pump as shown above and remove the electrical connector on the pump and replace it. Completely cutting power to the pump resets the electronics so that it will again operate all lines until the two time-out thing starts happening again. If you do nothing else, your pump may work for a while, but will again eventually shut down because of the same reasons that caused it to time out in the first place.
If your pump comes back to life, congratulations. You very likely don’t need a new pump. If it doesn’t, I probably can’t help you. Perhaps the electric motor has shorted out. It works hard and gets hot very fast inside the pump housing. Or maybe the brushes have simply worn out. It may be possible to replace the brushes, but I don’t know. If you’re electronically inclined, maybe you can tackle the problem. There is also a printed circuit board inside the pump. Plenty of components can fail there, but I cannot address this.
If you have a door that works intermittently (sometimes a door works and sometimes it doesn’t, seemingly at random), your problem may be a microswitch issue. Sometimes the microswitch gets tripped and sometimes it doesn’t when you close the door. Intermittent function is never a pump issue. Once the pump shuts down a line, it’s off for good (until you completely electrically disconnect it by pulling the connector).
Once you have the pump on your trunk floor, you can observe the two time-out then shut-down behavior of the pump. Disconnect the trunk line (HD) and leave the nozzle open. Now, simulate the closing of the trunk by operating the catch in the lower trunk latch with your thumb. The trunk lights will turn off, the trunk handle will retract, and the HD line will hiss loudly with air pressure from the pump. The pump will run for around 10 seconds and then stop. All seems well. Do it again. All seems well. Do it a third time, and nothing will happen with the pump any longer. Unplug the electrical connector and plug it back in. Simulate trunk closing again. All back to normal.
Now, check your pressure shut off mechanism. Put your thumb firmly over the HD nozzle preventing any leaking, and then simulate trunk closing, the pump will start up, and it should shut off almost immediately. That is because it pressurized the trunk line, and with your thumb over the nozzle, it reached the shut-off pressure very quickly and turned off. You can repeat this indefinitely. As long as the pump reaches its shut-off pressure and doesn’t time out, it operates just fine. If your pump doesn’t shut off when you do this, you’ve got a more serious problem with the pressure shut-off switch.
b. OK, My pump works. Now what?
Leak checking is next. As mentioned earlier, air leaks will cause the pump to time out because a leak may prevent reaching the shut-off pressure. There are a couple of ways to find leaks. You can use a pressure device with a gauge (mityvac or similar) to pressurize the line in question and watch for leak down. There are specific Mercedes factory specs for leak down. I don’t know what they are, but it is on alldata. Alternatively, you can simply operate the door or trunk once you’ve reactivated the pump and listen carefully for the telltale hiss of a leak. The pump really moves a lot of air. I believe that any leak large enough to prevent the pump from reaching the shut-off pressure will be audible. Just listen.
There are myriad opportunities for leaks. Lines can separate inexplicably from connectors. Lines can get pinched and crack. Lines can get chaffed and worn through. The actuators are also a source of leaks. These have rubber o-ring seals that can get hard and crack with age. There may also be leaks internally inside the pump. These may be fixable or maybe not. More on this later.
One boarder (beelootin) contacted me recently when his trunk stopped working followed by everything else. After he found his pump was fine, his mechanic turned down his pump’s shut off pressure per my instructions and also found the trunk line had a leak in it. Apparently the line was routed poorly and was rubbing somewhere every time the trunk opened and closed. They cut out the holed bit of line, rejoined it with a good seal, and his system is now working fine.
c. I’ve rejuvenated my pump by electrically disconnecting then reconnecting it, and I’ve confirmed that there are no major leaks. Now what?
It’s time to evaluate your pump’s pre-set shut-off pressure. You can use any door or the trunk. Partially close the door/trunk and carefully watch the closing action. You’ll hear an audible click when the latch catches, but then the door/trunk will be pulled shut for a period of time after the click of the latch. Finally, the pump will turn off and the door/trunk will relax to its normal fully closed position. First, time how long it takes for the door to audibly latch. This should be about 1-1.5 seconds. If it takes longer to latch the door, your pump is not providing enough pressure. Why? Leaks, or maybe a worn out pump head, or something else (??). If you can’t find a leak, then I probably can’t help you further.
Now, time how long the pump runs after the door or trunk has audibly latched (that is, how long it keeps the door “sucked� closed after it latches). I found my pump ran for almost 10 seconds after the latch had clicked, needing extra time to build up enough pressure to trip the automatic shut-off. This extra pumping time is unnecessary. If your pump takes more than 7-8 seconds to shut off, then you’re a candidate for adjusting the shut-off pressure.
The pump only needs to run as long as it takes to latch the door. The risk of your pump running for too long is that every once in a while, before it reaches its shut-off pressure, it instead reaches the time-out time and charges a time-out to that line. After one more of these, that line is dead. The pressure shut-off is a balancing act. You want to set the shut-off pressure high enough so that enough pressure is developed to latch the door or trunk, but you don't want it set too high so that the pump takes too long to reach the set pressure and times out.
d. Adjusting the shut-off pressure
Pull out the pump and disconnect all the lines. Label them before disconnection if they aren't already. Unscrew 6 screws on the top of the pump and pry off the cover.
Admire the dual manifold design with solenoid switching between pressure and vacuum for each door and trunk line. Kinda cool. The pressure shut-off diaphragm is visible in this pic.
OK, here's the business end. Notice the dual contact switch that is operated by the white piston from the pressure sensing diaphragm. That white piston has a screw in the end of it which can be adjusted to adjust the shut-off pressure. If you screw it out a bit, then the piston will have to move less to trip the switch, therefore the pump will shut off at a lower threshold pressure.
Just turn the screw to adjust it. I just used needle nosed pliers. Finally, here's my original vs. new setting.
Now, your pump will shut down automatically in a lot less time after closing the doors and trunk. You can play with the adjustment a bit so that the pump run time is to your liking. I’d say 3-4 seconds after the latch closes is all that is needed, and you could probably set it for less than that. My closing assist system has run with 100% reliability for almost a year now, after performing the shut-off pressure adjustment. The obvious fringe benefit of this adjustment is that the pump runs for a much shorter time each time it closes a door, so it should also extend the life of the pump’s motor.
e. A case of a bad pressure shut-off diaphragm
This finding convinced me that most closing-assist system failures that W140 owners experience may well be caused by either maladjustment or failure of the pressure shut-off mechanism.
A boarder (posix) posted that his pump had quit running. I responded insisting that he adjust his shut-off pressure. When he dug into his pump, he found that the problem was not that the shut-off pressure was set too high, but that the white piston of the shut-off switch mechanism wasn’t moving at all. So his pump was timing out every time it operated. The piston had cocked to one side and bound in the black case becoming stuck (see pics above). He went farther than I ever had and disassembled the diaphragm (the black housing that the white piston moves in) eventually finding the root cause of the sticking piston. It turned out that the rubber diaphragm that pushes the white piston had developed a tear. The tear was on one side of the diaphragm such that as the pump pressurized, the diaphragm would push one side of the piston only causing the piston to cock sideways in the black housing. Since the pump develops quite a bit of pressure, the piston would get wedged in place tightly once it got started crookedly and become permanently stuck. Posix was able to take apart the diaphragm assembly and use the tip of a latex glove finger to re-seal the diaphragm and make it air tight again. Very ingenious, indeed. He said everything was back to normal, and he had also adjusted his shut-off pressure appropriately in the process.
Here’s a pic from Posix’ post showing the 3 screws (green) and the 3 solder connections (cyan) that need to be removed to take the pressure shut-off diaphragm off the circuit board for disassembly.
V. Conclusion
This is everything I know at this time. Please, let me know if you have anything to add, and I’ll update this with more “case studies.� I hang out regularly as “Brett San Diego� on the W140 section of www.mbnz.org.
Brett