K6JRF at the mic K6JRF's Page
formerly W6FZC

My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
(Updated: Mar 1, 2013)

Analyze and Troubleshoot "Check Engine" MIL and Electronic Control Units (ECU)!

Fix a Door Closing Assist Problem!
Yesterday morning, I noticed that the passenger door was not fully closed. When I tried, it would close if I gave it a good 'push' but the closing assist function was not working. It's been cold here, around 34F in the early morning, so maybe that had something to do with why it's not operating. At least, that's what I hoped but that spell quickly passed and I realized something must have died so it's time to fix it!

How does Closing Assist Work?
MB Vacuum Closing Assist Pump There are two air pumps in the C140 (and W140), the Pneumatic System Equipment (PSE) pump (also called the central locking pump) and the Closing Assist (CA) pump.

The PSE pump does the locking and unlocking of doors, inflation of lumbar support bladders, operation of two (2) 'reverse' antennas on some of the early W140 cars (my car has this feature), extension and retraction of the trunk handle, release of the trunk striker 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 CA pump is responsible for performing the auto-close of the doors and trunk lid. It is located in the trunk on the drivers side tucked next to the gas tank.   The picture shows the various outputs and for my car (coupe), the "VL" and "VR" are the only two outputs that we need to trouble-shoot (T/S) and find what is wrong.

Victor from "RestoreYourMercedes.com" has recently added a video tutorial on the repair of CA pump and how to regulate the cutoff switch. I highly recommend this video here.

T/S the Problem
After reading a number of 'closing assist' threads, some people had success by simply pulling fuse #9 located in the trunk on the fender wall. Opening it and removing #9 and #3 did nothing to fix my problem. In other threads, many had success by removing the pump, pulling the power connector out for a few seconds and then reseat. Tried this and it also did nothing to solve my problem. So much for the 'easy' stuff!

Time for another approach. After consulting with my friend and tech, Pat at Exclusive Motors, a test-method was worked out. First we'll check out the CA pump's operation. If ok, then check the vacuum actuator and lines to the door. Here's how we did this.

The "pump" is located in the trunk on the left side in the foam-padded case next to the gas tank. There was no need to remove the trunk lining since the pump's foam-padded case had ready access once the side-covering is removed. The pump's PN is 140 800 18 48.

Finding the "Real" problem!
The pump does the closing of the doors and trunk with pressure, not by vacuum. For the door in question, it pressurizes the 'VR' (see CA Pump picture) line which pushes the piston in the CAA and causes the door to close. The pump usually runs until it reaches a preset shutoff pressure. If it doesn't reach the shutoff pressure, it runs for a set amount of time, turns off and 'times-out'. After pressurizing the line and doing the auto-closing, the pump then pulls a vacuum on the line to ensure retraction of the actuator piston. The door piston in the CAA is spring loaded and it retracts when the pump releases the pressure cycle.

Door Actuator - Vacuum Element 1)  To T/S, pull the vacuum line going to the right front door labelled "VR". Use a 'fork' tool that is used to 'pop' the plastic door fasteners used in the door panels. It works perfectly for this application since it applies equal pressure to each side of the line cap. Once the VR line is removed, activate the pump by closing the door. As the pump starts to produce air at medium pressure you can use your finger with a moderate amount of pressure to stop it lifting your finger. If the pump stops, then that means it has reached its shutoff pressure so the pump is probably ok! If it doesn't stop, then the pump is probably leaking internally so that may mean a new pump is in your future. Check out the link in the last paragraph before you junk it since it may only need an adjustment. My pump was fine and shutoff quickly so it appears to be working perfectly.

2)  At this point, Pat was convinced that it was the CAA but not me! It's still possible that there's a 'bad' valve in pump so I still need to positively determine whether it's the pump or the door actuator. I used a shop pump to pressurize the "VR" line going to the passenger door. Immediately a loud 'squeal' was heard coming from the door's leaking diaphragm. Case closed! Time for a new CAA, 140 800 19 75. Price is about $65 retail. Cheap enough. A new pump is $600+ retail.

How does the CAA Work?
CAA opened up Not being able to resist the temptation to see what's inside the CAA and to see how it works, I opened it but with caution! Why? If you pull on the plunger end, you'll see that the spring is under big-time tension! So that means that it will 'explode' when the tabs are pushed in. So I carefully cut the push-tabs using my B&W moto-tool and the CAA literally split-apart because of the spring tension.

Upon inspection, I thought that I'd see a tear in the silicon diaphragm but it was perfect. Couldn't see any defects anyplace in the diaphragm although the bottom section isn't removable so it could be there. Inspecting the base where the vacuum line (VR) attaches, showed nothing out of order and no cracks. However, it squeals loudly with air pressure so it must be escaping around the diaphragm/cover interface. For a 12+ year old part, everything looks to be in almost new condition!
Remove & Replace the CAA
Right Door Details of CAA Removal The first task that you need to do is to remove the door-backing material. Suggest you take your time using an 'Xacto' knife slowly cutting the rubber cement from the door without tearing the lining. This takes some time but it's worth it since the sound deadening properties are maintained. When done with the repair, use some contact cement to reseat the door backing.

The the actual 'fix' is not for a non-knowing-person! As you can see from the picture (that's Pat's arm!), getting the leaking CAA out is something of a puzzle since it buried inside the door panel near the door-latch and to make matters worse, it's behind the door-lock actuator. So it has to come out first so that you can remove the CAA. Since there's not a lot of openings to see what's going on you do this by "feel". Pat has done a lot of these so it was replaced quickly but for me it would have been an all-day-picnic-without-any-beer!
The comments here are from Pat's experience: If the other doors are operating ok, then it's NOT the pump, the problem is mostly the CAA (cracked diaphragm) and secondarily, a vacuum line that breaks and/or cracks. He says the vacuum lines in this car are very rugged. I inspected my 12+ year old lines are they are pliable and look as good as new, so I must agree.

The technique of using low/medium pressure device serves to quickly diagnose whether it's the CF-pump or vacuum line/CAA problem. This is handy when you don't have another Closing Assist pump to swap out as is the case with most DIYers.

If you are still having difficulties, then it's probably a worn pump. If so, for more information and details including some neat adjustments, check out Victor's Mercedes Repair page here .
Send me Email Icon with your comments