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 Hydraulics and pressure; how long?
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Ruud

Netherlands
66 Posts

Posted - 10/17/2010 :  15:56:53  Show Profile  Reply with Quote
I've done some testing lately in order to try to establish how 'healty' the hydraulic parts of my W100 are - so I know when I'll have to start saving money ;-) After killing the engine I can open and close the drivers window 23 times, which I think is a pretty good score. When I start the engine after this, the system is pressurized almost immediately. Also, so far I didn't find any leaks whatsover. But here is the question; when I park the car after a drive, the hydraulic system slowly depressurizes in 3 or 4 days. I kinda don't like that, I ususally take the car out for a ride once a week but like this it is always a hassle with the trunk - unless I start the car first. So; for how long should the system stay under pressure? And, as in my situation, the pressure in the hydraulics slowly decreases, where does it go?! Where is it leaking from? It certainly doesn't spill any hydraulic fluid in the process...... Any ideas? Thanks, Ruud

Chris Johnson

USA
3751 Posts

Posted - 10/17/2010 :  16:31:28  Show Profile  Reply with Quote
Hello Ruud,

In short, the system should maintain pressure indefinitely.

The fact that the system allows you to operate a window 23 times before the pressure falls off indicates that the hydraulic accumulator is working well.

Leaks in the system can be both external as well as internal. Considering the amount of fluid that the accumulator can hold, if you are not having to frequently add fluid, it indicates an internal leak. This is the type of leak that allows fluid to flow past a seal, but the low pressure side of the seal has a return to the fluid reservor rather than to out of the system. This is the most difficult type of leak to find, but in nearly every case this leak will be in a switch, not an actuator.

To narrow down things as far as possible before opening the tool box, do you notice anything in a different position after a few days than the position you left it in? Such as windows higher or lower, seat position, summer air flap, etc? Are there any other clues such as windows or seats, etc., that seem to move slowly while you are driving the car? In short, is there anything that you have noted that is not as you would expect?

Most typical switch leaks will not cause all the pressure in the system to dissipate completely, but rather cause the related actuator to move to one extreme end of its travel length. Once the actuator reaches its travel limit there is no pressure on that side of the seal so the leak effectively goes away. In this case where the system is totally losing pressure, it implies that there are at least two leaks: one from the high pressure supply to a device and another from a device to the fluid return circuit. I would have to study the hydraulic schematic to see if I could find any case where a single leak could fully bleed off pressure, but I cannot think of one at this time.

Regardless, if there is an internal leak, it can be confirmed by the slow transfer of fluid back to the resevoir. Remove the lid from the reservoir and look at the return port in the side. Also, look at the current level of fluid in the reservoir. Without changing anything over a few days, keep track of the level of fluid in the reservoir and if there is any detectable flow of fluid from the side port. Obvious flow of fluid in from the side port allows you to immediately confirm an internal system leak, but even if the leak is very small you will still note the rising level of fluid in the reservoir.

Here's the part you will love. If there is no obvious movement of a window or seat to offer a clue where to start, the ONLY clue you have is fluid flow back to the reservoir.

Let's assume that you do detect flow back into the reservoir. You can only try to eliminate large sections of the system to see if the parts of the system still functional are allowing fluid to continue to flow back to the reservoir. For example, you can disconnect the high pressure line to the driver's door and plug it. If the return flow to the reservoir stops, you can safely assume that the leak is somewhere in the part of the system that is disconnected. Once that is known, you can break that part of the system into smaller chunks, ultimately zeroing in on the faulty component.

This process requires lots of patience and a knowledge of the entire system, along with the proper parts to plug off parts of the system (included in the hydraulic spares kit).

Do you have the 600 service book, or at least a schematic of the hydraulic system?

Chris Johnson
If you aren't constantly impressed with your car, then it needs fixing.
100.012-12-000790
100.012-12-000867
www.300SE.org
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Chris Johnson

USA
3751 Posts

Posted - 10/17/2010 :  16:38:33  Show Profile  Reply with Quote
I generally think in terms of failures, but there is another possibility too.

Most of the switch levers are seriously worn after all these years. In an effort to put a "patch" on a worn lever, somebody will adjust the lever adjustment screws to eliminate as much of the slop in the lever as possible. If these screws are set only slightly too tight, it can prevent both valves in the switch from seating properly and you will have a case where both the inlet and outlet valves are open (slightly) simultaneously. Now you have the rare case of two faults in the same switch, and it can cause the exact problem you are having.

Has anybody been adjusting switch levers? There ALWAYS has to be a small amount of free play in the lever movement where neither valve is open.

Chris Johnson
If you aren't constantly impressed with your car, then it needs fixing.
100.012-12-000790
100.012-12-000867
www.300SE.org

Edited by - Chris Johnson on 10/17/2010 16:40:23
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Art Love

Australia
6139 Posts

Posted - 10/17/2010 :  17:13:09  Show Profile  Reply with Quote
I agree with Chris, you must have an internal leak. It must be quite substantial to depressurize the system in 4 days. In #178, I have had internal leaks in the front seat adjusters for as long as I have had the car, 22 years, and the front passenger seat will creep forwards over time and the rear of the driver's seat slowly lie back, but the system does not depressurize at anything like the rate you report and in fact still maintains pressure after several weeks and often much longer. I suspect it is going to be difficult to find if nothing is obvious.
Art
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Ruud

Netherlands
66 Posts

Posted - 10/17/2010 :  17:19:19  Show Profile  Reply with Quote
Hi Chris, thanks for the elaborate respons. I must say, really clearifying. When I just got the car, I noticed that every time I left the car for a couple of days, the drivers chair was in the most forward position when I got back... kind of strange, because I am almost 6 ft 7, en therefore always adjust the seat as far backward as I can! The strange thing is that the seat hasn't been doing this for the last couple of months... thus something must have changed.... But obviously, the seat won't have repaired itself, so there must be something wrong there. Not being familiar with the hydraulics, but for the rest pretty good with tools, how should I start handling this? thanks!
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Ruud

Netherlands
66 Posts

Posted - 10/17/2010 :  17:26:16  Show Profile  Reply with Quote
@Chris: I will check all the switch levers for some free play
@ Art: didn't read your respons before writing my own. But indeed, my front seat does exactly the same, but back doesn't move. I am most certain that the only 'auto-moving-part' is the front seat that creeps forward. Could that leak be so big that the system depressurizes after 4 days?
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Art Love

Australia
6139 Posts

Posted - 10/17/2010 :  17:57:07  Show Profile  Reply with Quote
Ruud,
All I can say is mine doesn't, but if the fluid is bypassing the seal quickly enough, then I guess it could. Does the seat still respond to the control lever now the seat has stopped creeping? At least with this fault, you have somewhere to start looking.
Art
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Chris Johnson

USA
3751 Posts

Posted - 10/17/2010 :  18:18:55  Show Profile  Reply with Quote
Hi Ruud,

I am firmly of the opinion that there must be two related leaks. The seat sliding forward means that the switch that controls that function has an internal leak in the valve to the high pressure side. That, by itself, will only consume pressure until the actuator below the seat is fully extended. At that point the pressure in the actuator will match that in the supply circuit and no more fluid will flow through the faulty inlet valve and the prssure in the supply system will no longer drop.

If a new leak develops in the outlet valve in the same switch, THEN you have a complete path between the supply side and the return side and the entire system will eventually lose pressure. How long this takes depends on the magnitude of the smaller leak, but it could conceivably occur over a fairly short period of time, i.e. minutes, or even less. The fact that it takes days indicates that the leak is very small, and also means that the leak will be harder to find.

However, since it would require two leaks for all this to happen, and you already know that one of the seat switches has at least one leak, that is where I would start. If you slide the seat all the way forward and then remove the carpet in the rear footwell behind the affected front seat you will see two metal lines and fittings that connect to two flexible lines under the seat. There are small "U" clips in the end of the fittings that lock the flexible lines in place.

Only after COMPLETE discharge of the pressure in the system (operate a window 30 times), the U clips can be removed and the flexible lines pulled out. Secure these lines in a bucket somehow so that any fluid dripping from them stays contained. Then take two of the plug pins from the hydraulic emergency kit and push them into the two open fittings and reinstall the U clips. Start the engine to repressurize the system and see what a few days brings. Hopefully the system will still have full pressure.

Note that the seat's movement will not be locked with the source and return lines disconnected! Inadvertently sitting in the seat in this state can cause the seat to move and possibly pump fluid out either of the two open lines, so be sure to secure these lines into a container to catch any fluid that does emerge. Note that the car is not safe to drive in this state since the seat's position is not locked in place.

Chris Johnson
If you aren't constantly impressed with your car, then it needs fixing.
100.012-12-000790
100.012-12-000867
www.300SE.org
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Ruud

Netherlands
66 Posts

Posted - 10/18/2010 :  05:06:22  Show Profile  Reply with Quote
Chris, I completely agree with your point of view. Just the sliding of the seat will depressurize the system a little. So there must be another leak that continuously looses pressure. I am not completely familiar with how the hydraulics work (not yet, but I am afraid this will change soon) but the first suspect in my opinion is indeed the outlet valve of the same switch. I do have the shop manual and look it up, together with your instructions. What I don't have, unfortunately, is the repair kit. Are these plug pins still available somewhere?

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Art Love

Australia
6139 Posts

Posted - 10/18/2010 :  08:21:13  Show Profile  Reply with Quote
Ruud,
Have a look in your boot (trunk). In the front wall there is a cupboard to the left of centre about 20cmx15cm in which there should be a "plastic" box. The cupboard has a woven strap that goes over the box that you pull to pull the box out. Inside the box are the parts that Chris is talking about. If the box is not there, then you have a problem. If the box is there, you should not have a problem.
Art
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paul-NL

Netherlands
4194 Posts

Posted - 10/18/2010 :  09:07:26  Show Profile  Reply with Quote
Art,
I believe some carowners did not order the set, because not all cars seems to have it:



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Ruud

Netherlands
66 Posts

Posted - 10/18/2010 :  16:39:37  Show Profile  Reply with Quote
@Art: I know where it should be, but here it comes (and please don't laugh) somehow I haven't been able to open cupboard, it seems to be really stuck! I never tried really hard because I was afraid to tear something apart. But now it becomes a bit more urgent, so I am going to try harder..... Hope to find something in there!

@Paul: that set looks nice!
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Art Love

Australia
6139 Posts

Posted - 10/18/2010 :  17:45:20  Show Profile  Reply with Quote
Ruud,
What you are looking at in the cupboard is the kit. It has a Mercedes Star embossed into the lid. There should be a braided strap that goes under it and appears at the bottom. If the strap is there, pull on it and it will lift the box out. If the strap has been left in behind the box, then you will have to try to prize he box out carefully. There is an edge (lip) about 2-3cm high across the bottom of the cupboard, but none at the sides or top, so you are best to try to pry it from the top. Here is the only photo I have showing the kit box in the cupboard and you hopefully can see the strap and the embossing on the lid of the box.





Art
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Ruud

Netherlands
66 Posts

Posted - 10/18/2010 :  18:05:13  Show Profile  Reply with Quote
Hi Chris, I've been reading the service manual, and found the following: If the drivers seat moves FORWARD, it should be diagnosed as a defective hydraulic ELEMENT. The sealing ring of the working piston is defective. If the drivers seat moves BACKWARD, they suspect the hydraulic SWITCH for double acting hydraulic element. But still, there is the oil leaking to the reservoir.. I think this can only be the case if one of the valves of the switch is leaking, don't you think? So should I look for 1] the Element (a broken seal makes the seat move forard) and 2] ALSO the switch for a leaking valve? It doesn't look like a faulty element alone can result in a total depressurization, or am I wrong here? What do you think?
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Ruud

Netherlands
66 Posts

Posted - 10/18/2010 :  18:13:31  Show Profile  Reply with Quote
Hi Art, in my car things look exactly the same as in your picture, including the Merdedes star and the strap eppearing at the bottom. The star is actually on the lid of the box? So this means I have the kit in there?! Didn't kown this, I thought the strap was for opening a little 'door' or something... I feel a bit stupid. But I did pull the strap quite fiercefully before, and nothing happened. That's why I quit, not wanting to break something. So I better pull a bit upward, isn't it?
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Art Love

Australia
6139 Posts

Posted - 10/18/2010 :  18:25:55  Show Profile  Reply with Quote
Ruud,
Yes, and I am wrong in the detail of my description of the cupboard in that it is only at the sides that there is no lip. At the top, the cupboard is open so that when you lift on the strap which is attached at its other end to the "back" wall (front in direction of travel) of the cupboard, it lifts the kit box upwards into that space until the bottom edge of the box is above the bottom lip of the cupboard after which it comes out of the cupboard. So you lift the strap upwards to lift the box out of the cupboard. Hopefully your box will look the same inside as Paul's.
Art
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