need help with 300td hydraulic suspension
 

i know that there are tons of threads on this topic but none of them I could find address the symptoms mine is showing. The ride feels great, its not bouncy bouncy and its not stiff feeling, it never bottoms out like i said it feels good when driving. if from park i put in reverse with my foot on the brake i can see it raise up, if i shift into netural it goes down a little and if if shift into drive it goes down a little more, so it is working somewhat. the thing is that if i have the car idling and the tailgate up and I have my buddy sit in the back it goes way down and does not raise up, it is also saggy looking when parked and i imagine when driving also, not way too low but noticably lower in the rear, what do you guys think ? plenty of fluid, no leaks.

Is it working?
 

>>so it is working somewhat...

The raising and lowering is simply the effect of torque reaction through the suspension - I suspect it demonstrates nothing about the operation of the SLS.

The most direct check would be to disconnect the link to the height corrector valve, and work the valve with the engine running, and see if the car goes up and down. Be careful under the car if you do disconnect the link, or do any suspension work - make sure the car can't come down low enough to hurt you.

Does it raise your buddy up if you raise the rpm's? Mine comes up with rpm's above idle.

As stated above, some up and down movement of the rear suspension when shifting between drive-neutral-reverse is typical of these cars. However, if the back end drops when weight is added and won't rise to the proper level even with revving the engine, the problem can lie with the positioning valve (leaking, worn or stuck), the positioning valve arm (loose on valve), the linkage that connects the valve to the suspension (broken heim joints, misadjusted), or the arm connected to the rear sway bar (loose where it attaches to the sway bar).

Check those components and report back.

will do, thanks

ok, everything is tight. if i seperate the linkage and operate the lever it goes up, sometimes, sometimes i push it up and the vave makes a noise like running water but it does not go up i have to take it back down and then push up again and it will go up. i have to move it at least 1/2 to 5/8 of an inch upward from where all the linkage is set now to get it to rise at all. with eveything being tight seems like that would be an odd way to have set it on purpose. i kind of suspect the valve may be funky. or should i just adjust it to where it looks right just sitting there - equal space from top of tire to bottom of fender as with the front end - also seems like if i do that then it will have the lever in the right place to have it rise more if load is added. what do you guys who know stuff think.

thanks again

Somewhere in a thread on the forum are the exact measurements from ground to wheel well for the proper setting.
I've never played with my valve but something does sound funky with yours.

well i went out there and adjusted the linkage, i had to adjust nbboth ends to their maximum making it the shortest possible bar it could be, tightened everything up. started it up and it came up and then settled a little bit, the neighboor lady was walking by and i had her sit in the back and it came back up - neat-o, but i still think somethings wrong having to adjust it so much and it still looks saggy in the rear, cant see top of tire. but it does seem to raise up to that level when loaded now ??? again i defer to those who know more.

now the ride seems a little bouncy. not bouncy like with bad shocks or balls which i have ridden in cars as such and that seemed dangerous to operate, but a little bouncy more so than before for sure.

Mine did the same thing.
 

The ride was fine, but it sagged overnight. After rebuilding valve body, the ride was extremely bouncy. I replaced the spheres and everything is great now. It is, by far, the smoothest ride I've ever owned. Rebuilding the valve body is pretty easy - take digital pictures of every little move. The hardest part was threading the lines back into the valve body. Be careful those aluminum threads strip really easy! On the adjustment arm there is a hole that when positioned parallel to ground a small screwdriver or piece of heavy guage wire can be put through it and align with a small indention (hole) in the valve body. This is the position that the arm should be in when everything is adjusted correctly. Good luck.

Brad,
Can you post your pix? I've got a similar problem with my TDT.
Mike

Where did you get the seals to replace those inside the valve body? My MB dealer has tried unsuccessfully to get them. A few members emailed me asking for help on this and all I could say is not available. Mine drops down a little overnight but after a few days its sitting pretty low but not on the grond so I think its just on its way out, I would like to be able to rebuild it so any ideas on finding suitable seals would be appreciated. Maybe Jadavis could hlep us, hes good at finding Viton O rings to replace the OE ones.

Somebody will post the location of the rebuild kit, as it does in fact exist.
On my current wagon and my long departed smoker, the linkage to the valve assembly was almost as short as you could make them.
Usually, if bouncy, it is the accumulators. Usually if it is very soft and sagging badly it is the accumulators.
If it leaks down overnight the valve assembly needs a kit. Struts (shocks) only seem to go bad by leaking. Make sure the plastic "Z" looking arms behind rear wheel hubs aren't broken. (sway bar links) That is how the valve gets it's info about ride height. ;)
When the car moves up, it does so very slowly, sometimes there may be a slight delay.
ALWAYS have the back of wagon on ramps if you are manipulating the linkage arm. The back end goes UP slowly, but comes DOWN fast! :eek:

Very very very rarely is the linkage the problem, and almost always, when you adjust the linkage, you CAUSE problems (like control valve failure).

The correct height (when all the rear suspension rubber is in good condition) is 13.75 inches plus or minus 0.5 inches, measured from the very center of the rear wheel (where the point of the star in the little hub cover is) straight up to the fender lip; hold a measure tape tight between these points, and where the tape touches the outer curve of the fender lip is where you should read 13.75 plus or minus 0.5 inches.

It sounds to me like either your control valve is failing or your hydraulic pump is weak, or your rear torsion rod (runs between rear wheels and is what the other end of the control linkage is attached to) has come loose from a mooring or two, and is not providing the correct twisting signal to the control valve.

I highly recommend you set the rod back to where it was (or as close as you can) and read up on procedure 32-501 in the MB service manual, which provides detailed info on how the system works. Very few mechanics understand it or are willing to spend the time learning, so you probably will be on your own (with help here of course) in solving the problem.

If the original rod setting is lost, try this: with engine running, disconnect rod and move the lever to the neutral position (stick a pin through the hole in the arm to the matching hole in valve body). Measure the wheel-to-fender height as described above to verify it is correct. Lengthen the arm as needed and connect. This is a very rough setting that will get you in the ball park. Note: if the ride height is incorrect with the arm held in neutral position and with engine running, this is a clear indication that you've got deeper problems that need to be fixed; adjusting the control rod to get a height increase is simply a bandaid that will almost certainly lead to control valve failure.

Here is a good post by David Bruckman:

From: David Bruckmann <dbruckmann@canadianliving.com>
Subject: FYI people with self-leveling rear suspension (long)

Greetings all,

A little while ago I spent a day sorting out my self-leveling rear suspension on the wagon, and thought I'd share a few details with the list (both lists, since this applies to gasoline versions as well). The car is a 1979 W123 300TD OM617.912. I gather the same principles apply to W124 and W126 models.

The problems were:
1. The spheres (MB calls them "pressure reservoirs") were shot, so the suspension was very hard.
2. Both before and after replacing the spheres, the rear suspension sagged within a few minutes of shutdown, or even while running after highway driving on warm days. The problem was an internal leak in the self-leveling valve, which maintains a base pressure at all times even without cargo. If your car droops it is NOT the rear springs, it's the valve failing to hold the base pressure. The steel springs are NOT meant to hold the car at driving height without some hydraulic assistance, so the car SHOULD droop if there's insufficient pressure in the hydraulic system.

As a preliminary note, machined aluminum body of the self-leveling valve was severely corroded where it came into contact with the steel bracket that is used to attach it to the car. It looked like an aspirin that had been dropped into water, presumably because some salt or other corrosive matter had nestled itself between the bracket and the valve body. The corrosion was so severe that it bent the steel bracket, deforming it by nearly a 1cm. It may be worth taking a look at that valve if you have a car with self-leveling and there's salt on your roads.

Replacing the spheres was very easy. You WILL need a proper brake/flare line-fitting wrench to disconnect the hydraulic lines
(11mm on the spheres); they are very tight and are easily rounded with a normal open-ended wrench. It is important to clean the entire area around the lines and fittings before beginning. I used brake cleaner, but wear eye protection and make sure there's lots of air moving or you'll get sick (ask me how I know). A lot of crud may be sitting on top of the spheres and brackets, and this will rain down into the hydraulic lines if you do not take precautions in advance.
It's a good idea to use plastic bags and elastic bands or some other method to cover the line ends.

There is a (small, expensive) repair kit for the self-leveling valve assembly, (MB part # 000-586-00-32, about $100CAD from the dealer), which includes a new valve piston, springs, ball bearings, o-rings and copper sealing rings. Give your VIN when ordering the kit, as there are different versions of the valve even within the W123 series.

The removal of the valve is very easy, but again it is critical to have proper line fitting wrenches to remove the hydraulic lines. I believe the sizes on the valve are 11mm for lines to the spheres, and 13mm for the pressure input line. Disassembling the valve is also quite straightforward (there are only about five pieces; just be meticulous about remembering where things came from and their relative configuration. Having the MB manual drawings of the valve assembly is very useful in ensuring that everything goes together properly. Take particular note of the relationship of the regulating disc to the attached control arm, which must be detached to permit disassembly and cleaning. There are two ball bearings that act as check valves (included in the repair kit). These will fall out upon disassembly. A third ball-bearing valve is located in the overpressure relief valve, which cannot be disassembled, and thus remains fixed to one of the assembly-halves.

I used varsol to clean the outside of the assembly BEFORE dismantling, then fresh varsol to clean all the pieces once disassembled. A toothbrush seems to work reasonably well as a cleaning tool. Final rinsing was done with brake cleaner, which leaves no residue but is pretty toxic; so make sure you have ventilation. I bathed components in hydraulic fluid after drying and then re-assembled. There's a note in the MB manual about not tightening the two halves of the valve until it is under pressure
(There’s a large o-ring that seals the two sides). I basically ignored that, since it's not possible to get a wrench into the space to tighten things after the fittings have been attached. No leaks so far, and the valve works perfectly.

Be VERY careful upon reassembly that you do NOT cross-thread the hydraulic fittings. If there's any resistance, do not force the connection. These are straight-threaded connections, and so must be perfectly aligned or you will strip the threads. Given that the fittings are steel and the valve body is aluminum, the expensive-to-replace valve assembly will lose to the fittings. I found it necessary on one line to disconnect it at the sphere end, fit it into the valve assembly, and then re-attach at the sphere end.
The spheres are relatively easy to move and angle to facilitate the introduction of the fitting, whereas the valve is more cumbersome/ rigid.

While you're at it you should drain, remove, and clean the reservoir and replace the reservoir filter (MB P/N 002-184-55-01 for W123 models, about $10 from the dealer). Note that the filter is in the
RETURN stream of the system (shoot THAT engineer), so any crap in the reservoir can flow unrestricted into the pump and system. I'm going to install a filter on the pump-input side; anyone who has done this and can suggest a filter p/n etc: I would appreciate the info.

Once everything is hooked up again, you should be able to start the car and watch it rise to normal operating height EVEN IF the actuating lever on the valve is set to release the pressure in the system. I left the actuating lever disconnected initially so that I could verify the function of the valve etc. The car should maintain a basic height/pressure with the lever in the neutral or "release" positions. Moving the lever to the "fill" position should result in an even higher ride (unless a corresponding weight has been added to the car).