300td 1897 self levelling control valve - PeachParts Mercedes-Benz Forum

plr986 · #1 08-23-2007, 12:11 AM

Hi Guys,

I am having a problem with my sls on my 300td. I notice that it has become a bit bouncey from the rear. I have replaced the aircells and when we were bleeding the system we noticed that when I control the height with the level control valve the car would not go lower but it would go higher. I was wondering if the problem here is the levelling control valve? I hope to hear from your expert opinions. Thanks.

Biodiesel300TD · #2 08-23-2007, 12:21 AM

A wagon from 1897!

I don't have a solid answer for you, but my inclination would tell me the leveling valve shouldn't lower the rear. It is built to lift the rear end up when the car is loaded. Usually the saggy rear end is a leaky system, and/or bad accumulators.

kerry · #3 08-23-2007, 12:24 AM

It seems to me that the valve should also bring it down. The additional pressure necessary to hold up a heavy load would need to be bled off once the load was removed or the car would be too high?

Biodiesel300TD · #4 08-23-2007, 12:31 AM

But it should only bring it back down to normal ride height, shouldn't it?

plr986 will the valve bring the level back down to normal height after lifting the rear? If it dosn't do this then something is up with the valve.

JimmyL · #5 08-23-2007, 12:39 AM

The valve will cause the fluid to be pumped in such a way that it will either raise or lower the back of the wagon. That is why that link is adjustable, so you can set the height higher or lower.
If you replaced the accumulators and you are still bouncy, then I would think the valve would be likely.
For W123's I know there is a valve rebuild kit. Not sure about W124's....

plr986 · #6 08-23-2007, 03:07 AM

Yes the levelling control valve would bring up the car and lower the car to its normal ride height. But it cannot go lower than that. I have seen other s124's that when we control the levelling control valve it can bring the car lower that its original ride height. Another problem of my car is that when I have heavy load at the luggage compartment the rear does not raise up to the specific ride height. If it does this what do you think the porblem is?

JimmyL · #7 08-23-2007, 07:03 AM

Quote:

Originally Posted by plr986

If it does this what do you think the porblem is?

When mine did that it was the accumulators. Since you replaced those, that would seem to rule them out. I will have to let others answer, as I have no first hand experience with your exact problem.
What was the wagon acting like prior to replacing the accumulators? What did replacing them change, if anything?

Douglas.Sherida · #8 08-23-2007, 08:23 AM

Warning: I am assuming that the w124 SLS system is functionally identical to the w123 system. (Having looked at pictures of the SLS valve it looks identical, except its a different shape.)

When you rotate the valve lever arm CCW it allows fluid to enter the lines running to the actuators (and from there to the accumulators). When the arm is in its neutral position the fluid path to the actuators is sealed off (this is how the system is most of the time). When the valve lever arm is rotated CW it allows fluid to bleed out of the actuators. The system does not actively pump anything to lower the car, as is evidenced by the fact that you can lower the car with the lever when the engine is off.

The car will only lower as much as the weight of the vehicle can compress the springs. The normal ride height of an unloaded vehicle is supported by the springs alone. Any car that goes lower than its normal ride height is either loaded down (and the SLS sytem doesn't work) or it has weak springs (and the SLS system doesn't work).

If I understand you correctly, your car will raise and lower (to normal height), but still feels bouncy after replacing the accumulators? Also that it doesn't raise up when the springs are compressed with a load.

If these are the correct (and only) symptoms, then I would guess that your pull rod (adjusting linkage) is too long and that the "normal" position for the SLS system has been set below the actual normal ride height.

Also verify that your actuators aren't leaking.

Biodiesel300TD · #9 08-23-2007, 11:29 AM

A rear that doesn't rise when loaded can be caused by two things, the suspention pump or the SLS valve. Does the rear settle or sag over night? If it's settling then the valve is probably leaking. Fix that then see what happens. If you aren't getting any sagging after sitting then your pump it the issue. You can get rebuild kits for the pump and for the valve from the stealership, atleast for the 123s anyway.

Might also check the lever arm while the car is sitting at normal ride height. It should be parallel to the ground. It not adjust it so it is. Then see if the rear rises when loaded.

Douglas.Sherida · #10 08-23-2007, 12:31 PM

Quote:

Originally Posted by Biodiesel300TD

A rear that doesn't rise when loaded can be caused by two things, the suspention pump or the SLS valve. Does the rear settle or sag over night? If it's settling then the valve is probably leaking. Fix that then see what happens. If you aren't getting any sagging after sitting then your pump it the issue. You can get rebuild kits for the pump and for the valve from the stealership, atleast for the 123s anyway.

Might also check the lever arm while the car is sitting at normal ride height. It should be parallel to the ground. It not adjust it so it is. Then see if the rear rises when loaded.

The original poster mentioned that the car will rise when he manually operates the valve, therefore the pump is working.

He also doesn't mention any leaks, suggesting that the valve is OK.

He also mentions that the car does not go lower than normal ride height. This suggests that he has decent springs.

Given these things and the fact that the car does not self correct the level when loaded, suggests that it is not receiving the proper input from the rear swaybar (either the z-links or the pull rod).

As an aside:

Once more, SAGGING OF THE REAR SUSPENSION INDICATES BAD REAR COIL SPRINGS. Yes, it also indicates a problem in the SLS, most likely the valve or the pump. But, if the springs are good an unloaded vehicle WILL NOT SAG. Of course everyone wants to fix the SLS, but nobody ever wants to address the springs. I would predict that this approach of only fixing half of the problem will create bigger problems later (by over stressing the actuators). Heck, chances are good that leaking SLS valves are in large part CAUSED by weak springs.

Sorry about the shouting, its just that everyone seems to ignore this.

Also I was under the impression that the w124 SLS valve has no rebuild kit and is not considered serviceable by MB (I could be wrong), and the w123 rebuild kit is no longer available from MB.

kerry · #11 08-23-2007, 02:18 PM

Quote:

Originally Posted by Douglas.Sherida

Once more, SAGGING OF THE REAR SUSPENSION INDICATES BAD REAR COIL SPRINGS. Yes, it also indicates a problem in the SLS, most likely the valve or the pump. But, if the springs are good an unloaded vehicle WILL NOT SAG. Of course everyone wants to fix the SLS, but nobody ever wants to address the springs. I would predict that this approach of only fixing half of the problem will create bigger problems later (by over stressing the actuators). Heck, chances are good that leaking SLS valves are in large part CAUSED by weak springs.

Sorry about the shouting, its just that everyone seems to ignore this.

.

I'm not convinced that this is true, so it's not just a question of ignoring it. There has to be some pressure in the SLS system at all times in order for the accumulators to function. On a brand new car, would the rear end sag if the hydraulic cylinders were removed?
Can anyone direct us to a part of the FSM where this is explained?

Douglas.Sherida · #12 08-23-2007, 03:25 PM

Quote:

Originally Posted by kerry edwards

There has to be some pressure in the SLS system at all times in order for the accumulators to function.

The accumulators do not need to be under positive pressure, they just need a sealed system that is completely filled with hydraulic fluid. Think about how the rear end works.

99.99% of the time the SLS valve is closed. It only opens (in either direction) when the car is loaded up or when that load is removed. The response time of the valve (and the pump) is far too slow for it to cycle open to adjust to road bumps. Therefore, you can think of the actuator/accumulator as a closed system. Its not possible for there to be negative pressure in the accumulator lines, just neutral and positive. It is the job of the accumulator to dampen road bumps. So long as the lines from the valve to the accumulators (through the actuators) are completely filled with hydraulic fluid, they will do their job.

The ideal setting would be to have the SLS set perfectly neutral at rest on the springs. In practice it may be easier to set the SLS to slightly lift the back off the springs, but only a very little (1/4 to 1/2 inch at the rear wheel wells). Over-compensating for weak springs by setting the ride height with the SLS is just putting unnecessary pressure on the valve, actuators and accumulators.

What I am suggesting to the original poster is that the SLS valve ride height is set below the spring height so that the valve is open to bleed off pressure from road bumps (bouncy) and load.

Quote:

Originally Posted by kerry edwards

On a brand new car, would the rear end sag if the hydraulic cylinders were removed?

We'll never know that. I can't quote the FSM, but my 22 year old wagon (with 22 year old springs) drops less than an inch when the SLS valve is removed. If you weren't specifically looking for the drop (laying under the car), you'd never notice it (as an overnight sag). If you have a noticeable sag, you have bad springs.

kerry · #13 08-23-2007, 04:56 PM

Quote:

Originally Posted by Douglas.Sherida

The accumulators do not need to be under positive pressure, they just need a sealed system that is completely filled with hydraulic fluid. Think about how the rear end works.

99.99% of the time the SLS valve is closed. It only opens (in either direction) when the car is loaded up or when that load is removed. The response time of the valve (and the pump) is far too slow for it to cycle open to adjust to road bumps. Therefore, you can think of the actuator/accumulator as a closed system. Its not possible for there to be negative pressure in the accumulator lines, just neutral and positive. It is the job of the accumulator to dampen road bumps. So long as the lines from the valve to the accumulators (through the actuators) are completely filled with hydraulic fluid, they will do their job.

The ideal setting would be to have the SLS set perfectly neutral at rest on the springs. In practice it may be easier to set the SLS to slightly lift the back off the springs, but only a very little (1/4 to 1/2 inch at the rear wheel wells). Over-compensating for weak springs by setting the ride height with the SLS is just putting unnecessary pressure on the valve, actuators and accumulators.

What I am suggesting to the original poster is that the SLS valve ride height is set below the spring height so that the valve is open to bleed off pressure from road bumps (bouncy) and load.

We'll never know that. I can't quote the FSM, but my 22 year old wagon (with 22 year old springs) drops less than an inch when the SLS valve is removed. If you weren't specifically looking for the drop (laying under the car), you'd never notice it (as an overnight sag). If you have a noticeable sag, you have bad springs.

I agree with almost all of that. It probably only lifts a half inch or so in its intended design position.

But, whether an overnight sag is an indication of bad springs depends on where the system is set. For instance, if the system is set(misset?) to lift the rear end an inch above the factory setting and it bleeds off overnight, a 2 inch sag would probably be visibly detectable in the morning, yet it wouldn't be an indication of bad springs. Same if the control valve was misbehaving and lifting the car too high all the time, above the correct setting.
So, I'm not saying that a TD can't have worn out springs, I'm only thinking that sagging butts are not necessarily an indicator of worn out springs.

Douglas.Sherida · #14 08-23-2007, 05:30 PM

Quote:

Originally Posted by kerry edwards

I agree with almost all of that. It probably only lifts a half inch or so in its intended design position.

But, whether an overnight sag is an indication of bad springs depends on where the system is set. For instance, if the system is set(misset?) to lift the rear end an inch above the factory setting and it bleeds off overnight, a 2 inch sag would probably be visibly detectable in the morning, yet it wouldn't be an indication of bad springs. Same if the control valve was misbehaving and lifting the car too high all the time, above the correct setting.
So, I'm not saying that a TD can't have worn out springs, I'm only thinking that sagging butts are not necessarily an indicator of worn out springs.

I absolutely agree. If it wasn't set right (set too high), you could notice a sag from a leaking SLS valve. That's how my 23 year old wagon was when I got it (over set by a couple inches). It was through the troubleshooting process of swapping out the valves on my 84 and 85 wagons that I figured out that the springs support the weight of the unloaded vehicle. With the SLS valve out the 84 sits about 2 inches below proper ride height. With the SLS valve out, the 85 wagon sits at very near proper ride height. After rebuilding the leaking SLS valve the 84 will now sit at whatever height I want (above the base spring height), but it will be getting a new set of rear coil springs soon. The 85 will get sold soon.

The degree of compression with aged springs is going to be infinitely variable, and the decision of when to replace them lies with the owner. My point being that if you have perceptible sag below proper ride height when the SLS system is not working (or when the bleeder valve is opened), you have worn out springs (hardly surprising on a 20+ year old car).

The original poster was worried that his car didn't sag when he bled the SLS valve. It shouldn't. It should return to proper ride height. The fact that he thought that it is normal for it to drop below proper ride height suggests that this is a common misconception.

The fact that his SLS doesn't respond to load is a problem, and I'm betting its cause lays in the swaybar (z-links or pull rod).

JimmyL · #15 08-23-2007, 06:55 PM

Quote:

Originally Posted by Douglas.Sherida

Once more, SAGGING OF THE REAR SUSPENSION INDICATES BAD REAR COIL SPRINGS.

I simply, and politely, don't agree with that. Now, I"m the first certainly to admit I could be wrong, but the wagons ride height is adjusted via the lever off the valve assembly. My belief is that the struts {shocks} hold the wagon at the desired ride height, and the accumulators provide the dampening, the "shock absorbing" if you will.
That has been my understanding, and I'm certainly open to being completely wrong. But, am I??????