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2004 SL600 84,000 miles
The front end of the car is very bouncy. The front right strut is weeping a little, but the ABC reservoir still has the correct amount of fluid.
I do need to repair my valve blocks this winter. (The car sags after 3 days, but the pump still puts out the correct pressure.
Could the springs be shot? Do I need to replace the 2 frot struts?
I would start with either a rebuilt valve block or doing it yourself with a rebuild kit. I bought a rebuilt one on ebay from Autohubshop; they are now $500. Even though my original valve blocks were working fine, I recently put in the ebay one in the back when I replaced the two accumulators back there. Works perfectly. Of course rebuild kits or eeven just o-ring kits are much less; some are less than $50.
My SL 55 has what I characterize as rough ride and was told that it’s most likely the accumulators there’s one in the front and one in the back. I have not replaced them yet, but I bought the parts. I also have a weeping right front strut, and I wonder that it feels stiffer than the others so I may end up replacing that also. Search accumulator and see if that matches your bounce. Apparently accumulators are a wear item and need to be replaced by 70,000 miles.
The ABC struts are often described as "hydraulic rams", so not at like like a traditional shock absorber with internal valving and orifices. The ABC computer controls the pressure and amount of deflection in these struts via the valve blocks. To provide a soft ride there has to be some cushioning of the non-compressible hydraulic fluid and the accumulators are an important part of this. If/when the accumulators fail (wear out), they simply fill with fluid and can no longer cushion the ride, causing a stiff ride. Worse (IMHO), pressure pulses, which were supposed to be absorbed by the accumulators, now get absorbed by the hydraulic hoses, leading to premature ruptures.
Therefore, a rough/harsh ride is a sign of fail accumulators, and IMHO they should be replaced ASAP to reduce wear on the hoses.
I have not yet disassembled an ABC strut and don't know if there is any way fluid can move around in it other than completely leaking out; therefore I would expect a bouncy ride to be due to either low ABC pressure, or the valve block not being able to maintain enough pressure to specific struts.
It seems our struts do have a conventional shock absorber within. Part 'V' on the diagram. Since the main failure point seems to be leaks, I wonder how much would be involved in disassembling them with spring compressors and replacing whatever seals and O-rings have failed?
What we call the ABC damper, or strut, is actually a hydraulic cylinder sitting on top of, and is connected to, a conventional damper. If you look at the above drawing, there is no fluid connection between the ABC hydraulic cylinder and the damper itself. This indicates the damper is a typical design closed unit with its own type of oil and contains pressurized gas to maintain an internal preload.
If a leak is from the damper, it will be seeping around the rod at the top of the damper. If it is coming from the cylinder, the fluid will be coming from above the damper. A leak from the damper means you will have to replace the entire strut unit. Dampers are not made to take apart for repair.
I wonder if the rebuilders replace that damper unit (V). Probably it's an off the shelf part used in other struts. I've got this idea to work out a way to rebuild our struts ourselves, and surely this shock absorber is available.
The ABC struts are often described as "hydraulic rams", so not at like like a traditional shock absorber with internal valving and orifices. The ABC computer controls the pressure and amount of deflection in these struts via the valve blocks. To provide a soft ride there has to be some cushioning of the non-compressible hydraulic fluid and the accumulators are an important part of this. If/when the accumulators fail (wear out), they simply fill with fluid and can no longer cushion the ride, causing a stiff ride. Worse (IMHO), pressure pulses, which were supposed to be absorbed by the accumulators, now get absorbed by the hydraulic hoses, leading to premature ruptures.
Therefore, a rough/harsh ride is a sign of fail accumulators, and IMHO they should be replaced ASAP to reduce wear on the hoses.
I have not yet disassembled an ABC strut and don't know if there is any way fluid can move around in it other than completely leaking out; therefore I would expect a bouncy ride to be due to either low ABC pressure, or the valve block not being able to maintain enough pressure to specific struts.
There is some misinformation in the forum. Pressurized fluid into the hydraulic cylinder causes it to extend and raise the vehicle. Relieving the pressure causes it to retract. Most cylinders have two ports that cause the cylinder (or a rod inside the cylinder) to extend and retract. The ABC only needs one port since the vehicle weight causes it to retract (shorten).
An accumulator has nothing to do with the stiffness of the ride. That is controlled entirely within the damper. The flow of the fluid inside the damper is controlled by restriction through orifices. In today's dampers, the orifices can be controlled electronically. In the drawing above, you can see a steel rod in the damper. The rod moves within the damper with the motion of the vehicle wheel. The orifices (called valving) are attached to the end of the rod and determine flow rate. There are seals inside the damper to make sure fluid does not leak past the valving. Wear of these internal seals is what causes the bounce (loss of damping) of the vehicle. That leaks internally, not to the outside.
The accumulator and valve block do not control damping. The accumulator stores some fluid to reduce pulsation of the hydraulic fluid as the ABC valves open and close. Its purpose is to keep the system pressure consistent. When the pressure is erratic, there will be vibration in the system that the driver will feel.
How dampers work: Damper and Awe: 6 Types of Automotive Dampers Explained
I wonder if the rebuilders replace that damper unit (V). Probably it's an off the shelf part used in other struts. I've got this idea to work out a way to rebuild our struts ourselves, and surely this shock absorber is available.
Tom, that is a great question about rebuilders. Dampers typically cannot be disassembled and are designed to be throw-aways to keep the cost down. Reusing dampers is risky since the internal piston seals are worn. Those seals determine the life of the damper. I doubt that Mercedes sells the damper alone. They don't want guys like us trying to rebuild their struts. That would be a safety risk. Maybe they sell them to trusted rebuilders. All dampers are designed specifically for a vehicle model. Size, mount type, and especially the valving are variables.
Keep us posted on what you find out.
Post #11 is the most concise explanation (that I have seen) of how the struts work. They're really pretty simple, with a single-acting hydraulic ram, level sensor and internal damper. Surely MB used off the shelf dampers that would be available somewhere - strut rebuilders must be able to source them from a supplier of these parts.
Then I imagine there are a number of hydraulic seals and gaskets which could be matched up from a hydraulic parts supplier.
Spring compressor kits are pretty cheap these days.
If anyone in Melbourne (Australia, not Florida...) has and old strut they'd like to donate I will pull it apart and see what's inside.
There is some misinformation in the forum. Pressurized fluid into the hydraulic cylinder causes it to extend and raise the vehicle. Relieving the pressure causes it to retract. Most cylinders have two ports that cause the cylinder (or a rod inside the cylinder) to extend and retract. The ABC only needs one port since the vehicle weight causes it to retract (shorten).
An accumulator has nothing to do with the stiffness of the ride. That is controlled entirely within the damper. The flow of the fluid inside the damper is controlled by restriction through orifices. In today's dampers, the orifices can be controlled electronically. In the drawing above, you can see a steel rod in the damper. The rod moves within the damper with the motion of the vehicle wheel. The orifices (called valving) are attached to the end of the rod and determine flow rate. There are seals inside the damper to make sure fluid does not leak past the valving. Wear of these internal seals is what causes the bounce (loss of damping) of the vehicle. That leaks internally, not to the outside.
The accumulator and valve block do not control damping. The accumulator stores some fluid to reduce pulsation of the hydraulic fluid as the ABC valves open and close. Its purpose is to keep the system pressure consistent. When the pressure is erratic, there will be vibration in the system that the driver will feel.
How dampers work: Damper and Awe: 6 Types of Automotive Dampers Explained
A couple of comments:
First of all, the purpose of the main accumulators is to ensure a stable pressure supply. This allows the pump and lines to be smaller and still allow both valve blocks to have an adequate supply of pressurized fluid when it is needed. They are on the supply side and don't affect the pulsation of the valves to the struts. The strut lines do have special sections of hydraulic hose for this purpose. They all have a couple feet of a textile reinforced hose which will expand more under pressure than the steel wire reinforced hose that is on the ends.
The return accumulator on ABC1 is there to prevent excess pressure build up in the long rear return line so that fluid can be released without excessive back pressure. In reality, you would need to be driving extremely aggressively to need it. Probably something the engineers saw on the road simulator, but not really needed in the rear world and eliminated for ABC2.
The info about dampers applies, but only to a point. ABC is a semi-active suspension. It does have springs and dampers, but they only do part of the job. If you drive a car in limp mode, it is quickly apparent that the dampers are not designed to do the job by themselves. ABC provides final ride height, 'stiffness', and damping of the system based on level sensors, acceleration sensors, speed, and either strut extension (ABC1) or strut pressure (ABC2). Unlike earlier systems that just maintain ride height, ABC actively controls the ride.
The info about dampers applies, but only to a point. ABC is a semi-active suspension. It does have springs and dampers, but they only do part of the job. If you drive a car in limp mode, it is quickly apparent that the dampers are not designed to do the job by themselves. ABC provides final ride height, 'stiffness', and damping of the system based on level sensors, acceleration sensors, speed, and either strut extension (ABC1) or strut pressure (ABC2). Unlike earlier systems that just maintain ride height, ABC actively controls the ride.
Mike, I realized that we have not defined "damping". I am sure some members do not know. Here is a brief definition that I copied from a website: Damping is restraining of vibratorymotion, such as mechanical oscillations, noise, and alternating electrical currents, by dissipation of energy. Of course, our application is mechanical. It achieves the dissipation by hydraulic restriction inside the damper. The oscillation is that of the vehicle mass that is produced by the springs. The bouncing is the result of reduced damping.
I have no problem with your discussion, but the limp mode screws everything up and is not worth discussing. When the vehicle is lower than desired, it compresses the springs and greatly changes the ride. The ABC does not control the damping but it does have some effect on it, as you pointed out. Thank you for your comments.
FYI.
Pumps Move oil.
Pressure is derived from this moving fluid flow being restricted or a LOAD for cylinders the lifted load for motors the torq required to turn shaft and whatever it is connected to think Hoist.
Accumulators act as storage device for fluid at some pressure greater than the AIR Side of the bladder or piston depending on design. This air side acts as a spring and as pushed back by incoming fluid under greater pressure the air pressure increases. Normal the air side is filled with Nitrogen and prefilled to some design pressure with ACC hydraulic port open to atmosphere.
So if pressure in system never rises above ACC Gas sde then it does not fill up.
One function well 2, is accumulators store energy, absorb pressure spikes and replenish oil flow to maintain system pressure if you want something to move faster then pump flow can deliver for brief periods.
NOt sure how the ABC system is plumbed and designed but my guess is the ACC help to absorb pressure spikes from dampers as car hits speed bumps that compress suspension and prolong like of pump and valving.
The invers would be true as well - when car hits pot holes and suspension drops the dampers need to be refilled fast to prevent cavitation inside so the ACC provides this quick supply of oil.
All this allows for small pump
Again most of above is speculation without seeing hydraulic system schematic.
yeah I am hydraulic engineer.
NOt sure how the ABC system is plumbed and designed but my guess is the ACC help to absorb pressure spikes from dampers as car hits speed bumps that compress suspension and prolong like of pump and valving.
The invers would be true as well - when car hits pot holes and suspension drops the dampers need to be refilled fast to prevent cavitation inside so the ACC provides this quick supply of oil.
Your second sentence is correct, but your first sentence is not how ABC is designed. When the struts are forced up by that speed bump, the ABC system senses the vertical acceleration and opens the control valves to dump oil into the return line (it doesn't back feed to the accumulator). Then the control valve reverses and refills the strut like you point out in your second sentence. The result is a 'softer' spring as the car starts up the speed bump to keep the car from jumping and a 'stiffer' spring to extend the strut back out and prevent the car from excess diving when it clears the bump.
Pressure on each strut is independent and significantly lower than the system pressure which is around 190 bar. Strut lines are only 150 bar rated. Accumulators are there to maintain system pressure at the control valves and would be precharged at something less than 190 bar.
Because the system actively responds to vehicle accelerations AND strut positions any mechanical resonance from the spring gets overridden by the active system. In other words, it has active damping as one of its functions.
sounds good to me after seeing schematic.
The accumulators are storing energy from the small pump.
Interesting they have some valve on the inlet of the piston pump since if you starve a pump of oil they tend to eat themselves up and send all kinds of metal particles into the system ruining everything downstream.
Dirt and contaminations are the death of hydraulics.
REplace that return filter once in a while
Nice it has oil cooler.
does not show breather on Tank which is a bad thing in long run as pressure can build increasing system back pressure. I hope it is just an omission and maybe the fill cap is the breather
Lots of Proportional valves for some CAN Module to control along with a mess of sensor to watch and make programming decisions on.
Thanks for Schematic - as I love viewing them to see how other peeps design their systems.
The accumulators are storing energy from the small pump.
Runs at nearly 3000 psi - tough little sucker - think running quarterback, or small forward in Aussie Rules
Interesting they have some valve on the inlet of the piston pump since if you starve a pump of oil they tend to eat themselves up and send all kinds of metal particles into the system ruining everything downstream.
That's the suction restrictor valve designed to regulate the inflow of oil into the pump, not to prevent starvation.
Does not show breather on Tank which is a bad thing in long run as pressure can build increasing system back pressure. I hope it is just an omission and maybe the fill cap is the breather
Look closely at the tank; there is a tiny breather hole in the top.
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