[Rod_84]

Aren't there only 2 accumulators?

There are three accumulators all mounted on the valve blocks: one for the front axle pressure supply, one for the rear axle pressure supply, and one for the pressure in the return line back to the fluid reservoir.

Arguably there is also a fourth accumulator mounted to the pressure supply valve unit. This is called the "Pulsation Damper".

 

[television]

The pulsation damper is in the main pressure supply line.

The output from the pump goes to the pressure supply valve unit, the pulsation damper is fitted here to iron out/compensate any hydraulic vibrations

 

[benz-SL500-newbie]

As to the question of whether or not the accumulators act as "shock absorbers", I think Rod_84 and I are both right and both wrong, depending on which accumulator we are talking about and the state of the valves that supply pressure to the struts.

The "center" accumulator or "return" side accumulator definitely serves this function. The pressure on the return side is 10 bars. The pressure in the struts is considerably more. When the valve opens to let fluid out of the strut, there are "spikes" in the return side of the system. The accumulators primary function is to even those out.

As for the accumulators connected to the front and rear valve blocks, I believe it depends on whether or not the valve that lets fluid into the strut is open or not. When close (which is most of the time), the strut pressure is isolated from the rest of the hydraulic circuit, so no shocks to the system should be occurring from the road surface. If the valve is open, then spikes in pressure from the road surface could travel back into the system and the accumulators would absorb it. Moreover, the act of the valves constantly opening and closing in the system would make for uneven pressure in the system, and the accumulators jobs would be to even that out. And I would expect uneven pressure in the system would produced increase wear and tear on components in the system, particularly the hoses and various o-rings in the system.

Am I making any sense?

 

[Rod_84]

The output from the pump goes to the pressure supply valve unit, the pulsation damper is fitted here to iron out/compensate any hydraulic vibrations

This is correct, and since it comports with the Wikipedia description of an hydraulic accumulator, I regard it as one.

As to the question of whether or not the accumulators act as "shock absorbers", I think Rod_84 and I are both right and both wrong...

I misread your original statement and was thinking of shock absorbtion with respect to the chassis, when you plainly stated the system. So I agree the accumulators reduce shock to the system. For that matter, so does the Pulsation Damper.

Am I making any sense?

To me, yes.

 

[benz-SL500-newbie]

So, just to recap what I have learned about the accumulators in the system.

The accumulators, often referred to as a "air cell" or "nitrogen ball", are black spheres that contain nitrogen gas (air) trapped behind a rubber membrane.
Hydraulic fluid is allowed to travel in and out of the sphere based on the pressure differential between the rest of the system and the air on the other side of the membrane. The compressed air in the system pushes back against the fluid and helps maintain an even fluid pressure.

These accumulators also act as a reservoir for fluid. When there are sudden demands for fluid (from the valves opening to fill the struts, the accumulators help supply that load. They assist the pump. The pump supplies a steady pressure to the system, whereas the accumulators handle the variable pressure demands.

The ABC system can be divided into two halves. The high pressure (~190 bars) or "supply" side of the circuit that extends from the pump through the pulsation dampener/pressure sensor/check valve assembly and on to the valves. The low pressure (~10 bars) or "return" side of the circuit extends from the valves through the check valve/temperature sensor assembly and then through the oil cooler and back to the reservoir.

There are three accumulators placed strategically in the system. One is attached to each valve block and their purpose is to even out the pressure on the supply side of the valves. The third, often referred to as the "center" or "return" side accumulator evens out the pressure on the return side of the valves.

Oddly, on the rear of the car, the return or "center" accumulator screws directly to the rear valve and the high pressure accumulator is some distance away attached by a hose to the rear valve. For the front valve, the high pressure accumulator is attached directly to the valve. So don't get the rear ones confused.

The "pulsation dampener" is arguably an accumulator as well. The design is the same. Its size and position in the system serves a different purpose though. It is to even out the vibrations in the fluid from the pump. When it fails you get this classic humming sound.

These accumulators wear out. Like any sort of wear part, how fast depends on a lot of factors. 60K-80K miles seems the norm from what I read. Your mileage may vary based on the age of the car, how clean the ABC fluid has been maintained, and so forth.

So, what can we take away from all this?

1) they play an integral role in the system. The act as "shock absorbers" in the sense that they even out spikes in the hydraulic system pressure. For that reason alone, making sure they are in good working order is important in that they will likely reduce stress (and eventual failures) on the rest of the system.

2) they also supply pressure beyond what the pump is capable of. Remember the pump supplies steady over-all pressure. The accumulators handle the sudden demand. If your accumulators go bad, you will get drops in hydraulic pressure. This is why the classic symptom of the brief ABC warning message on the dash occurs when hitting bumps or dips in the road occurs. There was a sudden demand for fluid beyond what the pump can supply, and the accumulator(s) were unable to meet the demand. Pressure dropped momentarily and the ECU reports that as a brief message on the dash.

My impression, from personal experience and from reading posts, is that the pressure drops (and resulting error codes being logged) that are a symptom of the problem frequently leads shops to replace the pump, when the accumulators were the real culprit.

So how do to tell if your accumulators are in good shape? That is a tough one.

One member mentioned there is a SDS test for them. That would be a good start. I don't know how reliable the test is.

Another member suggests a "bounce test" by pushing down on the bumpers and observing the response. But it makes no sense when looking at the hydraulic diagrams. When the car is parked, the valves to the struts are in a closed position, isolating them from the rest of the circuit. Pushing on the bumpers would only test the struts. Nothing more.

Observing the dip stick levels. This is just a theory of mine, but if an accumulator has failed or is extremely worn, then they would be holding on to more fluid than they are supposed to. When the car is shut off and the system is depressurized, this fluid should be pushed back to the reservoir. This is what causes the classic reservoir overflow symptom when the accumulators fail. So in theory, a difference in fluid levels from when the car is running and shut off could indicate the overall health of the accumulators (assuming the measurements are taken with fluid at same temperature and ride height setting). A differential larger than the distance between the "engine running" and "engine off" marks on the dipstick could indicate wear on the accumulators. Having to maintain your fluid reservoir levels lower than normal to prevent overflows may also indicate failing accumulators.

Visual inspection. Most of these accumulators are easily accessible and removable. If you have the system down for repairs anyway, it may be a good time to spend a little extra time or money and have them pulled and inspected. If they are good, the rubber membrane should be near the entrance to the sphere. If they are bad or "blown", you can reach a considerable distance into it. Be careful and don't use anything pointed that could damage the rubber membrane. Of course pulling any part in the system does introduce risk of contaminants getting into the system. The risk/reward decision is up to you.

To recap, there are two common symptoms that point to your accumulators failing. The brief "ABC Drive Carefully" messages on the dash, and the reservoir overflowing a few minutes after the car is shut off. A SDS test might confirm the accumulators as well..be sure to mention it to the mechanic as he might not be aware of it. It might be a good idea to have this test done whenever the car is in for repairs.

So do your car and pocket book a favor and make sure these components are all in good working order.

Comments welcome.

Darren

 

[television]

That is a very good assessment Darren, but there is all the electronic side to it as well, though in general this part is quite good, you have the valve blocks, on the front main block the sensors are no longer sold and if one fails you replace the complete valve block.

 

[Rod_84]

A SDS test might confirm the accumulators as well.

Using SDS I ran the test of the accumulators on my vehicle last week.

When you start the test SDS causes the struts to fill and raise the suspension to its maximum height. Next you shut-off the engine. SDS displays the system pressure, and apparently it is monitoring the decay in the pressure. After half a minute or so my test finished with a statement that the test had passed.

It is intuitive that the rate of decay will be vary dependent upon the condition of the accumulators, so no doubt SDS can reliably diagnose if one or more are faulty.

 

[benz-SL500-newbie]

Here is what I understand about the valve blocks. As you can probably tell by my previous posts, I believe it important to understand how things work, since it is the foundation for trouble-shooting.

The valve blocks job are to control the amount of fluid in each of the 4 struts. There are two valve blocks, one for the front struts and one for the rear struts. For each strut, there are two valves. The "control" valve is a 3-position valve. In its center position it blocks the flow of fluid to/from the strut; in another position it allows fluid to enter the strut; in the other it allows fluid to leave. The "shut-off" valve is a 2-position valve that blocks the flow of fluid to/from the strut as the center position of the control valve does. So each valve block has 4 valves in it in total. These valves open and close based on voltage being supplied to them by the ABC control unit, referred to as the ECU.

These valve blocks rarely fail outright. What happens is that contaminants in the hydraulic fluid builds on the seat of the valves, or the o-rings in the valve deteriorate. Either way, the valve no longer makes a good seal. As a result, fluid slowly escapes past the valve and the car sags while parked.

It is possible the car sagging is a fluid leak, such as the lines between the valve and the strut, or the strut itself. If the leak is severe enough to cause noticeable sagging while parked, there should be obvious signs of the leak such as puddles on the garage floor.

When the car is running, the ECU will compensate for any leaks in the valves. It is constantly monitoring levels and adjusting as necessary. The sagging would only occur when the car is parked and shut off.

If you are having issues with the corner of the car not being a the correct level, or exhibiting other odd activity, then you probably have a sensor issue (to be discussed later). If multiple corners can't stay at the correct height, you may have a pressure/pump problem. There is also a check valve in the system that regulate system pressure, and that will be discussed later as well.

Unless the sagging problem is severe, it is not an urgent problem that you have to rush to the repair shop for. You can monitor the situation over time and decide when it has reached the point you want to fix it. There is the off chance the debris holding the valve open will dislodge on its own. Going for a spirited drive might dislodge it. In the meantime be careful not to let the corner sink all the way down, as the wheel well may come into contact with the tires. Be sure to park with the wheels oriented straight ahead to avoid wheel well damage, and to start the car periodically to pump up the strut.

If and when you have to fix the valve blocks, you options are to:

1) Filter the ABC fluid (requires two filters) and perform a rodeo (a test that exercises the system) . Cost would be around $200-300. If the cause is debris rather than an o-ring, then it may dislodge some debris from the valves, but the results will be marginal and probably temporary. Try to imagine cleaning a fry pan by just running water over it. If the ABC fluid is older that 40K miles, many on the forum would suggest replacing the fluid as well (about $250+labor). If it doesn't work you will be out the $300-$500.

2) Overhaul the valve assembly. This is not an approved MB procedure. Many members have reported success in pulling the valves and cleaning them. Replacing the o-rings is also a good idea, but it may take some research to find suitable replacements since there are no MB part numbers for them. There are also DIY write-ups and a youtube video as well. Your local Indy shop may be willing to do this for you, with no guarantees of course. Parts cost would be minimal and labor cost around $1,000, not including a flush.

3) Replace the valve block assembly. This will run you around $2,500 including a flush. If saving money isn't a concern then this is the best option to fix the problem.

Anyway, valve blocks leaking is a very common problem and is also the easiest component to diagnose. If the car sags when parked, and there are no signs of fluid leakage, then you have a leaky valve block. There aren't any other explanations.

 

[television]

There is only the one pipe going into the strut, so any leak, the fluid can only come out and easily seen, I have had 2 struts do this at 18k miles. A 3rd strut was misting, this is allowed, but not dripping. The ball joints can wear early in life, and at £1350 per strut thats an expensive ball joint. You can add the pump to the list as well.

 

[Rod_84]

For each strut, there is a valve on the block that opens/closes to let fluid into the strut. A second valve opens/closes to allow fluid to leave the strut.

That is incorrect, as the two valves work differently.

The "control" valve is a 3-position valve. In its center position it blocks the flow of fluid to/from the strut; in another position it allows fluid to enter the strut; in the other it allows fluid to leave. The "shut-off" valve is a 2-position valve that blocks the flow of fluid to/from the strut as the center position of the control valve does.

If and when you have to fix the valve blocks, you options are to:

1) Flush the ABC fluid and perform a rodeo(a test that exercises the system).

According to Mercedes the first course of action is to filter the fluid -- not flush it. The cost is two ABC filters and 30 minutes of Rodeo.

 

[benz-SL500-newbie]

Thanks Rod_84. You are right on both counts. I'll edit the post to reflect that.

 

[Gary Knox]

Rod posted: "According to Mercedes the first course of action is to filter the fluid -- not flush it. The cost is two ABC filters and 30 minutes of Rodeo."

Yes, but didn't MB also state that there was NO MAINTENANCE required on the ABC system (essentially lifetime unit) for several years???

I may be too anal, but I'll do a flush every 2-3 years, <20K miles. Replacing a known hygroscopic fluid is for less than $100 (buying Pentosin CHF 11S from Amazon in 20 liter jug) is pretty cheap insurance as far as I'm concerned.

SL500 - newbie: Your summary of the system is very clear and easy to understand. You have become a 'guru' in a very short period of time. I'll combine your posts as a summary to put in my SL55 file. Well Done.

 

[television]

This fluid changing / filtering is in line with mercedes filled for life gearboxes, if the improvements changing/filtering the fluid is as marked on the ABC as it is in the gearbox, then perhaps we should all take this matter more seriously.

Thanks benz-SL500-newbie, it has been a very good thread, you have done very well.

 

[Gary Knox]

Malcolm,

Since my car was 10+ years old this past winter, while it was in storage, I drained (flushed in some cases) every fluid/filter in the car, from the differential in the rear to the radiator in the front (including supercharger). It now, with <50K miles, has completely fresh fluids/filters using only MB approved products.

My maintenance will be: engine oil and filter annually (<5K miles), brake fluid, coolant and ABC fluid/filter every 3 years max, and transmission/filter, differential and supercharger at 6-8 years.

 

[television]

This all makes good sense Gary after reading and thinking about it all.

I am thinking of a late SL63/5 now for me.

 

[Rod_84]

Rod posted: "According to Mercedes the first course of action is to filter the fluid -- not flush it. The cost is two ABC filters and 30 minutes of Rodeo."

Yes, but didn't MB also state that there was NO MAINTENANCE required on the ABC system (essentially lifetime unit) for several years???

For someone who has been following a regimen of maintaining clean fluid through fluid changes, filtering the fluid is a logical course of action to address sagging at a wheel. The key is the Rodeo routine which can remove deposits on the valves by heating the fluid to a temperature which would not typically be achieved through driving.

 

[benz-SL500-newbie]

Hi all,

Can anyone explain under what conditions the following error messages appear? I'm especially interested in these messages as it pertains to pump pressure. At what pressure levels and durations does the ECU decide to show each message?

Also, the drive carefully messages can be brief, often only a few seconds in the case of bad accumulators. Other times the message stays locked on until the car is restarted. Does anyone know the thresholds for this? How long does pressure need to remain low before the ECU shuts down the system for the remainder of that drive.

White "Drive Carefully"
Red "Drive Carefully"
White "Visit Workshop"
Red "Visit workshop.

 

[Rod_84]

Can anyone explain under what conditions the following error messages appear?

Each of those messages has many fault conditions which may trigger it. If you are really curious, you can easily recreate most or all ABC faults and determine which messages are associated with which faults and what thresholds are crossed.

 

[television]

Hi all,

Can anyone explain under what conditions the following error messages appear? I'm especially interested in these messages as it pertains to pump pressure. At what pressure levels and durations does the ECU decide to show each message?

Also, the drive carefully messages can be brief, often only a few seconds in the case of bad accumulators. Other times the message stays locked on until the car is restarted. Does anyone know the thresholds for this? How long does pressure need to remain low before the ECU shuts down the system for the remainder of that drive.

1, White "Drive Carefully"
2, Red "Drive Carefully"
3, White "Visit Workshop"
4, Red "Visit workshop.

1 and 3 are almost the same thing meaning some fault has been triggered.
2 and 4 mean that the car may be very low or a more serious fault..

Any message re the ABC usually remains until the car is re started, also once a message is up in the dash this usually renders the ABC controls inoperative till a re start.

If your car is constantly throwing up messages do get the codes read off.

 

[benz-SL500-newbie]

Hi All, I thought I would post my understanding of the ABC pump. As always, I welcome any corrections or additions. I'll edit the post based on the feedback. Eventually I will merge the summaries into one writeup.

The ABC Pump:

The power steering and ABC pump are integrated into one unit, referred to as a Tandem Pump. Although the two pumps share the same pulley and shaft, they are separate components otherwise. It is possible for the ABC pump to fail but the power steering pump is fine (and vice versa).

The pump is lubricated by the fluid, so it is important it never run dry. If you are having fluid leaks or sagging issues, be sure to keep a close eye on the fluid level in the reservoir. Should the fluid run dry and it is destroyed, it will likely shed debris into the ABC system. This could generate problems with downstream components for years to come.

There seems to be some consensus that the average life of a pump is around 60-80K. Like any component, some will fail sooner and some may last much longer. Pumps cost around $2,500 USD to replace.

Integrated into the pump is a suction restrictor or throttle valve. It is wired to the ECU and open and closes based on the voltage supplied to it. The opening and closing of this valve controls the rate of flow from the reservoir into the pump.

The pump may fail one of two ways. 1) it goes completely and cannot generate any pressure - somewhat rare. 2) The pump wears and cannot maintain steady pressure as it did before - the most likely scenario. It progressively gets worse, making error messages on the dash more frequent and more persistent.

The pump failing completely should be pretty obvious to diagnose. The ABC visit workshop message will appear shortly after the car is started, and the message will stay on. The car will not raise on command either. SDS error codes will indicate inadequate or no pressure.

It is important to remember that when the ABC warning message stays on the dash, the ABC system is disabled, effectively in "limp mode". The valves to the struts are locked closed, allowing you to drive to the workshop. The quality of the ride will be either rock hard or "tuna boat", depending on what height the struts were locked at. The switch to raise or lower the ride height will also be disabled.

If the pump is weak, you should also get ABC "Drive Carefully" and "Visit workshop" messages, but they will be intermittent in nature. If the pressure drops too low at any point, the ABC system disables itself and displays the message on the dash. It stays disabled until the car is shutoff and restarted. In many cases you can restart the car and the system will pressurize successfully, and the error message will clear. The car works normally again for a while. There will also be pressure related SDS error codes logged as well.

Some owners report the ABC system operates fine when the car is cold, but the error messages start appearing after the car has warmed up. The reverse also seems to be reported. Temperature does seem to be a factor.

It is also suggested the suction valve may be the culprit, and not the pump itself. But unfortunately the valve is not sold separately, it comes with the pump. Check to make sure +5 volts or more is present at the suction valve if the pump is not producing pressure in order to rule out wiring or ECU issues.

Some owners report a grinding or growling sound the pump goes bad.

It should also be noted that low pressure codes do not necessarily mean the pump. Accumulators can fail leading to intermittent low pressures (when hitting bumps). The suction restrictor valve could be malfunctioning. The pressure check valve could be malfunctioning. The pressure sensor itself could be malfunctioning.

The best way to know for sure if the pump is bad is to monitor the pressure while doing a rodeo. The rodeo will stress the system, and even a good pump will see about a 1/3 pressure drop at times. So if the car can get through the rodeo successfully, then the pump is probably fine. If there are still pressure related codes being generated after passing a rodeo, I would recommend ruling out some of the other possibilities mentioned earlier.

The SDS system may have tests specifically for the pump, and SDS may be useful to rule out other possible components as the cause of the pressure problems.