W124 300TE rear sports dampers / car sickness

[mark.porthouse]

My XM was an early one that didn't have anti-sink. Also, as the rear brakes are powered by the rear suspension hydraulic pressure (to give load proportional brake force distribution) and the hydraulics were fairly worn the rear end used to drop particularly quickly (an hour or two).

I'm still curious to know when the W124 does it's self levelling. Is it only once, shortly after engine start, or is it at any time with the engine running?

Cheers,

Mark

 

[andy_k]

I'm still curious to know when the W124 does it's self levelling. Is it only once, shortly after engine start, or is it at any time with the engine running?

it's constantly working

 

[Dieselman]

As andy has said if the SLS isn't working the rear will sit low, so it must be working ok. It just needs new spheres.

 

[mark.porthouse]

OK. I'll have to test. The height looks fine normally and about ten seconds after start up I hear a whirring sound for about one second which may be the system???
1) I'll set the engine running then load up the boot.
2) I'll load up the boot and then start the engine.
It didn't appear to respond to approx 27 stone of men before though.

Do you know if the weight distribution is close to 50/50? If so I'm imagining that the spring firmness should be similar front to rear and therefore feel similar when bounced by hand.

Cheers,

Mark

 

[Dieselman]

put weight in the boot before bouncing, just like on the Citroen, otherwise you haven't already compressed the gas in the spheres.

You should feel it go quickly stiff if they are expired.

I would expect the rear to be more softly sprung than the front as there is a fairly heavy piec of metal in the front end of the car.

 

[mark.porthouse]

Hi All,

Well, I've loaded it up a bit and I'm getting no levelling. However, I'm not sure what kind of tolerance the levelling has. On the XM if it was only fractionally out it would self adjust. Does the W124 need to be out by say 3 cm suspension travel before any adjustment takes place?

Also, if no levelling was taking place ever then surely it would just lose all pressure from the levelling system and go low at the back - which it isn't doing.

So really I'm assuming that perhaps to get SLS to 'show' I need to load it more???

Cheers,

Mark

 

[Dave Lewis]

Load it more, and drive it a few hundred yards. It's not as dramatic as a Citroen, quite subtle really. Basically, if the car sits level under all load conditions, it's working fine. I don't recall ever noticing it visibly rising. The maximum permitted load is pretty high, and the springs do a fair amount of the work.

 

[andy_k]

you won't really notice it drop because it has a valve to stop the fluid returning and the suspensionlevel falling.

however, what you will notice if you load the rear is that until you start the engine there will be a loss of "bounce" as the fluid will be under pressure. Once the engine starts and the valve opens, normal suspension travel will return.

Just because it looks similar to the Citroen system doesn't mean it behaves like it. The MB system is far more subtle.

Andy

 

[Dieselman]

you won't really notice it drop because it has a valve to stop the fluid returning and the suspensionlevel falling.

however, what you will notice if you load the rear is that until you start the engine there will be a loss of "bounce" as the fluid will be under pressure. Once the engine starts and the valve opens, normal suspension travel will return.

Just because it looks similar to the Citroen system doesn't mean it behaves like it. The MB system is far more subtle.

Andy

The lowering is due to the nitrogen in the spheres compressing. surely this must happen on any sphere sprung vehicle.
If the car is loaded with the engine off then it will lower and raise again when the engine is started, as the springs don't support the whole weight of the car and load.

 

[andy_k]

agreed, but it doesn't seem to drop much - a few cms at most.

I guess, that at standstill, the fluid in the system also comes in to play - due to the levelling valve that should be in a closed system and in theory shouldn't compress.

I'm basing this guess on what happened when the return valve broke on my old estate

Andy

 

[Dieselman]

Thanks Andy.
my point is that I think the spheres are shot, which is why the car has a bouncy ride.
New spheres will allow a lot more travel.

Mark, load the car up then try bouncing it. If it feels firm quickly the spheres are shot.

 

[andy_k]

fair comment but if the spheres are shot the ride will be bouncy but hard.

another dead giveaway of knackered spheres is that at standstill you have virtually no suspension travel as the nitrogen has been replaced by hydraulic fluid - the rubber diaphragm always fails.

Easy to spot by draining a drop or two of fluid from the rear struts - if it's all milky then the spheres need replacing

Andy

 

[mark.porthouse]

Hi All,

Well, today (at last) I managed to load the car up enough to trigger the SLS. I had to put around 250KG in to see it! Perhaps my trigger point is set too low - I think that it is just a matter of altering the position of the lever arm on the anti-roll bar so that it switches the hydraulics at a higher ride height.

From the Haynes manual it implies that the struts do no height/springing duty at all unless a load is in the car when they are pressurised via the accumulators. Until then all springing is done by the springs. This seems to ring true to experience. This would mean an inoperative SLS system wouldn't give itself away by a low rear ride height unless you loaded the car. I realise that this is to disagree with some other people here.

What I'm trying to figure out is the innards of the strut which I'm thinking must have two separate chambers - one for the normal damping function and one for the ride height/hydropneumatic spring function.

Here is a general diagram of the hydraulics (see also below):

The pump takes fluid from the reservoir and sends it to the control unit which then normally send it straight back to the reservoir. When the height lever is deflected (by a load in the car) then pressurised fluid is supplied into the accumulator and strut until a satisfactory pressure/volume is reached when the height lever is back to it's normal range, at which point fluid from the pump simply goes back to the reservoir. If the car is unloaded then the lever returns and allows fluid to freely move back to the reservoir thus depressurising the accumulator/strut.

The fact that the accumulator only gets occasional use is what gives it it's long service life of 100,000 - although to be honest I would imagine the service life to be measured in miles. After all when the accumulator is at low pressure the nitrogen will be at low pressure and very little nitrogen will leak across the diaphragm into the fluid - in a Citroen there is constant high pressure causing the spheres to deflate after just a few years.

Here are diagrams of some struts - the third strut is how I imagine a self levelling and shock absorbing strut is laid out (see also bottom):
http://www.mbclub.co.uk/forums/attachment.php?attachmentid=7318&stc=1&d=1183639056
The height strut is merely an extending piston.
The damping strut is a simple motion damper.
The combined strut would appear to do both.

Note that there is also a small flow restriction aperture in the hydraulics between the strut and the accumulator that damps the flow of that liquid as it moves quickly (as you go over bumps) between the strut and the nitrogen spring (accumulator). This is so that the additional pneumatic spring is damped - after all the fixed damper part of the strut is only designed to damp the coil spring.

Anyway, that's my understanding so far. It is open to correction!

Cheers,

Mark

 

[andy_k]

Hi All,

from the Haynes manual it implies that the struts do no height/springing duty at all unless a load is in the car when they are pressurised via the accumulators. Until then all springing is done by the springs. This seems to ring true to experience. This would mean an inoperative SLS system wouldn't give itself away by a low rear ride height unless you loaded the car. I realise that this is to disagree with some other people here.

May I suggest that you drain the fluid out of the system and then retype that paragraph - I suspect it will be slightly different

My experience of this system is that the rear of the car will drop 2 or three inches when just the springs are carrying the load.

I've no idea how the struts function but I think it's just on the quantity of fluid held in the pressurised section of the system. The viscosity of the fluid and the nitrogen in the spheres will provide the damping. I also doubt your twin cylinder theory as they are only fed by a single pipe

When the struts are off the car compressing them is very simple with little or no resistance.

This isn't based on the Haynes manual, this is from actually rebuilding the system on a couple of cars - I would suggest that in this case the Haynes book is not exactly accurate

 

[mark.porthouse]

So, if the self levelling is in action even when the car is unloaded why does my car stand at a sensible height and only self-level when the car is loaded with over about 200Kg and not before?

I would think that if a car is low when the hydraulics are not operational then there is something wrong with the springs.

Why make the system work when it doesn't have to? Why bother putting coil springs in?

I'm sure that there are plenty of self levelled estates driving round where the control unit lever has been incorrectly set to cause levelling all the time.

Is there a procedure for setting the control lever angle when the system is set up (perhaps after a rebuild)?

Cheers,

Mark

 

[andy_k]

So, if the self levelling is in action even when the car is unloaded why does my car stand at a sensible height and only self-level when the car is loaded with over about 200Kg and not before?

your car stands at a sensible height because there is a non return valve in the self levelling valve.

The initital setup weight of 120 kgs is just that, I've no idea what the figure is to check it.

the system works all the time because even a 50 kg load can easily become 200 kilo load when you consider the upward and downward movement of the rear suspension.

I would think that if a car is low when the hydraulics are not operational then there is something wrong with the springs.

why - see the point about the non return valve. The springs do not support the full weight of the car

Why make the system work when it doesn't have to? Why bother putting coil springs in?

again, see my comment, the system is constantly active and the coil springs lessen the demand placed on the hydraulics - the two work nicely together

I'm sure that there are plenty of self levelled estates driving round where the control unit lever has been incorrectly set to cause levelling all the time.

why should there be loads of estates driving around like that? because very few people will change the self levelling valve. It's a part designed to outlast the car - mine failed at 230k

If you do need to replace it there is a certain angle the arm should set to though in the manuals

I think what you are looking for is some dramatic rise and fall like the Citroen system and you are assuming because you can't see it then it's not working.

If it fails, you'll know when it's not working

Assume that when it's parked the suspension will stay at that level. It will take an awful lot of weight to compress that relatively small amount of fluid a noticeable amount but if you manage it the suspension will feel incredibly hard with no travel. Now, start the car, the level will only correct very slightly but you will once again have full suspension travel.


As I said earlier, please feel free to test/disprove them. All you have to do is drain the system


HTH

Andy

 

[mark.porthouse]

your car stands at a sensible height because there is a non return valve in the self levelling valve.

Sure, when the self levelling is active the pressure is prevented from being released. But imagine that the self levelling isn't active then there is nothing to stop the fluid running back - it all depends on the position of the control lever: Car is high=fluid leaves the strut/accumulator, car low=fluid is not free to drain from the strut/accumulator.

the system works all the time because even a 50 kg load can easily become 200 kilo load when you consider the upward and downward movement of the rear suspension.

So when the suspension is extended (a fraction of a second after it is compressed) you have a -100Kg load and the fluid is immediately free to lose it's pressure and drain back out. Basically the system cannot pressurise the strut/accumulator in the fraction of a second that the bump compresses the suspension - therefore I would not say that the self levelling was active.

The 120Kg setup weight you mention implies that self levelling is set to intervene only when 120Kg is exceeded. At less than 120Kg you cannot bleed the accumulators and struts because they are not part of the circuit yet.

If you do need to replace it there is a certain angle the arm should set to though in the manuals

That would be useful to know, but it is a little bit odd as it must be difficult to set an angle. With Citroens you set the ride height from the ground to a point on the underside of the car (front and back) and you set the control arm to meet that ride height requirement.

I think what you are looking for is some dramatic rise and fall like the Citroen system and you are assuming because you can't see it then it's not working.

I can see it - once the system gets in to operation when the car is loaded enough.

Assume that when it's parked the suspension will stay at that level. It will take an awful lot of weight to compress that relatively small amount of fluid a noticeable amount but if you manage it the suspension will feel incredibly hard with no travel. Now, start the car, the level will only correct very slightly but you will once again have full suspension travel.

I take it that you are talking about the situation when the accumulators are shot and the air spring has therefore gone AWOL - this is the only time when you lose the air spring and the only time you are trying to compress an incompressible fluid.

As I said earlier, please feel free to test/disprove them. All you have to do is drain the system

You are laying down the gauntlet there!

By the way the tyre to arch clearance is about 105mm at the front and about 70mm at the back - which seems rather high at the front to be honest). This is with standard 195/?? 15 tyres (I can't remember the profile off the top of my head). Anyone got any figures for their car?

Cheers,

Mark

 

[andy_k]

The 120Kg setup weight you mention implies that self levelling is set to intervene only when 120Kg is exceeded. At less than 120Kg you cannot bleed the accumulators and struts because they are not part of the circuit yet.

Sorry but you are talking rubbish there, the spheres and struts sit behind the valve in the system and constantly have fluid in them of course you can empty them.

The system is self bleeding

With Citroens you set the ride height from the ground to a point on the underside of the car (front and back) and you set the control arm to meet that ride height requirement.

Cheers,

Mark

How many times do we have to tell you It's not the same as the Citroen system?

You can spend as long as you like comparing the two but it won't help because they work differently.

Just because it has hydraulic spheres doesn't mean it's the same.

No more from me on this subject, it's going round in circles

Andy

 

[mark.porthouse]

Sorry but you are talking rubbish there, the spheres and struts sit behind the valve in the system and constantly have fluid in them of course you can empty them.

The system is self bleeding

Hi Andy,

Sorry, but I didn't say that you couldn't empty them and I didn't say the system isn't self bleeding. I was just pointing out that to refill the system after maintenance you should fill the struts and accumulators up before using the car by putting in a load of over 120Kg - in fact you mentioned this yourself.

How many times do we have to tell you It's not the same as the Citroen system?

You can spend as long as you like comparing the two but it won't help because they work differently.

I know that they work differently. I can see that - it's much much simpler for a start. I never said that they worked the same - in fact I'm saying that they work more differently from a Citroen than you are saying. I'm just trying to establish the facts. So far, some of what you are saying conflicts with itself.

However, I do appreciate your input and points. Early on Thursday I looked through a number of your old posts and got a lot of good info from them.

Cheers,

Mark

 

[mark.porthouse]

Hi All,

Well, I've just had a rear strut replaced, which means that I got to see the car with a depressurised SLS.

andy_k is correct, the car does go low when the pressure is down. This means that the SLS system is used all the time, not just when the car is loaded.

So I'm surprised that to bleed the system you are supposed to put some weight in the back of the car. Perhaps this is merely insurance - just in case, for some reason, your car's SLS isn't activated in normal use???

Also, you shouldn't need to load the car to check whether the spheres (accumulators) have lost all of their gas pressure, as it seems that the spheres are in constant springing service in parallel with the coils and when the spheres are absolutely hard there will be no give in the suspension at all.

Cheers,

Mark