Anti-sink sphere - car sinking, Has the sphere lost pressure?

[Jan-hendrik]

All the spheres on my XM were replaced with new ones - genuine Citroen - a little over 4 years ago. The suspension seems to be in fairly good shape, meaning no bouncing or hard suspension issues. However the car has begun sinking quite speedily of late when left. Even after just an hour it takes about 30 seconds or more to rise, the beep-beep-beep alarm sounding all the time and warnings appearing in the dot matrix display.
The vehicle is equipped with anti-sink, so this is not supposed to happen. I suspect the anti-sink sphere has lost pressure so that the rear anti-sink valve opens up again after shut down. The sphere is located in an uncomfortable and hot spot which may hasten the demise of it.
Does anyone have any experience with this?

[xmexclusive]

Hi jan

The "anti-sink" sphere is a misnomer.
It was added to the XM design at the same time as the anti-sink feature.
Its function is simply to hold a reserve of pressure for rear brake operation.
It is usually the longest life sphere on a later type XM and very unlikely to be the actual cause of your sinking problems.
There are quite a few threads on the XM and FCF forums (Xantias got A-S first) about sinking of anti-sink cars. Will see if I can find some links.

John

[robertmnorton]

Hi Jan,i'm sure you've seen this before, but uploaded nonetheless.As John notes the sphere only functions to provide reserve fluid to the rear brakes,the rear anti-sink valve does the isolation of the height correctors thus allowing fluid from the sphere to the rear brakes when closed.
robertm

[kenhall1202]

QUOTE (xmexclusive @ June 18, 2011 08:19 am) 

Hi jan

The "anti-sink" sphere is a misnomer.
It was added to the XM design at the same time as the anti-sink feature.

John

I agree with the above. Citroen originally described this sphere in their Xantia write up (see below) as the SC/MAC sphere derived from the French 'Systeme Citroen de Maintien de l'Assiette Constant'
This translates literally (via Google) as 'Citroen system of Maintaining Constant Plate'.
However my ancient French-English dictionary has an alternative translation of 'assiette' to mean 'stable position' which makes a lot more sense in this context.

As well as being connected to the rear brake circuit this sphere additionally has the vital function of maintaining a pressure differential across the purely mechanical SC/MAC valve thus keeping it closed whilst the vehicle is standing and 'locking in' the rear suspension cylinder fluid pressure. After a long while standing the fluid pressure in the SC/MAC sphere slowly leaks away via the brake doseur valve allowing the SC/MAC valve to open. The car rear will then gradually sink as the rear suspension pressure is now also able to leak away via the brake valve.

Ken

This post has been edited by kenhall1202 on June 21, 2011 05:52 pm

[Jan-hendrik]

QUOTE (xmexclusive @ June 18, 2011 08:19 am) 

Hi jan

The "anti-sink" sphere is a misnomer.
It was added to the XM design at the same time as the anti-sink feature.
Its function is simply to hold a reserve of pressure for rear brake operation.
It is usually the longest life sphere on a later type XM and very unlikely to be the actual cause of your sinking problems.
There are quite a few threads on the XM and FCF forums (Xantias got A-S first) about sinking of anti-sink cars. Will see if I can find some links.

John

The 'anti-sphere' is a misnomer. Hmm. I think these are my very own words I used on a forum previously. However this is the exact term Citroen uses for this particular sphere. Misnomer or not, it seems the purpose of this sphere is not just to provide fluid under pressure for the rear brakes. consider this:

The rear anti-sink valve is connected slightly differently (from the front):
in addition to feeding the rear suspension and the brake circuit,
as usual, it connects to an additional anti-sink sphere
as well. The function of this sphere is to maintain pressure
in the braking circuit. As the brake valve is the most leaky element,
it could exhaust the pressure between the piston
and the plunger while the remaining pressure behind the
piston (provided the high pressure and the front suspension
circuits do not leak that much) stays rather high. In this
case the anti-sink valve might open again in error—this additional
sphere ensures that this will not happen.

Source: Citroen guide Željko Nastasic´ and Gábor Deák Jahn

[Jan-hendrik]

Excuse me, Ken. What exactly is it you agree with? With the fact that 'anti-sink sphere' is a misnomer, while at the same time concluding it forms a vital element in keeping the anti-sink valve shut which prevents sinking of the vehicle?

[Jan-hendrik]

QUOTE

Hi Jan,i'm sure you've seen this before, but uploaded nonetheless

Thank you for that, but, indeed, I have seen this before and it is actually a file I have on my hard drive and have uploaded it somewhere to help someone before

My car does not suffer from frequent cycling of the pump. The issue described in the document is one particular to non anti-sink models - it seems. But I am open to and grateful for more information. Thank you.

[robertmnorton]

Hi Jan, the document is indeed applicable to non anti-sink but is also directed at sc/mac equipped cars.The rapid regulator cut in/cut out and engine not running are an "either or" by my reading so is very much applicable to all subsequent to RP 5928,the changeover to S2 H/A, although i believe actually from RP 5917 on the production line.In the absence of any other documented fix i would have thought this Cit document worthy as practicle as any to start diagnostics.It would be my first step being relatively straight forward to access the electro valves.
robertm

[kenhall1202]

QUOTE (Jan-hendrik @ June 18, 2011 02:19 pm) 

Excuse me, Ken. What exactly is it you agree with? With the fact that 'anti-sink sphere' is a misnomer, while at the same time concluding it forms a vital element in keeping the anti-sink valve shut which prevents sinking of the vehicle? 

Hi Jan,

You are right - 'anti sink' is not strictly a misnomer but the more easily understood anglicised derivative that describes the functional purpose of the 'SC/MAC' sphere eg as written in the Citroen Guide extract which you quoted.

Ken

[xmexclusive]

I have to dissagree.
The reason I descibed the use of the term "anti-sink" as a misnomer was because in the original english SC/MAC documentation issued by Citroen SC/MAC is used exclusively and there is not a single reference using the term "anti-sink".
It was only over a year later with the second SC/MAC system application on the Mk2 XM that the term anti-sink started to be officially used.

John

[Jan-hendrik]

QUOTE (xmexclusive @ June 18, 2011 11:29 pm) 

I have to dissagree.
The reason I descibed the use of the term "anti-sink" as a misnomer was because in the original english SC/MAC documentation issued by Citroen SC/MAC is used exclusively and there is not a single reference using the term "anti-sink".
It was only over a year later with the second SC/MAC system application on the Mk2 XM that the term anti-sink started to be officially used. 

John

OK, let's call it the rear brake accumulator
But the 'misnomer' is not likely to disappear

[Jan-hendrik]

Now here is what I have observed. When starting up in the morning the rear of the car sinks and after 10 seconds or so it begins to rise again and stabilizes at normal ride height.

When the SC/MAC valve is open the hydraulic pressure is equal for the suspension, brakes and the SC/MAC circuit. Switching off the engine and resulting lower pressure on the high pressure regulator side causes the higher pressure on the suspension, brake and SC/MAC side to close the SC/MAC valve. (The pressure differential, yes)
Internal leakage in the brake circuit (brake valve) would result in a lower pressure on the brake side than on the high pressure regulator side. That would make the SC/MAC valve open again (as mentioned in an earlier post) and the car sinks.
Here is where the SC/MAC sphere comes in, as it holds a reserve supply of pressure to slow down loss of pressure in the brake circuit. It keeps the valve shut and the vehicle retains height.
Starting the engine the mechanical valves in the stiffness regulators and the SC/MAC valves open by the build up of hydraulic pressure. The SC/MAC sphere supplies pressure for the rear brakes and the rear suspension to compensate for the decrease in pressure so the car remains at normal ride height when driving off.

If the SC/MAC sphere pressure is low, the rear will sink at start up, as there is insufficient pressure to compensate when the valves open and also the more or less empty sphere has to be replenished with LHM. This requires a little work from the LHM pump.

Please let me know your thoughts

[xmexclusive]

Hi Jan

I do agree largely with your analysis of the symptoms and effects.
I am tempted to run a trial with the next A/S sphere I change.
If I add a recharging valve to the sphere then I can play with the sphere pressure settings on the car and see how the hydraulic system behaves in service.

John

[robertmnorton]

Hi Jan,your first post stated that the rear began to sink soon after engine shutdown,your last post that the rear sank after engine start.
Are both circumstances true ?
robertm

[Jan-hendrik]

QUOTE (robertmnorton @ June 19, 2011 10:48 am) 

Hi Jan,your first post stated that the rear began to sink soon after engine shutdown,your last post that the rear sank after engine start.
Are both circumstances true ?
robertm

I don't think I said that, Robert. I wrote: "the car has begun sinking quite speedily of late when left". I had not observed the rear sinking, but when starting the car again (like after an hour or so), it took about 30 seconds for the system to pressurize and a bit longer for the car to reach proper ride height.

The rear sinking after start up I have observed.