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

[Jan-hendrik]

QUOTE (xmexclusive @ June 19, 2011 10:28 am) 

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

It will be interesting to know the results of your experiments, John.

[kenhall1202]

Hi Jan

QUOTE (Jan-hendrik @ June 19, 2011 03:39 am) 

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.
 

I fully agree with this interpretation of the SC/MAC / anti-sink / rear brake accumulator sphere's function.

However I would interpret the restart situation as follows:

On a car with the hydraulics in good condition, upon restarting, the SC/MAC valve opens allowing the LHM pressures to equalise between the SC/MAC sphere and the two suspension spheres. The resultant pressure will depend on the condition of the SC/MAC sphere and how much fluid pressure has been lost via the brake valve whilst standing. If this resultant pressure is substantially lower than the pressure in the rear suspension when the car was first stopped then the rear will sink noticeably causing the height corrector to supply HP LHM to restore the rear height.

Ken

[Jan-hendrik]

I had the SC/MAC sphere replaced yesterday by the main dealer and I am glad I did not attempt to do it myself. The mechanic had apparently never done this job on an XM before (XMs are very rare here), but he was very meticulous and very careful. Initially he tried to undo the sphere without dropping the sub frame and it seemed easy. But that was only for the first turns of the large nut that holds the sphere in place on its bracket. There was some rust and in the little space available it proved hard to operate two tools: the chain strap on one end and the wrench on the other. On my urging he then loosened the bolts of the sub frame on the left side and the ones on the right side just a little to get about 5 cm more space. He seemed afraid the frame would drop free although he had a jack under it. It took him more than an hour to get the sphere off and out of its restricted space. The man was very patient and we traded a couple of jokes about how Citroen put their cars together.

While the car was there up in the air I considered it a good idea to change the hydractive regulator - the centre sphere - at the same time. Which he did, managing without dropping the transverse exhaust damper. Good job! Now you try to do all this lying on your back under the vehicle - something I did on my former 1995 V6 XM, but that car had a different exhaust system.
After bleeding the system from the brake lines, the LHM level indicator was slightly above the upper ring, while it had been on the lower one before. Indicating that one or probably both spheres had lost quite a bit of gas pressure. One may think the anti-sink, alias brake accumulator, alias SC/MAC sphere lasts many years, but there is no hard rule.

Finally driving off three and a half hours later the car seemed better behaved at the rear. Leaving it overnight (17 hours) it did not sink at all at the rear and just 6 cm at the front. No sinking of the rear when starting up.

I seem to have proved the theory that the SC/MAC sphere is vital not only to supply LHM under pressure to the rear brakes, but also to keep the anti-sink valves closed, preventing quick sinking of the vehicle.

And now a few pictures.


Car raised.


Mechanic at work.


Rear damper removed.


Sphere removal in progress.


The old sphere and the new one.