This system is described in CitroenGuide, along with some pictures. Although it's a quite good description of this system, I think there are also some misunderstandings by authors, so I'll describe my view here.
This systems consists of 2 additional "anti sink valves" and one additional "anti sink sphere". Valves are fitted between height correctors and corresponding hydraulic cylinders and work on pressure difference between accumulator sphere and suspension. With engine working, pressure in accu sphere is 145-170 bar, which is higher than pressure in suspension, so these valves are open. As soon as pressure in accumulator falls below pressure in suspension (soon after switching the engine of or even some malfunction in pressure supply), these valves will close, cutting suspension cylinders from the rest of the system and also disabling automatic and manual height control. So, moving the height lever to the lowest when A/S valves are closed will do nothing. That's why different procedure is necessary to decompress the suspension - engine should be running while releasing the screw on the pressure regulator.
Now, how the system really works. In CitroenGuide, authors say "as soon as the leakage in the struts, height correctors and brake valve reduces the pressure in the accumulator below the suspension pressure, anti sink valves will isolate the suspension struts...". Well, in fact, scenario is slightly different. Car with this system will start to sink like any older citroen vehicle, mostly because of leakage in height correctors and brake valve, when older. After a while, height correctors will open, maintaining the ride height slightly below the normal position. This will soon exhaust the pressure in accumulator sphere, and finally it will equalise with the pressure in suspension. At that moment, A/S valves will close, with aid of a spring within them.
So, height correctors and their linkage play important role here. If the linkage is badly corroded, it might fail to open the correctors, so the car will sink to the bump stops before A/S valves close.
Authors claim that purpose of anti-sink sphere is to prevent rear A/S valve from opening again "in error", in case that brake valve is badly internally leaking. Well, I don't agree here. Even with worn brake valve and flat A/S sphere this system should work. In that case, rear end will soon start sinking, height corrector will open, pressure in accu sphere will drop below 80 bar (it can't stay "rather high", unless rear height corrector fails to open), and security valve will close (authors forgot this valve in the description), causing rear A/S valve to close also. Front A/S valve will close in the meantime, because pressure in front suspension is higher. In my opinion, purpose of anti-sink sphere is only to be a security item, acting as a pressure supply for rear brakes, when rear A/S valve is closed.
So, maybe this will be of some help in case A/S system is misbehaving.
Anti sink system - some thoughts
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monsak
- Posts: 22
- Joined: 21 Dec 2003, 09:02
Hi alexx
After read your comment about Anti-sink system.I have some question to ask you. I copy some sentences from your post here for refference.
"As soon as pressure in accumulator falls below pressure in suspension (soon after switching the engine of or even some malfunction in pressure supply), these valves will close, cutting suspension cylinders from the rest of the system and also disabling automatic and manual height control. So, moving the height lever to the lowest when A/S valves are closed will do nothing."
In my car , Xantia 95 with anti-sink system , When I stop engine and put height lever in normal driving position.After that if I push height lever to lowest position , my car will sink down to lower position.So I wonder that is this abnormal or not.
thank you very much
After read your comment about Anti-sink system.I have some question to ask you. I copy some sentences from your post here for refference.
"As soon as pressure in accumulator falls below pressure in suspension (soon after switching the engine of or even some malfunction in pressure supply), these valves will close, cutting suspension cylinders from the rest of the system and also disabling automatic and manual height control. So, moving the height lever to the lowest when A/S valves are closed will do nothing."
In my car , Xantia 95 with anti-sink system , When I stop engine and put height lever in normal driving position.After that if I push height lever to lowest position , my car will sink down to lower position.So I wonder that is this abnormal or not.
thank you very much
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AndersDK
- Posts: 6060
- Joined: 21 Feb 2003, 04:56
- x 1
Hi Alex
Agree -
The rear antisink sphere does not actually take part of the antisink feature.
The principle relies on the antisink valve changing state on pressure differences thus isolating the rear suspension cylinders.
The principle should in fact work with no antisink sphere fitted.
But alas - then no pressure at all is feeded to the rear brakes circuit - in case of pressure loss from the main system - hence the sphere with accumulated rear suspension pressure to supply the rear brakes circuit instead.
Thus you have a safety brake pressure accumulator separate for front & rear brakes - in case of sytem pressure loss (engine/pump stall).
Introducing the antisink sphere then makes possible for other odd problems - like the sudden rear drop on start caused by a flat - or (brake valve leak) drained - antisink sphere.
Agree -
The rear antisink sphere does not actually take part of the antisink feature.
The principle relies on the antisink valve changing state on pressure differences thus isolating the rear suspension cylinders.
The principle should in fact work with no antisink sphere fitted.
But alas - then no pressure at all is feeded to the rear brakes circuit - in case of pressure loss from the main system - hence the sphere with accumulated rear suspension pressure to supply the rear brakes circuit instead.
Thus you have a safety brake pressure accumulator separate for front & rear brakes - in case of sytem pressure loss (engine/pump stall).
Introducing the antisink sphere then makes possible for other odd problems - like the sudden rear drop on start caused by a flat - or (brake valve leak) drained - antisink sphere.
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AndersDK
- Posts: 6060
- Joined: 21 Feb 2003, 04:56
- x 1
Monsak -
This will happen if the main accumulator STILL has a pressure higher than the approx 100bar to keep the safety valve open - thus supplying the suspension with working pressure.
Try this :
Engine stopped - but with ignition on for control lamps in panel.
Before you touch the height lever - then press/release the brake pedal several times - untill the brake warning lamp comes on (o) (not just the STOP lamp).
Now move the height lever to lowest - what happens ?
This will happen if the main accumulator STILL has a pressure higher than the approx 100bar to keep the safety valve open - thus supplying the suspension with working pressure.
Try this :
Engine stopped - but with ignition on for control lamps in panel.
Before you touch the height lever - then press/release the brake pedal several times - untill the brake warning lamp comes on (o) (not just the STOP lamp).
Now move the height lever to lowest - what happens ?
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alexx
- Posts: 462
- Joined: 19 Nov 2002, 02:42
Hi Monsak,
As Anders said, car should normally sink when you, shortly after switching the engine off, move height lever to lowest, because A/S valves won't close immediately. Also, when you leave the car, it will raise several cm - it should sink back to normal height after that.
After you switch the engine off, car will need some time to sink a little, so that height correctors open and exhaust the pressure from the accumulator. On my Xantia ('98, only 49.000 km), this will happen in about an hour, and the car will sink between 1 and 2 cm during that period. After that, A/S valves are closed and manual (and automatic) height control is disabled. Then, car will maintain almost the same height for days, loosing maybe 1-2 mm per day.
On older car, internal leakage in height correctors, brake valve (and also other hydraulic elements) will be higher, so closing of A/S valves will occur earlier - maybe 5 or 10 minutes. If it happens within several seconds, there is a heavy internal leakage somewhere in the system, or the accumulator sphere is flat.
As Anders said, car should normally sink when you, shortly after switching the engine off, move height lever to lowest, because A/S valves won't close immediately. Also, when you leave the car, it will raise several cm - it should sink back to normal height after that.
After you switch the engine off, car will need some time to sink a little, so that height correctors open and exhaust the pressure from the accumulator. On my Xantia ('98, only 49.000 km), this will happen in about an hour, and the car will sink between 1 and 2 cm during that period. After that, A/S valves are closed and manual (and automatic) height control is disabled. Then, car will maintain almost the same height for days, loosing maybe 1-2 mm per day.
On older car, internal leakage in height correctors, brake valve (and also other hydraulic elements) will be higher, so closing of A/S valves will occur earlier - maybe 5 or 10 minutes. If it happens within several seconds, there is a heavy internal leakage somewhere in the system, or the accumulator sphere is flat.