Hydractive II spheres

[hirsty]

Despite going through the "sticky" and reading the excellent Citroen Guide, nowhere can I find anything that tells me what the extra spheres do. The role of the corner spheres is obvious, as is the anti-sink and accumulator, but what do the others do ? Can someone please explain.
I have two spheres at the front, one above the other. I believe the top one is the accumulator, but can't figure out what the one below does.
At the rear, I believe the one facing backwards is the anti-sink, but what is the one facing forward.

[Xaccers]

Gives you a soft ride.
They're active in normal driving as part of the suspension, when you go round corners or accelerate etc, they're excluded and so only the harder corner spheres are in use giving better handling.

[CitroJim]

That's spot-on Xac

In soft mode, the centre Hydractive sphere is the main suspension component and is so set up to be rather soft. The corner spheres remain in circuit but do little of any significance except at the extremes of suspension travel. The Centre Hydractive sphere is often a sphere with a bigger volume and at a much higher pressure than the corner spheres. The Hydractive sphere is effectively switched out of circuit in the hard suspension mode and this is achieved by the use of electrovalves on the Hydractive sphere blocks under the control of the Hydractive ECU that determines, on inputs from a number of sensors, when to switch between soft and hard mode. Hard mode will be selected, as Xac says, under heavy braking, hard acceleration, going round corners enthusiastically and if the suspension is at risk of bottoming out. The switching can occur at low speed whilst traversing speed bumps, making them very uncomfortable. A Hydractive car in hard mode is very hard indeed.

You are right about the sphere locations. The front Hydractive is the one below and forward of the accumulator up against the rad. You'll see the electrovalve on the block and its electrical connection. You can hear the electrovalve humming with the engine off and a door open. It hums when in soft mode.

The rear is the one that hangs down at an angle. again, you can see (and hear), if you look carefully, the electrovalve on top of the block.

Hydractive spheres can be tricky to change. They are not very accessible and have a habit of holding pressure even when the main hydraulics are fully depressurised. This makes them hard to unscrew and you can almost guarantee an LHM shower A Pleiades sphere removal tool is virtually essential for the rear Hydractive sphere and is useful for the front one.

Activas are Hydractive but you don't get the full benefit of the extreme ride comfort that a normal Hydractive car gives. The ride is absolutely sublime on a car with a good set of spheres and an ECU clear of faults. Smooth, comfy as a featherbed, beautifully damped and great handiling with surprisingly little roll. You might think an ordinary Xantia is a nice ride but a Hydractive is about 100 times better!