Why Cant I Get It Up

[Gav]

there seems to be a couple of symptons with my suspension which dont feel right.

when jacking the car up and lowering the suspension, the wheels did NOT rise into their wheel arches. i'd put the car on axle stands so the whole thing was off the ground, but the wheels didnt lift, front or back, when playing with the suspension setting. in the past when just lifting a single wheel it tended to jolt upwards very suddenly when lowing the suspension (quite alarming when working on the brakes). During normal use the raising and lowering of the suspension is smooth.

also, and possibly related, everytime i start the car the rear end sags. It stays up overnight (the anti-sink device working) but when starting the car, down goes the back end. After a short time the system pumps up and all is normal. I assume there is a leak somewhere at the back end which isnt contained by the antisink valve.

is this serious or familiar. its annoying having to actually wind the jack to change a wheel for the first time in 10 years.

[Peter.N.]

Hi Gav

The wheels wont rise of their own accord, only the weight of the car will push them up. If you have one wheel on the ground then the anti roll bar will cause the other one to lift. All my XMs have had different suspension characteristics, some stay up, some drop one end and so on, as long as it drives OK I wouldn't worry about it.

Peter.N.

[DerekW]

Hi Gav,

With regard to your sagging rear end, my explanation is probably going to be a bit confusing, but here goes.

The anti sink valve is a purely mechanical device that works on differing pressures. As long as the pressure from the pump and its associated accumulator (the supply pressure) is greater than the pressure in the rear suspension units (the suspension pressure), the valve is held open and the height corrector is able to send pressure fluid to or from the suspension units. If the supply pressure falls below the suspension pressure, the a/s valve snaps shut, isolating the suspension from the rest of the system.

Unfortunately this piece of Gallic logic produced an unwanted side effect. As you probably know the rear brakes get their pressure supply from the rear suspension, that gives the advantage of making rear braking proportional to the load on the rear wheels. Now, if when driving along, the hydralic pump fails or the belt breaks, the a/s valve will close very quickly (because another part of the system that need not concern us prioritises the stored accumulator pressure to operate the power steering). This isolates the rear suspension and you lose your rear brakes.

Our French friends came up with a solution. They put an extra pressure sphere into the system to store fluid under pressure solely to operate the rear brakes. The sensible name for this would be the rear brake accumulator, but because it is required as a result of the a/s valve it is actually called the anti sink sphere. Schematically it is fitted on a branch pipe between the height corrector and the a/s valve. Physically it is in an inaccessible position above the silencer an the left hand side of the car.

Now the preamble is over I can explain what I think is happening to your car:

When you switch the engine off, the stored pressure in the main accumulator is slowly lost through internal leakage in the system. When it drops below the rear suspension pressure, the a/s valve closes as previously explained. Overnight, the pressure stored in the a/s sphere also leaks away through the height corrector. In the morning when you start the engine, pump output pressure builds up quickly to equal suspension pressure (but with very little volume flow) and the a/s valve opens, connecting the suspension to the system, AND TO THE EMPTY A/S SPHERE. This needs pressure and volume to fill, so it grabs it from the rear suspension cylinders. Result - the rear suspension drops as the fluid is transferred to the a/s sphere. Meanwhile, the pump is manfully trying to recharge the complete system and as it succeeds the rear suspension rises up to normal ride height.

Sorry for the long rambling description, but as Peter said, nothing to worry about unless you fancy changing the a/s sphere, which is almost certainly flat.

Derek

[Gav]

thanks for the description. this all makes good sense. interestingly i was under the car recently and noticed this sphere above the silencer and thought to myself how much rustier it looks than all the others - barely any green on it. I guess it now explains itself. it doesn't sound like it even affects the ride at all.

[Niek]

I fully agree with DerekW's explanation.
I've done an anti sink sphere once, on my previous car. This had the same sagging rear end as yours (so does the 2.5 and probably every Xm out there). It's a horrible job to do.
You have to remove the rear silencers and the tray for the spare wheel first.
You then have to undo the hydraulic line that goes into the sphere, it's a different type of union which just unbolts. Now the sphere is only held on by a very large nut which is screwed onto the sphere. Once undone you move the sphere aside and undo the three bolts holding the bracket of the sphere to the subframe. (the sphere can't pass between the subframe and the bracket I couldn't believe it when I noticed that)
Once the bracket is off it's easy. Do everything again but the other way around.

Cheers,
Niek

[DerekW]

Niek,

I think your excellent explanation of how to change the a/s sphere should go in the self-help files.

Cheers,

Derek