W124 - Spheres

[Timster]

Hi.

The sphere's are knackered on my w124 estate (250TD) apparently.
I'm not 100% sure what they are, and what they do, if you could explain to me i'd be most grateful.

Thankyou.

Tim

 

[pint6x]

Just a guess from a mechanical numpty, but something to do with the suspension?

I seem to remember a Citroen of my acquaintance having a similar issue, and it had spheres that leaked as part of the suspension.

I'm sure someone who knows what they're talking about will step in soon.....

 

[Number_Cruncher]

The steel rear coil springs on the W124 estate just about balance the car at the right height when the car is unladen.

The devices under the rear wheel arches which look like shock absorbers *aren't*, they are simple hydraulic rams. No energy is dissipated in them, they are simply a means to augment the support force provided by the metal coil springs.

The spheres do two jobs.

The sphere is split in two by a diaphragm. Above the diaphragm is (or should be!) Nitrogen gas. As the wheel goes over a bump, hydraulic fluid is forced out of the hydraulic ram, and into the sphere, flexing the diaphragm, and compressing the Nitrogen. This part of the sphere function is to act as an auxiliary spring - the pressure rise in the Nitrogen being proportional to the extra support force being supplied by the hydraulic ram.

The other job that the speres do is to provide damping. This is done by resricting the flow of oil in and out of the sphere using valves and orifices.

There are 2 ways for the spheres to fail - if the diaphragm fails, the nitrogen spring is lost, and the suspension becomes very hard. If the damping valves fail, the ride becomes very bouncy.

Spheres are very much the most common failure on this system, the hydraulic rams can only fail by seizing or leaking, and this is comparatively rare.

The ride height is adjusted by the valve on the rear suspension opening and allowing more oil into the sphere / ram circuit. There are restrictors in the small bore piping between the spheres and the height corrector valve, which prevent rapid changes affecting the valve - the height correction is a very slow / low frequency process, compared to the springing and damping which happens quite quickly.

 

[andy_k]

they are a major part of your suspension.

behind the wheels you have two struts, they look like shock absorbers but in fact they merely transfer the fluid through a series of pipes to the spheres which provide a mixture of springing and damping. The level of the car is maintained by a pump under the bonnet and a self levelling valve attached to the rear anti roll bar behind the offside rear wheel.

If your suspension has gone hard then there is a pretty good chance the diaphram in at least one of the spheres has split allowing the nitrogen gas to escape into the system and the void left is replaced by hydraulic fluid which of course offers no damping.

They can't be repaired, you need to replace them - it's a messy job but not beyond the average DIY mechanic

 

[andy_k]

oooops - snap

 

[andy_k]

try this thread

http://mbclub.co.uk/forums/showthread.php?t=3256

 

[SilverSaloon]

try this thread

http://mbclub.co.uk/forums/showthread.php?t=3256

Superb "how to" there. It should be in the How to Section IMO!

I'm gonna print it out anyway as i am sure one day i'll probably need it!!

 

[Uberwagon]

When I did them on my car I purchased 2 new metal pipes going to the struts aswell as the fittings on these under the car was completely shot, so check these first when ordering parts.

Also to bleed the system, I popped an old fish tank in the boot and filled it with water and brought the engine to a fast idle, it was bled in no time.

Dave!

 

[andy_k]

Dave, the only issue I have with that method is that at some point you will be running the pump dry - not something I'd want to do

 

[pupsi]

When i bled my system, i topped it up with new oil and kept bleeding it until the old oil wasnt coming out anymore but the new oil was.
Stopped it there and finished the job by topping it up with more new oil.

The old oil was black, definitely in need of a change!

Theres no need to put anything in the boot as this will just make the oil piss out, you need the car on a ramp or elevated surface, with all 4 wheels on the ground.
With nothing in the boot and using this method, the oil was dripping out nicely and didnt take too long either

After this, one person in the boot jumping up and down and a second holding the revs at around 2-3k helped the oil go back round the system and into the rear struts once again.

Job done!