My '99 S600 has suddenly developed a problem in the rear suspension. It rides like I have 50 pounds of air in the rear tires. The car rides so bad that the CD Changer is having a job keeping on track! At the same time the rear has developed a grunting sound on the slightest road irregularities. I remember reading somewhere that the hydraulic accumulators may need attention. Is this a rebuildable part? Any other ideas?

The each of rear shocks have hose that go to "accumulators" or spheres. This is sort of a ball that has a diaphram inside one side is pressurized with nitrogen the other side is fed with hydraulic oil from a pump on the engine and contolled by a valve attached to the rear end sway bar. If the car is loaded it is pressurized, and the reverse if it is unloaded. These things go for about $100 each , and will give you a little oil bath as you open the fittings, they are straight forward bolt in. I paid a local foreign repair shop 2hrs ($120) to forgo the bath, he had to change his shirt also and dropped the exhaust hanger to make it an easier replacement.
Can you do it, yes, do you want to? you have to get it up in the air, ramps maybe!
I think your 600 has ADS which provides two levels of supension setting, comfort or sport check your switch. If the system fails it will go to the firmest setting, you should have a warning lamp, if it did

can't help you with the hard ride but the grunt is another matter.i have a grunt and i've isolated it to my left rear shock.what it is is the bushing that the shock mounts to the subframe on the lower end has dried out or worn out.the problem is that you cannot replace just the bushing-you have to change out the whole shock-the bushing is not available seperately.if you could lubricate the bushing(the outer rubber dia) i think it would go a long way to reducing the grunt.let me know how it goes

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'95 s500 (bought for wife but can't bear to share!!!) 125kms
'92 legend 180kms
'88 tbirdturbo(fantastic car-only regular maint.)120kms
'87 mustang gt(gone)
'86 tbirdturbo(gone)
'85 mustang gt(gone-but not forgotten)

I did the accumulator replacement. I have the factory microfiche library and looked the job up before starting. There is a control valve in the left rear that has a bleeder screw that can drain off the pressure on the system. I removed about a quart of fluid. I did this but the job was still messy since some of the fluid still drips down on you. Both sides were very hard to get at. There is no room to swing the wrenches. A lot of them were loosened with crowfoot sockets. The car has 101,000 mi. on it and the fittings on the left rear were very rusty due to the Northeast road salting. The rear suspension is really complex and all of the components are shoehorned into place. I still have the grunting but I will check it out at a later date and get back to the forum.