Hydropneumatic compensator - PeachParts Mercedes-Benz Forum

atexx77 · #1 07-21-2006, 03:36 PM

Hello!

I have to replace the Hydro...comp in my '72 280SE. I found a site to buy the Compensator spring kit and I read on this site is common to replace this animal. Does anyone know how difficult it is to do this? I am a weekend mechanich and I do not want to get myself into anything I cannot finish! Any advice?

Tex

Mark DiSilvestro · #2 07-21-2006, 05:13 PM

Last year, I did one on '66 250S. In my case, the old hydro-pneumatic compensator had lost all pressure and collapsed, so there was no tension or problems removing it. (I hear that the compensators sometimes will fail in a 'pumped-up' condition, so in those cases, removal may be difficult) For me, the tricky part was that I had to use a spring compressor to install the new coil compensating spring, as even with the axle halves hanging down, the coil spring was still too long to easily pop into place. The spring compressor I had wasn't an ideal fit for the small diameter compensating spring so even with the compressor, I had to fasten the replacement right-hand spring bracket with one bolt, then jack or pry the bracket against some spring pressure until I could line up the holes to install the second bolt.

Happy Motoring, Mark

nhdoc · #3 07-21-2006, 08:04 PM

I have heard one trick is to remove the top retaining nut off of one shock (I think left side) which will allow the swing axle to lower more when lifted to aid in removing or installing the spring. My '72 has the spring and I bought a PC for it but have not yet installed it. Next "rainy day" project I will probably tackle it.

Mark DiSilvestro · #4 07-21-2006, 08:35 PM

I thought about undoing a shock, but didn't want to risk dropping the swing axles too far and overstressing something or having the sliding joint inside the right swing-axle possibly come apart on me!

Happy Motoring, Mark

wbain5280 · #5 07-21-2006, 11:53 PM

The official factory manual shows a spring compressor being used to remove it.

nhdoc · #6 07-22-2006, 07:58 AM

Quote:

Originally Posted by wbain5280

The official factory manual shows a spring compressor being used to remove it.

This is true, but like many jobs, clever techs over the years have some up with ways to either make the job easier or do it without a special tool (the particular compressor is a special MB tool according to my shop manual). I know there was once a thread I started on this subject and one of the responses, from a person I have respect for, told me about the shock trick so I passed it along...now the old thread seems to have vanished!

I'm sure you could gerry-rig a spring compressor to fit the compensating spring, but if you could do the job without having to pre-compress it that would probably be safer...

Mark DiSilvestro · #7 07-22-2006, 08:51 AM

Having done this job, I believe loosening one or both shocks alone won't let the axles drop far enough to be able to fit the compensating spring without a spring compressor. The factory manual does mention loosening the shocks for some jobs but says to not leave the axles unsupported. With the spring compressor, freeing shocks might have made my job easier, but since I was already so close, I chose to press on, pry against the bracket, and move the bolt hole that last fraction of an inch into place.

Happy Motoring, Mark

Tony H · #8 07-23-2006, 02:08 AM

If you remove the shocks the alxe will drop enough to hang by the brake hoses. It will pull apart the hoses if you do not remove them.
Tony

Mark DiSilvestro · #9 07-23-2006, 09:42 AM

Quote:

Originally Posted by Mark DiSilvestro

I thought about undoing a shock, but didn't want to risk dropping the swing axles too far and overstressing something or having the sliding joint inside the right swing-axle possibly come apart on me!

As I was on the ground, under the car and didn't want to dissassemble more than was absolutely necessary or wrestle with 2 - 3 jacks and possibly risk messing up something, I installed the new spring with the shocks connected. In any case, I don't think this job can be done safely without some kind of spring compressor. With a better spring compressor than I had, one might be able to finish without prying the bracket that last fraction of an inch into place.

Happy Motoring, Mark

atexx77 · #10 07-24-2006, 10:08 AM

Thanks everyone!

Your advice helps! I will let you all know how it turns out!

Tex

Bob G · #11 07-29-2006, 02:03 AM

Some say that the peumatic compensator came on the last 280SL,s. I have yet to see one fitted with it. How expensive are these to purchase and while buying a used one may be less money , I heard their is no rebuild kits for these unites.

Bob Geco
1968 280SL

Mark DiSilvestro · #12 07-29-2006, 09:35 AM

Quote:

Originally Posted by Bob G

Some say that the peumatic compensator came on the last 280SL,s. I have yet to see one fitted with it. How expensive are these to purchase and while buying a used one may be less money , I heard their is no rebuild kits for these unites.

Bob Geco
1968 280SL

I never saw one on an SL. The units were supposed to provide a degree of auto-level to the rear suspension. I've only seen them fitted to the larger sedans, such as the W108, which were more likely to carry widely variable loads of passengers and luggage.
I've heard of prices in the range of about $1200 (aftermarket) to over $2000 (dealer) for new ones!

I haven't heard any complaints resulting from replacing the compensator with the spring kit, and today, our cars are less likely to be pressed into service as taxis or cargo haulers.
Awhile back, someone on this Forum said that the cylinders were rebuildable, but I've never heard of a rebuild kit being availible. I wouldn't trust a used hydro-pneumatic unit and prefer the reliability of the steel spring over having to buy an expensive gas/oil cylinder every ten years or so.

Happy Motoring, Mark