Hydropheunamatic compensating spring

[larryled]

Weather is warming now and will soon attempt to replace the old unit on my 1966 250SE Coupe.  I replaced the original unit in 1990.  This one has only 6,000 miles on it but is 30 years old; replacing it with a rebuilt one from MB C.C..  Would very much appreciate any help from all who have done this job successfully.  Thanks, Larryled.

[ja17]

These are easier to replace than the spring. Jack the car up so that the rear wheels hang down. Support the right side of the swing axle with a jack or such so you can un-hook the bottom of the right rear shock. Now gently lower the axle shaft until it stops. Do not allow the axle half to swing down suddenly or you can damage the casting of the rear end. You should now be able to attach the compensator. Use a rod or drift to help align the mounting holes as your installing the hex socket head bolts.

[wwheeler]

How do you know it has failed? Is it leaking? I am curious to know what to look for in the future.

I replaced mine a few years ago and wasn’t too bad. The coil spring was installed prior to my ownership and I wanted to try the compensator. If you are not a purist and want to save some money, you can install the coil spring substitute. I honestly cannot tell a difference in ride quality between the two. I don’t load my car up with passengers or luggage, so that leveling feature will never be used. Just a thought.

[larryled]

Joe/Wallace, thank you both for your comments.  Here’s the deal, I’ve put so much in this car since I bought it in 1978 in Naples that I decided to install it myself.  The first replacement in 1990 was done by a local friend at his shop, and the total cost then was about a third of just what the unit cost today.  We all can relate to that!  W. I had the spring replacement kit but sold that at Star Fest in Winchester, VA several years ago, thinking I’d never need it!  And my opinion of the Spring is that it has a tendency to raise the rear end out of level, whereas the Comp. Spring does not. I’ve owned 5 of these Coupes, and there is one thing, above all that makes them look bad, and that’s the rear being too high, not much, just a bit!  And Wallace, late last year I walked into my garage and noticed the rear end was drooping; later looked underneath and there was visible oil around the unit.  Now I’m assuming that’s the problem.  Makes sense since I drive it very little, which could be the issue.  If we don’t drive these cars, seals, gaskets etc. go bad for lack of use/movement!.  So, thank you again and I’ll keep you both posted on my progress, stay safe, Larry L.

[wwheeler]

I have heard that about the leaking and maybe from Joe. I just wanted to make sure as it has been many years since I heard that. The topic of the compensator doesn't come up much.

[waltklatt]

My '67 230SL has a Hydro in it and it's exercised regularly by pushing up and down on the rear.
Car is on blocks and covered up-hibernating for now.
Walter

[larryled]

Here’s an update on the installation of the Hydropheunamatic compensating spring on my 1966 W111 Coupe.  After procrastinating to no end about doing the job myself, and after reading every bit of information available about installing the unit I decided to do the job alone this past weekend!  It took me two days using a heavy duty floor jack and good jack stands.  Thanks to all those above for your inputs; they were helpful.  Why the unit went bad is unknown but it certainly wasn’t high mileage or rough driving, apparently just age; very likely one of those seals inside that protects the high pressure necessary to operate.  New units are no longer available, only rebuilt ones from Classic Center Parts, and they ain’t cheap!  Bottom line, the car is back level!  LLLL

[ja17]

They will go bad with time. Not letting it set too long without use will help extend the life.

[Mike Hughes]

Not letting it set too long without use will help extend the life.

Sounds like yet another justification for driving 'em 'til the road wears out. ;^)

[Fonzi]

This topic is of concern to me because I had the option to get an LSD rear from a 280se 4.5 that was being parted, but had to give up my standard 280se 4.5 rear end in the swap.  It needed to be done in a hurry, and I kept the hydro compensator with the LSD and sent my spring with the standard rear end.  I liked the high look in my 4.5 and will probably want to swap back to a spring when given the opportunity.

I did recently swap to a spring on my 1967 250se 4-speed because the hydro compensator was bad.  The job was pretty quick and simple having a scissor lift.  It was done as said above, and the main concern with hanging suspension is usually the brake hoses, but since they run down the axle on these swing axle rear ends, that’s not a concern like it is on most cars. 

I now have two bad hydro compensators, one from my 250se, and another in a parts 1969 280se.  I wonder if anyone has rebuilt their own.  It looks to me like the rod end of the piston might unscrew.  That would take one big set of pliers.  LOL. 

[ja17]

These compensators are nitrogen filled as well as well as having hydraulic fluid. Someone is re-building them for the Mercedes Classic Center, so it is possible to have them rebuilt. I believe additional specs may be in the Technical Data Manual for pressures etc. ? Finding a seals may be a challenge.

[wwheeler]

My understanding is that these units are pressurized at high pressures. They are rebuilding these in Germany and possibly where the Classic Center is getting theirs with a core. Probably not advisable to do it yourself.

I had mine rebuilt in Germany years ago and the contact info I had is below:

Gisela Weller
ZF Sachs AG
53783 Eitorf, Deutschland/Germany
Telefon +49 2243 12-681, Telefax +49 2243 12-480

gisela.eudenbach-weller@zf.com

[larryled]

Wallace/Joe, my unit was purchased from C.C. Parts but was rebuilt in Germany by ZF, a global company that does many things in the automotive industry.  It seems Boge, the manufacturer of the unit, no longer makes them, nor do they rebuild them.  There was 4 pages of instructions that came with the unit; (see attachment), unfortunately it is written in German!  I e-mailed the company, ZF, and got a reply back, but that went to an empty hole stating they did not have instructions in English and referred me back to MBCC.  At that point I went ahead with what I had in the way of info from several sources and installed it.  I don’t think ZF is tuned to working with consumers.  The unit cost me $1270 but with tax and shipping final price was $1375.88, ouch!  And would have been more had I not sent CC  an old one, they  require a core and w/o one it’s more!  As a parting shot, I also don’t find it unusual that MB does not rebuild parts; they leave that to the many Vendors making parts and components bearing the MB name.

[wwheeler]

ZF bought Boge and are the same now. Since the Classic Center does sell the rebuilt units now, ZF probably does just refer end user customers to them. It is a huge hassle on both ends to deal with ZF from the US. I had to have a bank transfer, and many years ago, was not a trivial thing. I think I paid around $900 including shipping the core to ZF. I probably saved $200 at the time but was a huge hassle.

[larryled]

Here’s a final note on the Hydropheunamatic compensating spring that I installed on my 1966 250SE/C in 2021!  Boge no longer makes this unit but some company in Germany is re-building old ones, and that what I installed, purchased from MBCC.  Cutting to the chase, the re-built unit was faulty and when driving it pumped up to near normal, but overnight the rear would sag, or droop, not good!  Differential Mounts and other rubber bushings do not cause this gradual lowering.  No need to go into conversations with CC about all this but they finally accepted a return and I am replacing it with a Spring. Definitely not what should be there!  Am having considerable problem finding a spring compressor that works and if anyone has a solution let me know.  Happy New Year all!  Larry L.

[Benz Dr.]

I've posted how to replace without using a spring compressor but it follows pretty much as Joe describes. I remove the screws that hold the right hand spring retainer bracket but leave the outer one installed loosely so it can swivel upward. Hook a long crow bar into the cup that's inside the bracket and compress the spring by prying upward with the crowbar. Having a helper ready to start the inside screw on the retaining bracket helps a lot. It's easier to do and more effective than it sounds.