down but not out

[vplager]

My 6.9 suspension is down in the front. It has gotten progresivly worse and now it won't get up after running. The suspension light is off. It works though as I have done some testing on it. I am wondering if the central accumulator is bad. I replaced all of them 10 years ago, but they might be on their last legs. Very bouncy ride. Maybe the nitrogen is gone. Any tests or suggestions? Thanks.

[John Olson]

V Plager,
My apologies for not replying quicker. Since only the front is dropping the first thing I would "suspect" is air trapped at the front end of the system. The W116 Body Service Manual [available from MBUSA Technical Manuals Department in New Jersey (1-800-for-Mercedes) via credit card] describes a bleeder-type fitting in the front and rear plumbing expressly for purging air that is not getting out of the system by normal oil circulation at the filtered reservoir near the radiator. If you have this manual reread the dozen or so pages with photos and charts describing the Hydropneumatic suspension. It took me about six readings to realize it's all there. Everything you ever need to know is really there including this air purging process with photos of where the bleeder fittings are front and rear. As it advise\s, don't bleed the system too far as car's level drops as the air is purged. Stop, run the engine for a minute to re-level the car and then purge again until no air bubbles come out with the oil. If this was the problem, it is fixed. If not the Manual will offer other tests. I own one of the rather pricey factory pressure testing kits mentioned in the Service Manual, and will loan it out with an escrow deposit if you contact me privately.
.
A "bouncy ride" may indicate one or two of the fice nitrogen bulbs needs replacing though I'd take it in steps, and that first step of purging air bubles doesn't cost anything. Also I've not bought into the rationale that of one or two nitrogen bulbs are bad they're all bad, or if one strut is leaking they're all about to go. Quite the opposite, my 1977 vintage is still running well on three of it's original four hydraulic struts.
.
I like my 6.9 immensely. People usually think of them as the second generation 6.3s. To me the 6.9 gives the feel (strong body) and smoothness (suspension and trannie) of a very fast Grand Mercedes 600 rather than being a successor to the 6.3. The 6.9 is a Teutonic pooling of the best of both models. When the 6.9 was developed Stuttgart already knew sales of the 600 (barely 2,000 in the first 10 years) could not justify another generation.
.
I've also owned the W126 (500SE and 560SEL); it is a logical "next generation" but the 5.6 liter V8 is no match for the 6.9's magnificent low RPM torque (in any gear). Contrary to legend it has not been a high maintenance affair either. During 18 years of ownership I've averaged about $1,000 a year on service and repairs with essentially zero depreciation. I believe the car is worth about the same today as the day I bought it with a promising future. If I'd have owned a newer "flagship" Mercedes I would have still had $1,000 or more per year in service plus sizable annual depreciation.
.
Where do you live? I'd like to meet you V. Plager, and your 6.9, at a 2006 MBCA event!

[vplager]

John,
Thanks for your reply. I will try bleeding the system. I have the shop manual for the car. I might be interested in using your gauge and fittings. I live in California.

How would air collect within the system in the first place?

[jyardley]

I had a 79 6.9 come in today that was down in front. I was surprised there even was one in Alaska! Anyway, the owner's first thought was to fill the reservoir with NAPA jack oil, which did not help. Can you tell me where the bleeder is for the front, and, more importantly, what fluid should it have, and will the NAPA jack oil do the usual damage? Unfortunately this car does not even have an owner's manual with it, and I haven't even seen one up here since 1981!
thanks
Jason Yardley

[gerryvz]

Jason,

Are you prepared for some diagnosis time?

The 6.9 suspension is a hydropneumatic system, in which five spheres (a.k.a. accumulators -- one at each corner of the car plus a fifth "master" sphere up front) act as the coil springs of the system, working in conjunction with hydraulic struts at each corner. The system is pressurized by a hydraulic pump at a very high pressure level. There is a height-control function as well that is built into the system, with one level-control valve located on each axle and a master control valve sitting underhood atop the hydraulic fluid reservoir, which is connected via cable to the level-control knob located just above the steering wheel and under the instrument cluster in the center of the dashboard.

The first thing you should do is to raise up the car and check for any obvious hydraulic fluid leaks. You can check for these around all of the spheres and struts, including fittings and such. Often you can see fluid leaking on the outside of the struts but internal leaks are possible as well. Check the level control valves located near the center of the car at leach end. This is a squarish valve with a lever protruding upward from it.

If you don't see any leaks, that's a good sign.

Next, open the hydraulic fluid reservoir and check the level of the oil there. At the proper level it should be around 1/2 full. If it is more than 2/3 full it is TOO FULL and fluid NEEDS to be removed.

It is likely that the reason that the car is down in front is due to a lack of pressure in the system. The pressure is developed hydraulically via a radial piston pump which is located on the front of the block, on the driver's side (it's the round finned pump behind the radiator, about the size of your hand.

Check for leaks around this pump. There are two lines going into it, one from the hydraulic reservoir (behind the driver's side headlight, wiht the round metal cap on top) and the other is the output (under pressure).

The pumps get old and eventually are unable to generate enough to pressurize the system. Sometimes this is as simple as the gasket between the engine and pump leaking, but generally it's because the pump is just old and worn out. There are several o-rings inside of it that give way.

The pumps can be rebuilt (there is a kit to do this) but it is difficult to get everything right and aligned correctly, and 75% of the time they leak after being rebuilt anyway.

The other thing that can cause the front to go down is if the front self-leveling valve is defective. This valve is exposed to the very hot and large 6.9 engine, and also to grease and grime that is attracted to the underside of the engine and suspension. These valves do tend to fail. They are available rebuilt from Star Motors at the link below for around one-half of the price from MB, new.

By the way, ONLY use the Mercedes-Benz hydraulic oil that is specifically intended for the hydropneumatic suspensions. It is available at the dealer. One good alternative exists and that is an oil called "Lowtemp" which is sold by John Olson (a moderator of this forum) out of Minneapolis, which works well in the systems and is considerably cheaper than the MB fluid.

You will need to flush the system to remove all old oil and the NAPA oil and replace it (eventually) with the MB or Lowtemp oil.

There is some documentation and troubleshooting information about the 6.9 hydropneumatic suspension at the following URL:

http://www.300sel.com/pdf/HydroSuspGuide.pdf

Best of luck to you!

Cheers,
Gerry

[jyardley]

Gerry,
Thanks for the help! It is much appreciated. I do a lot of Rolls and Citroen work, so I'm familiar with the basic system, but it's my first 6.9 with a problem, so I'm sure I'll be back!
Thanks again
Jason

[gerryvz]

Well, if you have Citroen experience, it should hold you in good stead. The systems are essentially the same, the MB system being a modified version of the original Citroen system. Unlike what Citroen mechanics commonly do, don't try to recharge the accumulators -- if one or more of them are bad, just replace them with new factory parts. They are not overly expensive to purchase and are readily available.

It's pretty easy to tell on a 6.9 if an accumulator for that corner of the car is bad (and they too often are) -- just press down on a corner of the bumper sharply. If it provides a smooth up and down damping motion, then it is probably OK. If it feels harsh and is anything but smooth, it needs replacement.

Cheers,
Gerry

[jyardley]

Gerry
I forgot the KISS principle! I reattached the link on the front level valve and up it came! Thanks again,though. I now have lots of info if it comes in on it's knees again.
Cheers
Jason

[gerryvz]

Yes, it's true that often the situation is caused by the most obvious problem....in this case the visual inspection for leaks should catch it. Glad to hear that it was that easy.

I wonder how the lever was disconnected in the first place. I'd still do some more visual inspection for leaks and at the rear axle lever as well.

Good luck!

Cheers,
Gerry