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Old 02-10-2005, 05:48 AM
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Join Date: Oct 2002
Location: Australia
Posts: 44
Hydro Suspension 450sel 6.9

Could anyone explain the basic operating function of the hydro suspension system. This vehicle has been sitting for some time and after starting it up the suspension would not rise. So I changed the oil pump,the vehicle shot up to it's max hight and will not come down. Only if I drain the oil from the spheres will it lower but only to rise again to it's max hight once the engine is started up again. The system has 5 shperes, why? Please help.

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Old 02-10-2005, 06:53 AM
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Join Date: May 2001
Location: Northern Va.
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Hydropheumatic, water/air, or to be more accurate, oil and nitrogen. The pump suplies oil under pressure, the spheres have a membrane seperating the nitrogen from the oil. There are leveling valves front and rear.

Check to make sure the leveling valves and their linkages are properly adjusted.


Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL


Not part of the in-crowd since 1952.
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Old 02-10-2005, 06:55 AM
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Join Date: Feb 2001
Location: Saint Marys Ga
Posts: 103
Thumbs up 6.9 suspension

you should have a 3 position switch that raises and lowers the suspension.(it is called the valve unit adjusting switch)s is for lower n is for normal and h is for high.if this switch is not working,its a good place to start troubleshooting.if your warning light stays on after the car has lifted,began checking the system for leaks.haynes manual #63030 has a good diagram of the system.good luck.
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Old 02-11-2005, 12:31 AM
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Join Date: Oct 2002
Location: Australia
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6.9 Suspension

Thank you Robert for your reply. Could you send me a diagrame of the hydraulic piping etc.
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Old 02-11-2005, 01:42 AM
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"Unhinged Troll" - Jim B.
Join Date: Dec 2001
Location: The Woodlands, TX
Posts: 1,268
The 6.9 suspension is an evolution of the hydropneumatic system that was pioneered by Citroen in some of its models.

The system as found in the 6.9 (as well as certain W126 models as an option, but not in the USA) operates on the principle of gas pressure with hydraulic level control. The vehicle load is supported by four suspension struts which simultaneously serve as shock absorbers. The individual suspension elements consist of one suspension strut and one pressure reservoir (accumulator), each of which are interconnected by a hydraulic line. The vehicle load is hydraulically transmitted to the pressure reservoirs via the suspension struts. Suspension is accomplished by the compression and decompression of the gas (nitrogen) cushion in the pressure reservoirs.

To regulate the vehicle level, the hydraulic oil volume in the suspension struts is increased or reduced by means of a hydraulic system. The hydraulic oil system is comprised of an oil pump (located on the front of the engine block); a pressure regulator (atop the oil reservoir); and an oil reservoir. (behind the left front headlight). The pressure regulator and the level selector valve of the control system are combined into one valve unit.

The main pressure reservoir supplies the system when the engine is not running.

The control system controls the oil volume by means of one leveling valve each at the front and rear axle and by the level selector valve of the valve unit. The following adjustment of the system can be made by means of the level selector switch (directly above the steering wheel on the dashboard):

N - normal level - control knob pushed in or control disc at the front stop
S - locked position - control knob pulled out completely, or control valve at first stop
H - high level - control disc at second stop, only with cable disengaged
M - assembly position - suspension system pressureless with control disc pulled to rear stop with cable disengaged.

The hydraulic oil pump, driven by the engine, delivers oil from the oil reservoir to the main pressure reservoir via the pressure regulator of the valve unit. When the max. oil pressure in the main pressure reservoir is reached, the pressure regulator of the valve unit will reverse the oil flow. The oil now delivered can flow back into the oil reservoir.

If the pressure in the main pressure reservoir drops to the minimum pressure as a result of consumption (lowering the level by opening the levelling valve), the pressure regulator will reverse and oil will be pumped into the main pressure reservoir until the max. pressure again is reached.

The hydraulic oil in the main pressure reservoir is connected to the level selector valve of the valve unit and to the levelling valves by means of hydraulic lines.

If the vehicle level drops due to increased load, the levelling valve will open the passage to the suspension struts. The oil flowing into the suspension struts will lift the vehicle until the normal level is attained and the levelling valve has again closed the connection. If the vehicle load is reduced, the levelling valve will permit oil to flow from the suspension struts until the vehicle is back to its normal level. The oil flowing out of the suspension struts is returned to the oil reservoir via a filter.

The system operates at a pressure of 3,000 PSI.

The "fifth" pressure reservoir (accumulator) serves an equalizing function and is the reservoir that is connected to the red light on the dashboard that warns of inadequate system pressure. It supplies both levelling valves with hydraulic oil pressure independent from the hydraulic oil pump. With the engine stopped,it also serves the purpose of compensating for the volume changes in the pressure reservoirs caused by temperature fluctuations.

I hope this helps answer your questions. I can post diagrams if you wish, to help illustrate how the system operates.

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Old 02-11-2005, 06:59 PM
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Join Date: Oct 2002
Location: Australia
Posts: 44
Hydropneumatic 6.9

Many thanks Gerry, this is a great rundown of the setup and it's function. Reading this, it seems my main regulating valve must be faulty since it is not possible to lower the suspension by moving the lever on the main valve to any position at all. (I can lower it by draining fluid from the accumulators) But when I restart the engine it will go up to max. hight again, I guess only the shocks stop it for going all the way to heaven. Any thoughts on that?? Once again thanks for your time.

The diagrams would be handy.
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Old 08-16-2011, 09:17 PM
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Join Date: May 2011
Location: Sydney, Australia
Posts: 17
Can Penrite MB15 suspension oil be used in a 1979 450 se 6.9 mercedes fitted with hydropneumatic suspension
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Old 08-17-2011, 12:32 AM
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"Unhinged Troll" - Jim B.
Join Date: Dec 2001
Location: The Woodlands, TX
Posts: 1,268
I would recommend just using the hydraulic suspension oil that is available at your local MB dealer, rather than trying to utilize a replacement that may or may not be suitable.

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