K6JRF at the mic K6JRF's Page
formerly W6FZC

My Mercedes Benz
S500 Coupe

K6JRF's MB S500 Cpe
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(New: Sep 11, 2019)

Hydraulic Suspension Problems & Repair

My '94 S500 coupe's suspension developed some problems and I wasn't aware of it until AFTER I fixed it! I know that sounds strange but since I've retired my driving has been limited . . so it appears that the suspension has gradually degraded and I didn't pay any attention until I found a small "puddle" on the garage floor by the back right wheel. That got my attention quickly! This is how it was repaired.

Diagnosis and Repair of the 1994 Hydraulic Suspension System
The suspension system consists of two shock absorbers in the front of the car and two hydraulic struts in the rear of the car. The front absorbers are the traditional type and there's no need to explain them since they work 'by-themselves' as they are conventually oil filled and self contained.

The rear shocks are hydraulic struts and there are a lot of components in this part of the system. These struts have a supply line feeding them with hydraulic oil pressurized by the power steering pump.

Under the hood on the right side of the engine under a removable cover, there is a small reservoir that holds the hydraulic oil as shown in the picture. You should REGULARLY check the fluid level with the car fully warmed and running by removing the cap which exposes that it's a dipstick. There are two marks; MIN and MAX. The fluid level should be between these two marks.

I have not checked this level in 5+ years! And the last time the suspension system was flushed was in 2010, 9 years ago! Since I don't drive it alot these days, why would I need to check these things???

About 2 months ago, I noticed that the rear end was low, barely sitting above the back tires. This is somewhat normal b/c if the car sits for a long time (a month or more), the hydraulic pressure will drop, causing the rear end to sag. Of course, when I started the car, the rear end would quickly rise and so all was OK, so I thought. I didn't notice that it would not come up all-the-way to where it used to rise. It would rise comfortablely above the rear tire width so there was no "rubbing". However yesterday, I saw the small puddle of hydraulic fluid on the garage floor near the right rear wheel.

Pneumatic Suspension System Operation
The power steering pump is used for two systems, one for the steering, and the other for the rear suspension oil. The pressurized oil is sent to a valve near the rear of the car. This valve is attached to the under body of the car. The valve lever is attached to the rear axle. As long as the car is at the correct height, there is no movement of the arm relative to the valve. However, as soon as extra weight is put into the car, the body squats down. This motion causes the arm on the valve to move, and that opens the supply port for the pressurized hydraulic oil. The oil passes through the valve, and eventually makes it's way into the struts. Since oil is being forced into the struts, they begin to fill up and lengthen. This raises the car until the valve arm is back into normal position, which stops the supply of oil. This is how the vehicle maintains the proper ride height in the rear.

When the weight is removed, the same thing happens in reverse. The valve arm moves in the opposite direction, which causes the valve to open a drain port. Oil is allowed out of the struts back into the system, and they get shorter again. Motion stops once the proper ride height is reached.

With a hydraulic system like this, there is no shock absorbing capability. Since the fluid will not compress, something else is needed to soften the ride! To accomplish a soft-ride, Mercedes used nitrogen accumulators as they are called. I have replaced both apx 8 years ago. In the diagram, this is just the basic system with no-frills!

When I saw the fluid puddle on the garage floor, I checked the reservoir's dipstick and saw it was below the MIN value. So looking down into the reservoir with the filter removed, the level was quite low . . . maybe 6 - 8 ounces low. That's not enough to account for a rupture of an accumulators' diaphragm. So maybe it's just time and wear?? So what to do??

The "FIX"!
I have a bottle of the recommended fluid (MB Hydraulic Oil, PN 000989910310) and some Lucas Power Steering Stop Leak. Since the fluid hasn't been flushed, I needed some not-to-expensive hydraulic fluid so I purchased one gallon of Lucas Hydraulic Fluid for a very reasonable price. I used this as the sacrificial fluid to be able to thoroughly flush the system using the complete gallon if necessary. And was it dirty! And took quite a bit of the Lucas HF gallon before it was clear *.

After flushing, I pumped the reservoir down until it was almost empty. Then I added apx 15 oz of Lucas Stop Leak. I should note that it is compatible with all oils and hydraulic fluids so that's what gave me the security to do this as is shown on the back of the bottle.

I filled the reservoir with the MB fluid + Lucas SL + a few ounces of Lucas hydraulic fluid to bring it up to the MAX level.

I went for a long ride . . . 50miles . . . and when I returned, the rear end was up to where it used to be . . . which I had forgotten. And the best part, there was no leaks!!! Can you say . . pleased and happy!

So over the 9 years of my car's minimal driving, the hydraulic fluid had deteriorated and some had evaporated due to a leak in the system which caused the rear-end to drop and not be able to fully inflate the struts to gain the needed height. I guess you can blame that on my neglect and non-driving. The ride is a bit 'stiff' but not any worse than it was before.

The picture showing the right rear tire was taken in 2007 and shows the oversize tire, 255x45x18 mounted with just enough clearance even for that large oversize tire. Today, 48 hours later, there's no detectable sag in the rear end.
Power Steering Repair
This is not the end of the story: I thought that this article would be a good place to explain my power steering "fix" of apx 6 years ago. As you were reading, you might have wondered why would I have Lucas Power Steering Stop Leak available?? It's simply b/c I used it to fix a PS leak that had many 'spots' on the garage floor before I put a pan under the area.

It was diagnosed by my tech during a routine oil change. You could see the flow of PS fluid in the area. It looked to be the PS pump, not the rack and pinion. So when I got home, I cleaned it up and added the required amount of Lucas PS Stop Leak fluid. Not sure of the amount but I completely drained the reservoir, removed and cleaned the filter and then filled it up with Lucas Stop Leak. Apx one year later, when I went for an oil change, there wasn't even a drop of PS fluid to be found. And that was at least 5 years ago!! Today, my S500 coupe's power steering is still as good as new and there isn't a drop on the garage floor!

If this appears to be a blanket recommendation for Lucas Oil Products, you would be correct! And why wouldn't it? In the two (2) instances that I have used their products, I'm batting 1000! I recommend that you try this if necessary before replacing parts! Who knows . . . you may have similar results.

*: To perform a hydraulic fluid flush, unscrew the Return Line fitting and then add apx 4-5 ft of clear plastic tubing (makes it easy to see the color change) and put the other end into a bucket. Also remove the FILTER element by unscrewing the plastic nut. Remove the filter assy so that you can see the fluid level.
Next, start the car and observe the old fluid being sent into the bucket. Add hydraulic fluid as needed to keep the reservoir full. When the fluid turns 'clear', continue to pump until the reservoir is almost empty. Turn the engine off. Next add the chosen amount of Lucas Stop Leak - apx 15 oz - and fill the reservoir up to the level that is just above the metal-holding-strap. Remove the plastic drain hose and re-attach to reservoir fitting after replacing the filter element. Add any needed amount of hydraulic fluid to get the dipstick level between MIN and MAX.

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