PeachParts Mercedes-Benz Forum - View Single Post

stevebfl · #15 11-25-2002, 08:32 AM

Let me point out some theory on the accumulators. Knowing how they work is a real benefit to understanding their fault.

First, one must understand that their function is to be a hydraulic "SPRING". In the box the chamber has some high pressure - say 1000psi. The diaphram is plastered against the sides of the sphere. It is now installed and the motor is started. The pressure of the fluid instantly raises to the pressure of the nitrogen. This is because once filled the fluid is not compresible.

The fluid is now at 1000psi (forgive me I don't remember the actual pressure but 1000 works for understanding). The pump is running and fluid is displacing the diaphram while squeezing the nitrogen. Somewhere at say 1500psi the car is rising and leveled. As this car is now driven the shock is forced up and down by the action of the road. As the shock is compressed the fluid goes into the accumulator and further squeezes the nitrogen. The pressure now hits 2000psi and more than supports the car. The suspension rebounds and the pressure in the sphere drops as the shock rebounds. All this activity is dampened by the regular shock activity of the strut.

On cars that I have owned (2 shop wagons - 280TE, 300TE current) we have replaced the accumulators every 50-75k. The reason behind this goes to the rest of the story. The diaphram holds the nitrogen inside like a tire holds air; only the pressure is tremedous and the acitivity continuous and there is no way to add air. Gradually the nitrogen passes through the membrane and the entering fluid pushes the diaphram further with each year. Eventually the diaphram reaches its stop during some form of shock compression. At first only the big bumps hit bottom, eventually with no nitrogen the hydraulics are non-compressable and the only suspension left is the tires and seat springs. The nitrogen that leaks is dissolved at high pressure in the hydraulic fluid and then in the reservoir at low pressure it is given off into the atmosphere.

BTW, Nitrogen is somewhere around 78% of the air we breath and is used because its available and non reactive.