The self-leveling rear suspension on some W123 models can seem like a bit of a dark art to those who have not had the chance to learn it. Once you know the system, it is not all that difficult to understand or work on. Here is a really good breakdown of the system with some help from Peach Parts member Biodiesel300TD
What does SLS do?
First a little explanation of the Self-Leveling-System suspension in the rear. The Self-Leveling-System (SLS) adjusts the ride height of the rear end when it is loaded to keep the rear end at the proper riding level. The springs and the SLS maintain the unloaded height. The SLS maintains height when loaded by pressurizing the system which lifts the rear. The springs, struts, and accumulators work together to dampen road bumps. The system is made up of several parts; here are the main components and what they do.
1. Pump - supplies pressure to lift the rear.
2. Fluid Reservoir - for maintaining fluid level.
3. Leveling Valve - the brains of the operation, it tells the system whether to lift or lower the rear end.
4. Accumulators (pressure reservoir) - Helps dampen the changes in pressure due to bumps on the road.
5. Struts - Works much like a normal shock for dampening bumps but is able to be adjusted by pressure from the system to change the ride height.
How it all works?
The pump is always creating pressure and pushing fluid through the system. The leveling valve maintains the level of the rear end. It does this by maintaining pressure or diverting it to raise or lower the rear. When the car is unloaded and sitting at the proper ride height the leveling valve is in the Neutral position. In the neutral position the struts and accumulators are still pressurized which maintain the unloaded height along with the springs. The valve maintains the neutral position pressure in the struts and the accumulators by not allowing the pressure to bleed off and also directs the pressure that the pump is creating to back to the reservoir. When a load is put into the back, the lever arm on the valve is deflected into the fill position which diverts the pressure and fluid flow to the struts and accumulators. This pressure expands the struts which lift the rear until the lever arm is in the neutral position again. A check valve in the leveling valve keeps the increased pressure from bleeding off until the arm is deflect into the return flow position. When the load is removed, the arm on the leveling valve is moved to the return flow position which allows the increased pressure in the system to drain off, until the valve returns to the neutral position and the rear of the car to its normal unloaded ride height.
Troubleshooting Failures and Symptoms:
Pump failure - Rear doesn't rise when loaded.
Leveling Valve failure - The rear settles or sags after sitting for many hours, and in extreme cases the rear doesn't rise when loaded.
Accumulator failure - Rides bouncy and/or hard, rear bouncy like when you have bad shocks.
Strut failure - Rides bouncy but not hard.
Diagnosing a failed part:
Many of the problems that arise in the SLS are caused by leaks, so the main thing is to look for leaks.
Leveling Valve - This valve is located just in behind the rear axles and differential, slightly on the left side. The valve can leak internally; in this case you won't see fluid on the valve. It can also leak externally; visible moisture on the outside of the valve. If it is leaking please see our article on rebuilding your leveling valve
Accumulators - Diagnosing this isn't as easy, the accumulators have a rubber bladder in them, with suspension fluid on one side and gas on the other. They can get holes in the rubber and then the fluid gets into both sides and the accumulator fails. By poking the eraser end of a pencil into the accumulator you can feel around the diaphragm for a tear. The pencil should only go half way into the accumulator. If there is a tear in the diaphragm you'll be able to poke the pencil all the way in.
Struts - Struts aren't often the source of problems with the SLS, they rarely go bad. When they do, they usually start to leak; so look for suspension fluid on them (there shouldn't be any).
Pump - The pumps generally don't break, but they can start leaking. They usually leak internally and cause one of two things to happen. They allow engine oil to be introduced into the SLS system, or they allow the SLS fluid to go into the motor oil. If the SLS fluid in the reservoir is black it has motor oil in it, or its really old fluid. Either way it should be changed. If you keep losing fluid but it's not leaking anywhere else, then it is going into the motor. The pump is always creating pressure, which is just sent back to the reservoir when the car is in the neutral position. When the car is loaded the valve then sends the pressure to the accumulators and struts which expands the struts and causes the rear to lift until it hits the neutral position again. The other thing that can happen associated with the pumps are the hoses. There is one going from the reservoir to the pump and one going from the pump to the leveling valve. Make sure they aren't leaking. The one going to the valve is high pressure so if it gives out you'll have a big mess on your hands.
The pump (yellow arrow) and reservoir (red arrow) are located at the front of the vehicle. The reservoir is easily accessible to add or check fluid levels but if you are working on the pump you will need to remove the radiator hose and perhaps shroud.
With a few components removed you can see the pump (red arrow), fluid supply line (blue arrow) and the high pressure output line (yellow arrow).
In this photo along with the pump you can see the reservoir (red arrow), the pump supply line (yellow arrow) and the return line from the level controller
This photo illustrates the leveling valve unit (red arrow) with the adjustable leveling bar (yellow arrow) and how it attaches to the sway bar (blue arrow).
From below you can see the leveling valve unit (red arrow) with the input lines as well as the output lines to the accumulators (yellow arrows).
This photo illustrates the accumulator (red arrow) as well as the strut inside the spring (yellow arrow).