What does SLS Do?
First a little explanation of the Self-Levling-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, there is a diagram at the end of this post that shows all the 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 wether 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 hight along with the springs. The valve maintains the neutral position pressure in the struts and the accumulators by not allow the pressure to bleed off and also directs the pressure that the pump 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 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 it's normal unloaded ride height.

Now Some Troubleshooting-

Failure - Symptom
Pump failure - Rear doesn't rise when loaded.
Leveling Valve failure - The rear settling or sagging after sitting for many hours, and in extreme cases the rear doesn't rise when loaded.
Accumulator failure - Rides bouncy and/or hard, rear bouncys 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 drivers side. The valve can leak internally, in this case you won't see fluid on the valve. It can also leak so there is visable moisture on the outside of the valve. If it is leaking I have available a o-ring kit to solve the leaking problems. Visit here for more information - SLS Valve O-ring Kit Info - Also make sure to visit the DIY on replacing the O-rings - O-ring DIY
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 is shot. By poking the eraser end of a pencil into the accumulator you fan 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, the rarely go bad. But when they do the usually start to leak, so they will have suspension fluid on them and they shouldn't.
Pump - The pumps general don't break they usually start leaking. They leak internally usually, 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 loosing fluid but it's not leaking anywhere else then it is going into the motor oil. Either way you've got to rebuild the pump. There are kits available for this. 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.

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