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Rear Hydraulic Leveling Unit Replacement
 
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Pelican Technical Article:

Rear Hydraulic Leveling Unit Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$500

Talent:

**

Tools:

10mm, 8mm wrench, 12mm, 11mm flared nut wrench

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

New hydraulic self-leveling unit

Hot Tip:

Be prepared for some spilled fluid

Performance Gain:

Working leveling system

First a little explanation of the Self-Leveling System suspension in the rear of the W124. 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: The Pump - supplies pressure to lift the rear; Fluid Reservoir - for maintaining fluid level; Leveling Valve - the brains of the operation, it tells the system whether to lift or lower the rear end; Accumulators (pressure reservoir) - Helps dampen the changes in pressure due to bumps on the road; 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.

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 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 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.

Failures and symptoms can be: 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 bouncy like when you have bad shocks; Strut failure - Rides bouncy but not hard.

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 driver's side. The valve can leak internally; in this case you won't see fluid on the valve. It can also leak so there is visible moisture on the outside of the valve. Please see our article on replacing 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 is shot. 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. 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 losing 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.

If your leveling unit needs to be replaced, begin by safely raising and supporting your car off the ground. Please see our article on jacking up and supporting your Mercedes-Benz. You will be working under the car with fluid dripping on you so give yourself as much room as possible.

The reservoir for the self-leveling system is located in the front left of the engine compartment right beside the ABS pump (red arrow).
Figure 1

The reservoir for the self-leveling system is located in the front left of the engine compartment right beside the ABS pump (red arrow). You are going to have to bleed the system after replacing the unit and this is where you will add fluid.

The self-leveling hydraulic unit (red arrow) is mounted to the sub-frame on the right side just in front of the differential.
Figure 2

The self-leveling hydraulic unit (red arrow) is mounted to the sub-frame on the right side just in front of the differential. The level sensor arm (yellow arrow) is attached to a bracket mounted to the sway bar.

Brace the back of the bolt on the adjustment arm with an 8mm wrench and use a 10mm wrench to remove the Nylex nut (red arrow).
Figure 3

Brace the back of the bolt on the adjustment arm with an 8mm wrench and use a 10mm wrench to remove the Nylex nut (red arrow).

Leave the arm attached to the sway bar bracket.
Figure 4

Leave the arm attached to the sway bar bracket. There is an adjustments on the arm to set the level position once the car is back on the ground (red arrow).

You are going to need an eleven and twelve millimeter flared nut wrench.
Figure 5

You are going to need an eleven and twelve millimeter flared nut wrench. The hydraulic line fittings are in difficult to reach places. The fittings on the lines are also made from a softer material and can round off easily. If you end up rounding off a fitting you will have to replace the whole line. Be smart and use the right tools.

Use the 11mm flared nut wrench and remove the two upper lines (red arrows) and the 12mm flared nut wrench top remove the two lines on the side (yellow arrows).
Figure 6

Use the 11mm flared nut wrench and remove the two upper lines (red arrows) and the 12mm flared nut wrench top remove the two lines on the side (yellow arrows).

This photo illustrates the flare on the end of the hydraulic line (red arrow) the fitting end is a softer metal (yellow arrow) and if you round it off you cannot get a replacement fitting over the line.
Figure 7

This photo illustrates the flare on the end of the hydraulic line (red arrow) the fitting end is a softer metal (yellow arrow) and if you round it off you cannot get a replacement fitting over the line.

Remove the two 10mm bolts securing the unit to a bracket on the sub-frame (red arrows).
Figure 8

Remove the two 10mm bolts securing the unit to a bracket on the sub-frame (red arrows).

Remove the unit from the vehicle.
Figure 9

Remove the unit from the vehicle. Installation is the reverse of removal. You will need to use the adjusting arm to set the level arm (red arrow) at the right height. You can bleed the system using the bleed valve on the top (yellow arrow). The procedure for bleeding is the same as bleeding the brakes or hydraulic clutch. Please see one of our articles on bleeding your brakes or clutch system for additional assistance.

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