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Old 08-01-2007, 06:43 PM
Douglas.Sherida Douglas.Sherida is offline
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Join Date: Nov 2004
Location: CT, USA
Posts: 472
DIY: Rebuild the self leveling suspension SLS valve on w123 300TD wagon sagging rear

First let me apologize for the overly long thread title. I wanted to make it amenable to searching the archives.

This post is meant purely as a DIY. Time to give back to the forum.

The car: 1984 euro 300TD turbo, but I think this will apply to most w123 wagons with self leveling suspension (SLS).

Problem: The car rides fine, not too bouncy or bone jarringly stiff *. When the engine is running the car sits level, but after being parked for a few hours the rear end of the wagon sags down a bit. Its most noticeable because in the morning when the car is started you can feel the back end up the car raise up. The SLS valve (figure 1) is wet with hydraulic oil. The SLS actuators (rear struts) are NOT leaking (they are dry).

Causes: 1) One or more of the o-rings in the SLS valve is leaking and 2) the rear coil springs are either worn or broken and should be replaced (or possibly shimmed if it doesn't sag too much), and/or 3) the SLS ride height is adjusted too high. In my case all three are true, but if you can feel your car raise up when it is started at least two of the above are true (unless you just loaded it down with about 400 lbs). If the car has good springs, the unloaded car won't sag even if the SLS is disconnected.

Fix: 1) Rebuild (replace the o-rings in) the SLS valve. 2) Replace the rear springs (beyond the scope of this thread). 3) Adjust the ride height.

Rebuilding the SLS valve:

Tools required: jack and (2) jack stands or (2) ramps, (2) 17 mm wrenches, 12 mm open end wrench, 11 mm open end wrench, (2) 10 mm wrenches (one open end), 8 mm allen key, a small bent pick (dental pick) or very sharp forceps) 1 foot piece of 1/4" ID hose, clean dry plastic bottle, drain pan.

Parts Required: metric o-rings (buna-nitrile or viton) dimensions are listed in mm as cross section (CS) x internal diameter (ID)
(1) 2 x 58, (2) 2 x 7, (1) 1.6 x 10.1 , (1) 1.6 x 7.1 (** see below for availability), 1 liter of MB hydraulic oil (to replace lost fluid, available through Fastlane) the total capacity of the SLS system is 3.5 liters.

Time required: 2-3 hrs

Difficulty: No more difficult than adjusting the valves

1) Chock the front wheels securely and jack up the back end of the car just enough to get under it. Place jack stands under the trailing arms and carefully lower the car. Make sure that the car is secure before you climb under it. Alternatively, back it up onto ramps, then chock the front wheels. The idea here is that the rear suspension should be compressed before you open the hydraulic system and release the pressure, this will minimize the amount of hydraulic fluid lost. (If your springs aren't too bad and you don't eat too much you may be able to do the entire job without jacking up the car.) Ideally, the suspension should be compressed and the car should be sitting level (If you had good springs you could do this).

2) Disconnect the SLS height adjusting rod (figure 1.A, looks like a throttle linkage rod) from the valve lever arm (figure 1.B). Use the open end 10 mm wrench to hold the ball stud and another 10 mm wrench to undo the nut. (The picture for figure 1 was taken with the car jacked up and the wheels hanging, that is why the lever arm is pointing down from "normal".)



3) Start the car and crawl back under it. VERY CAREFULLY release the pressure on the SLS actuators by rotating the SLS valve lever clockwise (CW) about 10 degrees. WARNING: The rear end of the car will come down to its lowest point (resting on the springs), and if your springs are really bad it may squish you. My springs weren't too bad and I could do this with the car on the ground, so I had fun playing with the valve lever for a while (car goes up, car goes down, car goes up, car goes down, etc.). Don't let the car raise too far. I don't know that anything bad will happen, but it seems like a bad idea to overextend the actuators. Turn the car off.

4) Attach a hose to the bleeder valve on the SLS valve (figure 1.C) and put the other end in the bottle. Loosen the bleeder valve (11 mm ? open end wrench) to drain any residual pressure in the SLS system.

5) Place the drain pan under the valve. Disconnect the hydraulic oil feed and return lines from the valve (figure 1.D, 12 mm open end wrench). They may leak oil when loosened, but it should stop in a minute.

6) Disconnect the hydraulic lines to the actuators from the valve (figure 1.E), 11 mm open end wrench). They may leak oil when loosened, but it should stop in a minute.

7) Remove the two bolts holding the SLS valve to the bracket (figure 1.F, two 17 mm wrenches).

Last edited by Douglas.Sherida; 08-01-2007 at 07:01 PM. Reason: Perfection
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