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Pierre Hedary

SLS Hydraulic Leveling System: Schematic

Old School – Pierre Hedary
Keeping It On the Level
Understanding the SLS Hydraulic Leveling System

The Mercedes-Benz 450SEL 6.9 first appeared on our shores in 1977. The discreet 6.9 badge signified that this car was special. In addition to its marvelous engine, the 6.9 was the first Mercedes-Benz with an optional hydraulically operated active suspension system. Known as SLS (Self Leveling System), the 4-wheel system revolutionized the way the heavy 6.9 and the successor 500SEL handled. The 1979 300TD offered a more practical version of this technology to level the car by adjusting ride height just at the rear axle when needed. Mercedes continued to use this rear-wheel system on all of its models well into the late 1990s, when systems such as Airmatic and Active Body Control [ABC] replaced it. From its introduction, SLS was available as an extra-cost option on every Mercedes passenger car line, except for the R107 roadster.

How does it work?

The SLS system, diagrammed above, is driven with hydraulic pressure, which is different from the air compressor system on the W100 and W109 chassis. The SLS pump, which is driven from the timing chain on most models, pressurizes a set of variable-height shock absorbers. This fluid pressure must circulate through the leveling valve. Ride level is controlled with an arm positioned from the rear sway bar to the leveling valve. As the sway bar moves, it raises the arm on the leveling valve, allowing more pressure to be allotted to the shock absorbers, thus adjusting ride height.

The SLS valve is always responding to changes in road surface, weight transfer during cornering and, of course, the amount of weight over the axle. This makes it an active suspension system, especially in cars with 4-wheel SLS.

SLS components

The best way to understand the SLS is to understand what each component contributes to the system, and where it is located within the system.
• Hydraulic fluid reservoir: On vehicles with rear-wheel SLS, this is a clear plastic bottle, located in the front right or left fender, under the hood, just behind the headlight. It has a fill hole, with the lid for the fill hole doubling as a dipstick. A metal return line goes into the top, and a supply line exits at the bottom. The reservoir also contains a system filter. On cars with 4-wheel SLS, the tank is metal and is in the left front fender.

• The hydraulic pump: This pump is engine-driven, with the exception of units on M103-, M104-, M119-, and OM60X-series engines. (On these chassis, it is integrated into the power steering pump and is driven off the serpentine belt.) The standard hydraulic pump is silver and finned, and is mounted at the very front of the engine, pushing fluid through hydraulic pressure hoses to the respective leveling valves. These hoses are known to be common sites for leaks, so inspect them often.

• Main leveling valve: This valve is only present on cars with 4-wheel SLS. It has a cable attached to it so that the height of the vehicle can be adjusted on the fly. It has a pressure-regulation function that keeps fluid pressure elevated in the entire system. Needless to say, failure will cause the entire vehicle to sink.

• Front axle leveling valve: Again, this unit is only available on vehicles with 4-wheel SLS. On 116 and 126 applications, it is at the back of the engine bay, attached to the front sway bar with a linkage, with a mess of lines going through it. This valve rarely fails, but it if does, the front end squats down and does not rise when the engine is started.

• Rear axle leveling valve: The rear valve is located just behind the differential mount, and is tied to the sway bar with an arm that adjusts rear ride height in response to sway bar movement. In 1986, this valve was updated so that it was smaller, and consequently more difficult to repair.

• Main accumulator: This acts as a pressure reservoir for cars with 4-wheel SLS. It is located in the front left fender, by the wheelhouse, and is quite prominent. If it fails, the car may stay up in operation, but will sink down as soon as the engine is off. Also, system response will not be nearly as healthy with a failed unit. New units are expensive, but rebuilds are available.

• Accumulators (pressure reservoirs): These are smaller and located underneath the car by each wheel. They have a metal pressure supply line going in, and a line going out to the shock absorber. Failure of these units will be discussed under the troubleshooting section. Replacement is easy and parts are cheap. Accumulators have a membrane separating a nitrogen cell from the fluid pathway. If this membrane bursts, then the accumulator fails.

• Front hydraulic shocks: These are only present in 4-wheel SLS applications. They are not spring assisted, and they replace both the gas-filled shock and the spring itself. These shocks can leak and should be checked often.

• Rear shocks: These are spring assisted and located inside the spring, or next to it, taking the place of a conventional shock. They also have a ball joint in the bottom, which can wear out and lead to an unpleasant thud on bumps, even while the rest of the system works well.

Diagnosing the SLS system

 SLS is easy to work with on rear-wheel applications, but a bit more complex (and expensive) on 4-wheel systems. Begin diagnosis by checking fluid level. Low fluid level may indicate a leak, most likely at one of the following places:

• The hose from the reservoir to the hydraulic pump

• The high-pressure hose that runs from the engine to the rear axle or to the main height control valve

• The main, front or rear leveling valves

• The cylinder of each shock absorber

• The hoses that feed each shock

If you add fluid and it disappears, yet no external leaks are visible, then the hydraulic pump may be leaking fluid back into the engine. If this is the case, you may notice your engine oil level rising. A burst accumulator can consume fluid as well.

Once you have established the source of leaks, the check writing begins. Front shocks are available rebuilt from Star Motors, while rear shocks are available new for somewhat less. The leveling valves can be rebuilt with kits from Mercedes on early applications. On 201, 124 and late 126 cars, the rear valve cannot be rebuilt, so if it leaks, you will need to install a new or serviceable used part. The main valve on 4-wheel cars can be rebuilt. All hoses are available new and reasonably priced. Hoses with removable ends can be rebuilt successfully.

Diagnosing height and ride issues can be a bit more involved. Height issues should be addressed by inspecting the linkage from each sway bar to the respective leveling valve. The linkage has tiny ball joints and when they deteriorate, the linkage becomes detached. Buying a new unit is cheap and easy, so plan on doing so with any car that has not been running for a while. You must adjust the linkage with the car running and on the ground – or on ramps. Although there is a factory setting for this, many must be fine tuned to their respective leveling valve. Oftentimes, the linkage for the rear valve on a 123 cannot be shortened enough, necessitating the manufacture of a bracket leading from the rod back up to the arm on the valve.

The rod should be adjusted so that it is just barely off the threshold of picking the car up. This means that under any circumstance of change, it will immediately react.

If the car bounces while driving or refuses to handle bumps well, then the accumulators may be at fault. Try opening the metal line slightly near each accumulator. If a bubbly mixture like shaving cream squirts out, then there is nitrogen in the system and an accumulator has failed. If you can afford to, replace all of the accumulators so that the result is uniform.

Finally, if the car bleeds down after it is parked, then you have a leveling valve that is dumping pressure. On 4-wheel SLS applications, this could be the main valve or an accumulator. On rear-wheel versions, it is often the rear leveling valve that is  at fault.

Bleeding and servicing

All SLS systems have a filter. Changing the fluid and filter every 30,000 miles is a good idea. To do this, disconnect the metal line at the reservoir. Run a hose from said line into a catch pan and start the engine. As the engine pushes old fluid into the pan, add new oil to the reservoir to replace it. Do this until the oil is clean. Make sure to use the Mercedes or Febi hydraulic oil. Anything else will ruin the system.

The filter is inside the reservoir. Most reservoirs use a removable twist lid with the filter hanging down from the bottom of the cap.
To bleed the system, jack up the car or put it on ramps. Make sure that the vehicle is absolutely stable! Start by disconnecting the linkage on each sway bar and then start the engine. Undo the bleeder screw and move the leveling arm up and down slowly with the screw cracked until no more bubbles are present. Next, go to the fluid supply line at each shock and bleed the system out where the hose bolts to the shock itself. When you have finished, set the car on the ground and use some ballast to determine if the system is rising when needed.

SLS in daily use

Given the number of 123 and 124  station wagons in service, the SLS system has  been shown to be durable and helpful. Before you fault the system or think of removing it entirely, remember that it was designed with serviceability in mind, and that all the parts are still available. While some items, such as height control valves and shocks can be expensive, the alternative is dismal: installing a plain old suspension on these fine cars. Before you (or your mechanic) decide to dump the SLS system, remember that with time and dedication, anything can be repaired.

In the meantime, my hat goes off to those of you who pursue the perfection of originality built into your vintage Mercedes-Benz.

Happy motoring to all! 

Looking into the engine bay of a 450SEL 6.9. The Blue Hose seen in the cengter runs between the hydraulic pump, the reservoir, and the main control valve.

Close-up view of the front SLS hydraulic valve as installed on the 450SEL 6.9

Engine bay of the 300TD. Note the location of the hydrawulic bottle on the right and the hydraulic pump on the cylinder head, just below the uppper radiator hose.

Black, an accumulator(pressure reservoir) in place.

Example of an SLS hydraulic pump.

An SLS hydraulic shock absorber.

SLS Rear suspension layout: 123, 124, 126, and 116 series.

SLS-Four-wheel system; Models 126.033 & 126.037