W123 300TD SLS Delete DIY & Review

In the past I used to own a W123 240D, so when I went to look at this 300TD and it appeared that the rear shock absorbers were gone. At the time I thought to myself, “that’ll only be $100-$160 to replace them with some nice Bilsteins.”

Oh but was I wrong… It turns out the W123 Wagons used Mercedes’ self leveling suspension (AKA “SLS”) I originally wanted to retain the useful suspension. But through diagnosing the suspension it looked like the nitrogen filled accumulators, shocks, lines, and reservoir had failed. My sub-$200 fix could very quickly approached the thousands. 

After scouring the internet, it looked like no one would answer how to do it. The response to be found was:

“sell it and buy a sedan” – The Purists. 

But I didn’t want to sell my wagon, instead after weeks of research, and taking a gamble I decided to pull attempt to delete the expensive SLS.

First off, I’d like to make some comments about this conversion.

  1. It is surprisingly time consuming.
  2. This can be very dangerous, even lethal if you are not careful.
  3. You can buy cheap shocks (such as the Sensen), but from experience I would highly recommend going with the Bilstein Heavy Duty Shocks. The Sensen feel very under-damped for this car.
  4. The Klann (Replica) Spring Compressors are necessary, I tried two other styles of Spring Compressors, and they didn’t work. At all. I used the eBay Replica ones, and they worked well. You might be able to rent/borrow a set.
  5. You absolutely need to install smaller Spring Pads / Insulators. Others have installed the smallest OEM Spring Shims with good results. It appears that they were discontinued. Fortunately NAPA has a Spring Insulator that fit well, and came out to about $8. If you leave the Original ones in place, the car sits VERY high, and causes a lot of positive camber.

Here’s a list of parts I accumulated during my conversion, I also threw some part numbers of what I used in there to make it easier to find!

W123 300TD SLS Delete Parts

  • W140 Non-SLS Heavy Duty Springs | Lesjofors 4256869
  • 300D Rear Shock Absorbers | Bilstein HD 24-007146
  • SLS Pump Block Off Plate | http://www.slsconversion.com
  • SLS Pump Gasket (Optional) | MB# 114 236 00 80
  • Klann Style Spring Compressors
  • Spring Pads
  • Shorter Bolts for the SLS Block Off Plate (included w/ slsconversion’s block off plate.

First, you need to start by disabling the SLS system. Without disabling the SLS pump, you run the risk of pumping all the fluid out of the system, and the possibility of the pump seizing and causing damage to the engine! 

Remove the Fluid Reservoir

The first step is to drain the hydraulic fluid from as much of the reservoir and pump as possible. Try to spill as little as possible. As the fluid can eat through the car’s paint. If you spill any, clean it up immediately. 

Start by pulling the lid off of the reservoir and removing as much fluid with a turkey baster. Then removing the line off of the bottom of the reservoir (which feeds into the SLS pump). Use a container to catch the remaining fluid. From here you can remove the return line, dismount the reservoir, and remove it from the car.

Remove the SLS Pump & Install the Block Off Plate

Mercedes 300TD SLS Block Off Plate

The SLS pump is located on the front of the engine. Start by removing the two lines that run to the pump. One of the lines feeds fluid to the pump, the other is the line the runs to the rear of the car.

Once the lines are removed, continue to unbolt the Pump from the engine. There are four 5mm allen head bolts holding it on. Be sure these are clean to avoid stripping them out. Once these four bolts are removed, the pump can be removed from the engine. There will be a “key” that joins the pump to the drive on the car, remove this. The four bolts you removed will be replaced with shorter ones.

At this point you can install the Block Off Plate. Clean off the mounting point on the block, removing all oil, grease, and old gasket. Do the same with the plate. I ended up using the new OEM gasket to draw an outline on the block off plate for the gasket maker. The OEM gasket is made of paper, hence why I used the gasket maker. Once you’ve put the gasket maker on, carefully install using the new short bolts.

Remove the old Hydraulic Shocks and Nitrogen Accumulators.

Now to the rear of the car!

Carefully disassemble the trunk of your 300TD to expose the tops of the struts, and the access holes for the pressure lines to the hydro shocks. I found that flipping the seats forward, and removing the three small screws that hold the cover down works well.

Jack the car up and get it on jack stands! Chock the front wheels so it doesn’t kill you! You’re going to want to get the car high enough to lower the differential. Remove the wheels

Go ahead and undo the fittings for hydraulic lines going into the shocks, mine were accessible from the interior of the car once you removed the stiff access hole covers.

Once that is undone, remove the bolt (might be nuts on some years) that hold the top into place, then get under the car are remove the two bolts holding the bottom of the shock in. The shock should be removable from the car. Go ahead and throw those oily turds in a bin to drain.

From there you can remove the nitrogen filled accumulators. Remove the lines going to and from them. Then remove the little nuts holding them to the body. Also toss these in a bin to drain.

Now for the scariest part.

You need to be very careful during this step. This is where things get sketchy.

On my car, in order for me to be able to get the massive springs in and out of the car, I needed to lower the differential. Which gives you more room by lowering the rear spring perch.

If you cannot get enough room with the diff in place. Start by removing your rear calipers and suspending them safety so they’re not being held up by their brake hoses.

With this completed, there are four bolts holding the rear suspension assembly in place. Support your rear differential with a jack and a block of wood. Undo the four bolts, and slowly lower the assembly a little bit.

From there you can then take your KLANN (clone) spring compressors and compress the springs. Carefully remove the springs from the car, and slowly untension them on the floor. Then remove the original rubber spring perches from the car. I found that with the original spring perches in place the car sat way too high, and actually had quite a bit of positive camber.

Now you can take your new W140 Springs, and compress them, install them in the car, using your new spring insulators to correct the ride height.

Now you can carefully raise the differential back towards the body of the car, and reinstall the bolts. From here the jack can be removed from the underside of the car.

Now you can install your new shock absorbers. I’d highly recommend opting for the Bilstein HD’s, as I originally installed some cheapo Sensen shocks, that were very under dampened for the weight of the car, and blew out immediately. I originally used the bushings they provided in the kit, but I will attempt to reinstall the 300TD style mounts in the future as I believe they are a better design.

Torque everything down, and reinstall your brake calipers.

From here, the leveling unit can be disconnected from the sway bar and unbolted from the chassis.

Your car is now ready to drive!

I’ll update this post, with better details and pictures over time! Feel free to leave any comments with questions or feedback!


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