I just completed a seat repair using a technique that I'm not aware has been tried before. My '85 w123's driver seat had 5/6 springs broken and poorly repaired (welded with coat hangers, etc) when I bought the car, while the passenger seat was in perfect condition. This seems to be a pretty common problem. I set out to repair the springs while minimizing the risk of breaking any trim, as my car has the apparently rare blue interior, which has been entirely replaced by the previous owner and is in incredible condition. I did not want to try the pool noodle or splicing technique, as I am trying to keep this car as original as possible.

Your results may vary, you do this at your own risk, I take no responsibility for damages, wear proper safety gear, etc.

Essentially, the idea is to swap individual "springs" between 2 or more bases to repair your springs. The disassembly steps will need to be completed on both your original and donor seats. Typically for a donor seat, you would like to try to find passenger seats from a lower milage car. I was able to use a driver's seat from a rusted out 240d that had ~100,000 miles. I believe you can also get parts from a w126, though I can not vouch for that personally.

To do this, you will need a good set of seat springs from the junkyard. In my case, the front 5 springs on my original seats were broken. There were 6 springs total, the front two are unique, the remaining 4 are similar if not identical and can be swapped around using this technique, so you just need to try to get enough parts to repair your seats.

In my case, I chose to remove the seats from the car - as I could not get the seat covers off due to the clearance between the console and the inner seat.

Next, I removed the seat back. Depending on how your seat padding is attached to the springs, you may be able to skip this, however my car has replacement "horsehair" pads which were hog ringed into place, which meant I really needed to get the backrest off to gain access to the hog rings. Fortunately while removing the seat from the car, the outside and inside hinge covers popped off on their own accord, so I was able to skip removing these. Once the cover has been removed, there should be two (10mm?) bolts holding the back to the base. Remove those carefully and set the seat back aside.

Now you'll need to remove the covers. Press down on the seat, and pull the cover away from the frame. It's just held down by a strip of material locked into a grove. Make note of how this is attached. You may also have hog-rings holding the seat fabric to the springs and/or pads. My car did not have those.

Next you can remove the foam/"horsehair" pads to gain access to the spring. You might need to cut hog rings, which I used a rotary tool with a cutting wheel for due to not being able to get good leverage with my angle cutters.

Now comes the fun part, swapping springs between seats. You'll notice that each spring is connected in at least 4 places. Where the spring attaches to the metal base, there are tabs which you will need to separate. Where the spring attaches to the "band" that forms the top edge of the seat, there are 2 or more metal straps that are wrapped and crimped around the spring and band to attach them.

First, lets look at how to remove the springs from the bases. You'll need a screwdriver you don't mind damaging, a small pry bar, and a hammer. simply start by working the screwdriver or pry bar in to the tabs, then pry the tab up enough to loosen the spring. In my case, it made sense to pull the entire donor assembly out intact, instead of swapping springs. Your technique may vary.

Now, if you need to piece together two spring assemblies to make one, you'll need to remove the straps that hold the individual springs to the bands. I used a cold chisel to lift one corner of the strap, then worked a screwdriver and eventually pry bar in to pry the strap away from the band and spring. Locking pliers may also be of assistance here. Remember that you need to preserve these bands in order to reassemble the seat.

Once you have enough good components to build your new spring, start by assembling the springs to the base. Just slide the springs back into the tabs you pried open earlier, and make sure that they are sitting properly (there are small bumps which will stop the spring from moving). Now you can use a hammer and drift to push the tabs back in place, and lock the springs in.

Then it is time to lock the springs to the metal band. Reinstall the bands you saved earlier, using locking pliers or c-clamps to wrap them back around and crimp them very tightly. I recommend following this up with a rotary tool and a grinding disk, or a file, to remove any sharp edges caused by prying against the bands.

Finally, the improvement that will make the seats better than new (at least in the case of the w123). You need to source four "Foam Seat Support" parts from a late model w126. These look like pool noodles, but are made from a dense foam similar to carpet padding, wrapped in a rubber-like coating. Insert these into your coil springs, I suggest leaving the uncoated end pointing up - as I suspect the metal seat frame might abrade this with time. If you are not repairing broken springs, but want to improve them, you can remove the cover from one side in the car, and work these in to place carefully, though doing so is not a particularly fun job and you might get some scratches on your hand.

Finally, reassemble everything, and enjoy sitting in your 'like or better than new' seats. This is also a great time to try to lubricate your seat tracks and mechanisms, if the seat is out of the car.

Obviously, this is not as long-term of a solution as buying two new seat bases, however total cost in my case was well under $100, including the foam inserts, hog rings/pliers and a junkyard seat base.

Will try to post some pictures later to better explain this process, unfortunately I didn't think to take any while I was doing the work.