Dealing with the suspension has been a struggle. The Mercedes-Benz dealership didn't want to do an alignment because the front of my car was setting up too high and the steering damper is leaking. I loosened the upper and lower control arm bushing nuts and bolts, lifted the rear of the car as high as I could get it with the factory jack for a few days, and then lowered it down nearly all the way before tightening the bushings.
I then drove the car over some speed bumps and to a level spot of a parking lot to check the stance. The front end had settled down so the front wasn't so high, but it still seemed a little too high.
I didn't understand it. I installed new springs and pads front and rear, and they were all genuine Mercedes and the correct part numbers. But, something wasn't right. It seemed the bushings and front springs weren't going to settle much more than they already were. I looked up some posts about this problem, and found this interesting thread: https://www.benzworld.org/forums/w126-s-se-sec-sel-sd/2366609-89-300se-new-oem-front-springs.html
In short, someone else installed new front and rear springs and pads which were the correct part numbers, but they made the front of the car set too high. It turns out that there are several variations of springs within the same part number. So, springs which share the same part number may have different lengths and spring rates, and when you order new ones, you don't know what you'll get, so you have to compensate for it with different height spring pads. My 300SD uses the same part number front springs (116-321-32-04) as a 450SEL. But, not a 280S (116-321-33-04). The rear springs (116-324-08-04) seem to be the same throughout the W116 lineup (except the 6.9).
My original front springs have a yellow and I think two blue dots (I'm color blind). The new springs have four green dots. Hmmm... so, that must explain it; while the front springs I got were the correct part number, they are either a different length or have a different spring rate than the originals. Since you apparently can't choose what type of spring you get when you order them, apparently you have to just order them in and figure out what type of spring pad you need to order to compensate, according to a points system for which you add a point for each accessory your car has (sunroof, power windows, etc.) and then calculate your pad based upon the color code of the spring.
I didn't feel like trying to figure out what type of spring pad I needed to make the new front springs work, so I just removed them and put the old springs back in. It's very unfortunate because now I feel it was wasted money buying new front springs, which apparently last forever anyway. The new rear springs are the same part number as the old ones AND they have the same color coding (one yellow dot and one green dot), so they should be just fine and might actually bolster the rear end. Rear springs can sag with age, though I'm not sure mine were sagging at all.
When I got the new springs out, I put them up against the old springs and they seemed to be exactly the same length and wire thickness. Hmmm... maybe they hadn't fully decompressed yet. I dunno. The old front springs are back in, and I don't think I can use my spring compressor anymore because the threaded rod is starting to strip out even though I've greased it. One of the sway bar bushings at the upper control arm got deformed, so I had to push it back into place. Then I jacked up the rear of the vehicle again and let it set for a day.
The service advisor from the Mercedes dealership recommended an alignment shop in Glendale (closer to here than Phoenix) which works on vintage and custom cars, because he had them do his 1965 Chevy Nova. They only charge $40 for an alignment and they said they were currently working on a 1979 W116 300SD. So, that was encouraging. Unfortunately, the alignment feels worse than before I went in. Before I went in, the steering wheel was perfectly centered when going down the road, it didn't pull at all, and it would return to center on its own. Now the steering wheel is turned to the right when going straight, the car feels like it pulls on both sides, and is all over the road. Very disappointing.
I don't think they know how to align old Mercedes, and I'm pretty sure they didn't use a spreader bar or the centering pin. I told the guy I had the centering pin for the steering box, and he ignored me. I need someplace that will do it by the book, but right now I can't afford to put any more money into this car. At least putting the original front springs back in seemed to fix the ride stance. Assuming they got the camber and caster correct, I might adjust the tie rods so the car at least goes straight down the road and the steering wheel isn't crooked.
Here is how the car is currently riding. It seems like the front end is slightly lower than the rear now, but probably not much more than when the new front springs were in. I wouldn't mind going up a pad size in the rear from a 2-nub to a 3-nub to lift it up a little (5mm).