-W126 locked out of trunk: if the trunk linkage becomes loose & sloppy, you can't open it by pushing the button. Take a 2x4 about a foot long and hold it so one tip is on the trunk button. Whack the other tip with a hammer (seriously). The trunk should pop open. Make SURE the lock is unlocked first (you can lock & unlock the drivers door to make sure it is in the unlocked position). This won't work if something else is wrong with your trunk. Now pull off the top plastic cover and pull back the carpet. You can adjust the linkage from here. Make sure it works before closing the trunk!!
-W126 rear brakes: Make SURE you keep on top of the pad thickness. Once you're down to about 20% pad or less, the pads will cock themselves in the caliper diagonally. Then the caliper siezes. You will know this happened when you pull the pads and can't push the piston back into the caliper. It's time for a rebuilt caliper at this point. I'd change the rear pads (they're cheap and last for yeras!) when they're down to 35-40% just to be safe.
-W126 parking brake light stays on: There was a recall for the parking brake cable on this car. It involves removing the entire cable from the pedal to the rear brakes, including a ton of labor. Here's the easy fix: Put a wire tie around the parking brake pedal arm (near the top) and clip one end of a spring (from Home Depot) to it. Clip the other end under the dash to keep the pedal pulled up. Everything works as normal, but the little bit of slack in the pedal is eliminated, so the light will stay off.
-W126 dripping ATF: Most likely this is the ATF lines going to the radiator (transmission cooler). Replace these lines if they're dripping -- you just need a crowsfoot wrench. You will know they are leaking if there is any ATF on the outside. You can find these easily by looking at the bottom of your radiator (look from under the car) and find two hoses that look like they have a "spring" covering the outside. It looks like a hose with a piece of wire wrapped around in a coil-like shape. You'll see what I mean.
-300SDL glow plug replacement: pull off the intake FIRST. Makes the job much easier. Careful around the injector lines - these are fragile. While in there, replace the following items: 1) intake gasket (both ends - there is a large one and a small oval one), 2)shutoff valve (easy to do while you're there) 3)replace ALL glow plugs.. sucks to have to do this job again in 6 months! Remember all plugs are the same age, so if one goes the other 5 aren't far behind.
-300SD rough shifting between 1st & 2nd gear: This is NORMAL. It is called the 'neck-snapper' shift by many in reference to the feeling you get when it shifts
There is a bowden cable that can be adjusted to soften it, but the tradeoff is between long transmission life <--> soft shifts. By softening it, you'll shorten the transmission life. Careful doing this if your transmission is old and hasn't been rebuilt.
-300SDL trap oxidizer: Look under the hood (passenger side) for a large metal thing that looks like a football that is plumbed into the exhaust system. You'll see it immediately if you have one. If you do, go to the Mercedes Dealer and they will replace it, along with your exhaust system and possibly even your turbo if it's bad. Search forum for more info. If you have trouble with the dealer denying knowledge, keep bugging them or try another dealer. Be assured this IS covered by a recall and it's still in effect if you car has the trap ox in it. Note that your car (or car you're potentially purchasing) has most likely NOT been to a dealer in the last 10 yrs! Many "dealer maintained" SDL's on eBay still have the Trap Ox
-W126 hood pad: Buy a new hood pad but be CAREFUL how you install the new one. Search the forum -- there is a special 3M glue and you MUST scrape off ALL the old gunk first. If not, you'll be doing the job again in 2 weeks. Make sure to do it right the first time!
-W126 battery: The MB battery is made by Interstate and this is the best battery for the car. Try an Interstate before buying anything else for this car!
-W126 tires: Many opinions here, but Michelins have always been EXCELLENT for me on a number of these cars. One tire expert told me to remember the phrase "Michelins on Mercedes" because that's all they recommend after years of being in the business.
-Clunking from front end going over bumps: Check your brake support bushings (search forum for info). These are common problem areas on the W126. If they haven't been replaced, you will be changing them for sure.
-W126 windows or seats not working: The switch is dirty. Clean it out with contact cleaner or similar (TV tuner cleaner from Radio Shack works also). Most times the switch is just dirty. Keep working the switch back & forth while cleaning.
-W126 radio scratchy noise in some speakers: The fader (roller dial in the center console) corrodes a bit. Roll it back and forth as far as you can in each direction. I do this about every 3 months and the problem never reappears.
Neal