Althoug the W126 received excellent corrosion prevention at the factory, it still is advisable to look for signs of corrosion damage when buying one. Especially when the cars have been driven around in a moist environment and when service records are not available.
Likely corrosion areas are depicted in yellow in the illustrations below. Always check the trunk area of the car. Whenever there is serious corrosion damage, the trunk will be the first place where it is revealed. Always remove the trunk lining from the car to get a good look inside. The car was designed to redirect the waterflow from the front of the car towards the rear of the car, rather than across the sides. This construction means that the rear window and the trunk area have to be watertight. Older cars can develop leaks through the trunk seal, the rear window seal and the taillight seals. Water can make it's way down into the trunk area and cause severe corrosion damage.
Feel around the underside of the package tray area (it can be reached through the trunk) and check for signs of rust.
Engine
The engines are known for their durability. If an engine is properly maintained, it can easily run for 300.000 - 400.000 kilometers without a rebuild. The V8 engines are alloy engines and the walls of the cylinder housings are coated with a silicium-based layer. Expect to change a distribution chain every 250.000 kilometers. The early Bosch K-jetronic continuous fuel injection systems can develop a rough idle over time. Typically, a V8 should run about 650 - 700 rpm at idle when hot. If the car will develop a jumpy idle (jumping from about 1500 rpm to 500 and up again), the electronic control module is likely to be at fault. Post-1985 models were equipped with Bosch KE-jetronic pulsed fuel injection systems with a completely electronic engine management system.
Electrical system
One of the problems older car can suffer from is disfunctioning electrical systems. Usually the remedy for such problems is some good contact cleaner at the right place. The switches in the doorjambs are a good example. These switches can easily be removed and cleaned, curing problems like disabled interior lights and courtesy lights, strange behaviour of seatbelt extenders (in SEC's), light warning buzzer and factory alarm systems. Resoldering certain electronic components such as the electronic climate control unit can cure heater problems as well.
Delamination
An incurable problem with Mercedes-Benz cars from the 1980's tha cannot be fixed is the 'delamination' process. MB front and rear window are made from several separate layers of glass. Over time, these layers become separated, eventually fracturing the glass. First sign of trouble is a white, milky discoloration of the window sides. In case of the rear window, the process will eventually take out the rear window heating as well. On a W126, usually the first spots appear in the lower corners of the rear window.