Exterior:

It is reported that some of the double glazed electric windows can fail overtime due to the excessive weight putting additional strain on the rails and motors.

The double glazed glass itself is sealed with air in between. Because of this, and leaking moisture can build up inside them which then creates a foggy or mist-like effect in the window. The only solution is to replace the window, this will not be inexpensive.

The clear coat on the paint can suffer from peeling mainly around the sunroof, bonnet or boot. It’s not highly common but it will be more noticeable on examples that have been neglected.

The Pneumatic System Equipment pump which operates things like the door locks and central locking, rear parking sensor antenna guides and boot lid release can suffer from vacuum hose and connection leaks or a failing pump motor which can result in all of the features mentioned becoming faulty.

Also the equipment that controls the soft closing doors and boot lid can also experience problems and failures.

Thanks to the wiring harness being biodegradable, things like the door locks and lights can glitch out and fail.

Both the front and rear windscreens can delaminate. This can usually be diagnosed by either off-white or brown stains or marks around the perimeter of the glass. Again the only real solution to this is to get them replaced.

Electric sunroofs are also prone to a host of different malfunctions, ranging from leaks to faulty switches or just overall failure resulting in the sunroof being stuck open or closed.

They can also suffer from tail light discolouration, this can be identified by the lower plastic cover being exposed to excessive sunlight and UV resulting in them turning a pale shade of pink.

The headlights and taillights themselves can have moisture build up inside them if the lens covers have any leaks or cracks in them. This can lead to a milky or foggy effect which can also affect the overall usability and strength of the headlights.

While the W140 seems to be relatively resistant to rust, poorly maintained examples, or poorly repaired examples can see some rust around the wheel arches and around the boot lock area and on the front quarters where the metal meets the front indicator light, but generally speaking, good ones are rust free.

Interior: 

The HVAC or air conditioning evaporator unit is notorious for failing and to repair it generally requires a labour-intensive dashboard-out procedure, which will be understandably expensive.

The electric seat motors are known to wear out over time, so make sure that the seats are adjustable in every direction because replacing the motors is generally the only solution.

The centre console woodgrain trim can be damaged depending on how well kept the car was by its previous owner. If the woodgrain is damaged at all then there’s a good chance it’s been sitting in the sun more often than not, a less common culprit of this damage can also be linked to improper removal/reinstallation of the console.

Although there aren’t many of them, the interior plastics can be particularly brittle, with any exposure to heat or humidity assisting to show their lack of durability as there has been evidence of these plastic components cracking or just coming off with relative ease.

Remember how I mentioned the wiring harness can fail over time, this can result in the interior also suffering from all sorts of electrical gremlins like gauges giving false readings.

Mechanically:

The W140 S-Class is often regarded as being superbly reliable thanks to an immense level of over-engineering. However, there’s a high chance it’s more a case of they are engineered just about right, and that Mercedes-Benz has since been engineering unnecessarily complicated cars, therefore making more recent models less reliable, and in turn, making the W140 appear to be a reliability dream.

Engine wise, in Australia the vast majority of engines will be either an inline 6-cylinder or V8 petrol.

There are a handful of V12 models on the used market and while many are well maintained, it takes a very committed enthusiast to sign up to ageing V12 levels of maintenance.

The V8 petrol, mechanically speaking is overall pretty reliable. They do have some timing chain complications (not actually the chain as it’s a double row chain) but the tensioners and guides can be problematic. They can break and cause a horrible noise.

There are also some valve train issues with oil supply as there is 16 little oil feed lines with o-rings within the head that leak and cause hydraulic lifter issues.

Oil leaks and complicated old school fuel injection and ignition systems can be a bit of a weakness too.

The 6-cylinder petrol engines (also found in the Ssang-Yong Rexton & Musso)., mechanically is also quite reliable with no one big expensive issue that causes catastrophic failures. They all leak oil but otherwise, it’s just mix of every other issue you receive on any car of this age.

The transmissions are also generally ok but there are a few issues.

Valve body and solenoid issues occasionally occur, but more often the electronics within the 5-speed transmission fail, as does the main electrical connector to the transmission.

Also, inside the car, the actual selector can be a bit temperamental- especially if its had coffee spilt on it which is a common problem.

The W140 is ageing so the reliability issues are mostly going to be from old age, the biodegradable insulation used in the wiring harness and electronic complications are going to be a nightmare.

Also, the PSE (pneumatic support equipment) systems (which is basically the air system that operates thing like soft close doors, door locks and seat comfort functions) are extremely problematic.

Overall, mechanically, most W140 S-Class are great as long as you’re an enthusiast that loves fixing small and large problems frequently, or enjoys paying someone else to do that for you.