Some common problems associated with the Mercedes-Benz W140 model include wiring harness issues, problems with power door locks and power windows, and inadequate power output from certain engine models. Transmission defects attributed to the W140 can lead to a range of troubles, such as oil leaks, fluid contamination and slippage.
Problems with the wiring harness, a central collection of cables responsible for delivering electrical power throughout the vehicle, may lead to prematurely blown fuses and connector damage, which can, in turn, damage dashboard warning lights and quickly drain otherwise healthy batteries.
Other complaints cite faulty power locks and windows as a frequent issue, especially as the vehicle gets older. The W140 employs a vacuum system with a compressed air pump to control door locks; over time and depending on usage, the mechanism's electric motor can weaken to the point of complete failure. Door lock problems may also stem from leaks in vacuum lines or pump connectors.
The W140 model offers four engine options, two of which are cautioned against. The 3.2-liter inline is infamous for notoriously weak power output, especially considering the car's considerable heft, and the available V12 suffers from an extensive history of high-priced problems that are often hard to remedy.
The car's 5G-Tronic automatic transmission gets bad grades from owners for oil leaks resulting from deteriorating connection plugs; pipe corrosion, which allows engine coolant seepage into the transmission fluid; and transmission slippage caused by the erosion of internal seals and breakdown of clutch packs.