Arguably Mercedes air conditioning systems have left much to be desired. One can argue that with the moderate summer climate of Germany and with the engineers focused on quality, performance, and safety, the HVAC systems may have been overlooked.
The first Mercedes imported to the U.S. in the 1960's did not have air conditioning, and if you did wanted it, it was an afterthought. This included a hang-down inside unit and an almost home-made engine bracket to accommodate the inefficient York compressor. No auxiliary fan to cool the condenser was available until the early 1970's. Early AC and heat controls were strictly manually controlled with a blower speed control switch and a switch for temperature adjustment. A lever would control the direction of air flow, i.e.,whether warm air would go to either your feet or the windscreen. Manual controls are okay for most as long as the desired effect is realized. Heating was okay, but air conditioning left a lot to be desired.
As the U.S. became more important to Mercedes sales, MB USA needed to respond to the demands for climate control systems for the U.S. market. The first "climate control" system for Mercedes debuted in 1976 in the S class 116 chassis 450SE, SEL and the 280S. The fact that this system was only used in the U.S. market provides evidence that the engineering strength for climate control systems by Mercedes left a lot to be desired. As a result, the CC system appeared as a convoluted system which was expensive and unreliable and this continued through 1981. In 1982 the climate control system was completely changed to a much more reliable and inexpensive system.
One would think that the heart and soul of any cooling system would be the compressor but I strongly believe that for a Mercedes owner, the evaporator would be the most important. The evaporator is the device inside the dash which gets cold and provides the cooling inside the car. There are no moving parts to break, just a piece of metal about the size of a loaf of bread, with small tubes and fins to help dissipate the cold provided by the freon.
The evaporators of most Mercedes through the 1980's were made of brass and copper and very seldom failed. Automobile manufacturers are always faced with government mandates to improve fuel economy, which means weight is always a consideration, and the durable copper based evaporator was replaced with aluminum as the base material. Unbelievably, the model which suffered the most from defective evaporators was the high line S Class models, of which many were priced above $70,000. This evaporator replacement is very expensive (about 25 hours of labor) and with about $800 in parts, at a dealer it would total well over $3000.
With this in mind, what percentage of these evaporators would fail within 10 years? Unbelievably, it is close to 100%. Why would this part, a part that is so important and costly to replace, be so poorly designed?
So what do we take away from this blog/article. If you are purchasing a W140 chassis car, you must check the evaporator for leaks. The fact that the AC is operational is not enough. (It may be a slow leak.) When you do a pre-purchase inspection, make sure you check the condensate tubes for dye which is from the refrigerant oil. This is always a tell tale sign of a defective evaporator.
Good luck and make sure you do your pre-purchase inspection due diligence.