I drive every car that we repair A/C on. Good job in a non airconditioned shop in N Central Florida. I do this to evaluate the cooling efficiency as a quality control issue and in a long running evaluation of how different models do with R-134a conversions.
I have a 4 lane divided road that I can drive 2 miles at 50mph. I place a digital thermometer in the center duct and turn on high blow and recirculate. The length of the drive allows the system to achieve a stable value or cycle if good enough. I grade systems by how many degrees off of ambient temp is achieved or by the length of the drive it takes to achieve cycling. In other words if the day is 96, I consider the A/C to be OK if it reaches 40 degrees off ambient or 56 degrees. (This will not cause cycling). If it was 80 out then the same car should hit 40 and probably would cycle first.
Most cars running in R-12 will cycle during this test even when its 96. That means probably a fifty degree drop. Some domestics have such good A/C that they hit low forties before I even get to the 2 mile stretch.
We have retrofitted hundreds of MBs and consistantly I see about a ten degree difference in performance. But there are abberations. I drove a 89 300e on the test last week. It was in for an intermitant lack of compressor (was the Klima relay). This car we had replaced the compressor and evaporator two years ago and converted (before I decided not to do any more pre 1990 124 bodies). To get to my 2 mile stretch I have to drive through a maze of back streets. This car was cycling at 44 degrees in high blow with the ambient temp at 96 and it did it in the stop and go section before the higher speed run. I have never seen an early 124 with such good air and it was 134.
I also drove a 300SE (the air should naturally do better on this car) this week that had been converted at a dealer (not locally). Even after pulling all the 134a out and evacuating and recharging; tuning the presssures for maximum efficiency I could only get 57 deg and it was a 92 degree day (only 35 degrees off ambient, no cycling in high blow). When I first tested it, it was doing 69, only 23 degrees off. The car probably still had a recirc door problem or an evaporator covered with leaves as often happens in the "Tree City". It was noted that the system didn't meet our minimum standard of 40 degrees off ambient.
Try it on yours and see what you get!
Bosch Master, ASE Master, L1
33 years MB technician