Opening the Heater Box - 1973-1977

Doing what "can't" be done! 
Now that the heater box has been removed, it is time to give it a good overhaul. This is not a job you want to do again for another 40 years. 

Between 1973-September 1977, the heater box at least in the SL series chassis, was partially sealed from the factory. Yes, as crazy as it sounds, it is true. 

The only part you can get to easily is the heater matrix (core) for renewal or repair. The A/C evaporator however is the part that is sealed into the case from the factory. According to the Mercedes workshop manual - (Section 83.1—538/1) "In the event of leaks on evaporator, the air conditioning unit [heater box] on vehicles (up to September 1977) must be replaced, since the removal of the evaporator out of air conditioning [heater box] unit is impossible without damaging the housing."

Image from Mercedes-Benz workshop manual

After doing a considerable amount of research on the web, the local Mercedes service center, and independent service centers, no one would agree to work on this system. Ironically, either no one believed it was sealed or they knew and said it is impossible and required a new box. A new box however, would be just as useless considering an old evaporator would be sealed in that unit.

On occasion, there is a benefit to being incredibly stubborn and this was one of those times. After thorough study of the heater box, I discovered the secret of how Mercedes fit this unit together and the ease by which it can be separated and refit just as it was 40+ years ago.

Sadly, the one or two online suggestions of how to get into the system were horrendously destructive for no reason. Remember, once the box is destroyed or weakened the only option is to source a used box as Mercedes no longer makes the system. While a later system can be installed, it would be the electronic climate control system that would need to be retrofitted to the older chassis and is not nearly as reliable. 

The other major benefit of separating the entire system is to remove the 40+ years of dirt and other crazy items you might find inside. 

What the heater box looked like when it was first opened.

The match case was lodged in the evaporator drain area. 

Below is what the system looks like separated in it's entirety. Once the system was completely disassembled, I thoroughly cleaned it and renewed the foam padding on the flaps with a waterproof 1/8 in. (3.175mm) foam and adjusted the flaps for proper movement. In addition, by removing the "sealed" section, all the deteriorating foam flap seals can be renewed. The only part that was not separated for the photo immediately below is the evaporator from the case. It is however ready to lift out for testing and cleaning.

1973-1977 Mercedes-Benz heater box separated. The evaporator was not removed for this photo

Photo shows "sealed" component section of the heater box and what it looks like when separated.

Ventilation flaps with new waterproof foam and flaps adjusted