You should not be offended by Paul's comments. We have had a number of posters here that have asked basic a/c questions that were alarming. The problem with a/c service is that a number of people over the years have been killed or maimed themselves when trying to charge an a/c system through the high side, causing the can to explode. I think Paul's comments were meant for your safety, not to belittle you. We don't want to see or hear of any of our mercedesshop siblings being hurt.
That said, there is a lot more to a/c service than might meet the eye. Also, you will need some tools and equipment that may make it economically impractical to accomplish yourself. If you still want to learn and do for your own personal satisfaction, there is no one here that understands that thinking more than myself.
Since your compressor locked up, you are most definitely dealing with what most a/c people call a "compressor burnout" situation. This requires the complete flushing as I mentioned in the earlier post. This is best accomplished with a flush gun that allows you to pour in solvent, pressurize and then use the blowgun to force the solvent through the component being flushed. Flush each component in both directions. Flush, flush, flush................ Mineral Spirits works for initial flush, followed by brake cleaner or a/c flush which is a treated alcohol solution.
To flush, disconnect ALL components and flush them individually. Each line should be considered a component and be thoroughly flushed along with the condensor, expansion valve and evaporator. You should also test your evaporator for leaks. This is the big achilles heel of the 124 a/c system. It is about a 15 hour job to replace.
You should replace all o-rings upon reassembly, and if you are going to use 134, you need the green o-rings. Use a nylog lubricant on all the o-rings to ensure a good seal.
Replace the filter drier and put everything together after thoroughly flushed and after you have poured out as much oil from the compressor as you can get. Pour the correct amount of Ester oil for 134 or mineral oil should you decide to go back to R12(this is what I would do.) Pour the oil into the compressor. After everything is together, turn the compressor several times by hand so that there is not a huge slug of oil directly in the compressor upon startup.
After the system is completely reassembled, evacuate the system thoroughly and if the weather is cool, you need to heat the components with a heat lamp to boil out the moisture. In the summer time, you typically do not need extra heat to cause adequate boiling. If ANY moisture is left in the system, it will combine with the refrigerant to make an acid. Believe me, with an evaporator that takes 15 hours to replace, you don't want ANY acid in the system. The evaporator is almost always what the acid will eat up first.
After evacuation, let it draw in as much refrigerant as it will take. This should actuate your low pressure switch allowing the compressor to kick in. This means you should not have to short any pressure switches. After start up, you can then complete the charging.
Necessary supplies and equipment to properly carry out the job will be:
O-ring kit with Nylog lube
Cannister of refrigerant or can tapper and individual refrigerant cans
Ester or Mineral oil
Additional parts you will need besides the compressor is the filter drier. This is a MUST after opening the system for major work, not to mention the fact that your filter drier is contaminated with junk from the "compressor burnout."
Don't be fooled when you open the system and don't see large particles in all the components. The contamination consists of very fine particles that are not normally seen by the human eye. After a burnout, however, you can be guaranteed that the contamination is there and MUST be removed.
There are some good books on auto a/c. The aircondition.com and ackits.com are excellent sources of such books as well as parts and excellent advice. The guys are really good and patient with all that post.
Feel free to keep posting questions and comments as you progress through the project and we will help you through it.