Hello I will help you with your question,
The first thing to do is to find out why your system does not work. If it is an electrical problem find and repair this first. If it is that the system has a leak this needs to be determined before any further action and the leak repaired. If the compressor has failed that can be delt with along with the conversion.
There are some things you need to know about R134a conversions. First R134a does not cool as well as R12 (Freon). The mineral oil used in the R12 systems to lubricate the compressor is not compatable with the R134a oils and if any is left in the system will form acid that will eat through the aluminum from the inside causing more leaks so all of the old oil will have to be removed. The service valves will have to be changed and the accumulator/reciever dryer replaced.
The steps to conversion are
1. Have a good working R12 system
2. Remove the compressor, accumulator dryer and open all the hose connections
3. Flush the evaporator, condensor and hoses to remove all traces of oil and R12.
4. Hand flush the compressor of all oil (Replacement is preferred)
5. Flush solvent from components with dry Nitrogen
6. Reassemble the system installing the correct amount of PAG oil in the various components and install a new orifice tube
7. Install a new reciever dryer/accumulator and service valves.
7. Pull a greater than 500 micron vacuum for 2 hours and let sit overnight.
8. If vacuum is maintained install 80% of the R12 charge amount using R134a
9. Performance check the system
If you have any questions please ask