Cadillac Repair: ac, r 134 freon, r 12 freon

Question
why is it suggested to remove oil from r12 system to conver to 134 as well as to remove all the r12?  What is the harm in just adding 134 without doing one or the other or both?  thanks

Answer
1. Remove ALL the R-12 from the system.

2. You can purchase a small can of refrigerant oil from any parts place and add it to the R-12 refrigerant oil which makes the R-12 oil compatable with the R-134 freon.

3. Add R-134 freon to your system.

NOTICE: Converting an R-12 system to an R-134 system will most likely give you a check engine light and or a SERVICE AC SYSTEM message on your display due to the fact that the low pressure switch is calibrated for R-12 freon and NOT compatable for R-134 freon.

There is NO new low pressure switch yet available to fix this problem.

Suggest you stay with the R-12 system.