Just bought a 93 Sierra, 5.7L. AC is not working and before I try to work on it I need to know how you can tell if it's been upgraded to R134a from R12.

Help

 

It should have bigger core valves compared to the R-12 valves.

 

That is a good way to tell, but not 100%. Alot of cheapo's out there will convert them and keep the same R134 Adapter for the next one on their charging set-up. Most people are just pumping 134 in an r12 system to get them over the curb anyway. It will work, but will probably leak out shortly. If your system is full I would have a shop evacuate it and assume it has not been converted. Unless like RUFFNECK4LYFE said it has the 134 fittings on it already. If it doesn't it really isn't as complicated as once thought to convert them. I have had great luck with just swapping out all the o-rings (cheap) and the accumulator (cheap) and flushing out the rest of the system really well with denatured alcohol. Get a set of 134 fittings, vacuum it down and charge it. Don't forget the PAG oil. Good luck...it's getting hot out there!

 

Yeah I agree. Dont chance guess it thinking it has 134 in there. Just pull out the freon, retro fit it, vacum, then charge it.