Thanks Guys!

 

First you need to figure out what coolant the system requires.
It will be either R12 or R134.

If it's R134, the procedure is pretty simple.
http://autorepair.about.com/od/regul...C-recharge.htm

 

1st I guess we assume yours is empty? If so you need to find the leak. Next you would need to determine if you want to fill it with R12 or 134 freon. In either case you would need to replace the receiver/dryer and the orifice valve. I would also suggest you flush the system. When you are ready to refill the system you would need an oil charge and then the freon. You can fill it at the rear of the compressor or you can fill the low side at the receiver/dryer. Wear the appropriate safety equipment and follow the instructions on the freon cans.

 

Given the elevated price of R134a these days, I'd really recommend having a pro do it. As has been mentioned, the previously existing refrigerant went somewhere. A shop will evacuate the system, extracting any moisture that has accumulated. They then do a leak test which basically consists of pulling a good healthy 25in. of vacuum and monitoring it for 10-25 minutes and observing if the vac holds or whether there is a gross leak. Your 90 model was R12 originally. Check the accumulator on the passenger firewall to see if the fittings are Shraeder valve (like a tire valve) or are larger and of the push/pull variety. The latter are R134a fittings. After the shop leak checks it and assuming they find no gross leaks, they can just add an oil charge and fill it up w/ R134a. I'm in Texas where a/c is a must, There is no mystery to converting from R12 to R134a. All that is needed is to extract all the R12 (usually has been gone for years anyway) as well as the mineral oil lube used w/ R12. Getting every last drop of mineral oil isn't necessary anymore as the industry developed PAG and ester oils that are compatible with trace amounts of the mineral oil. Flushing a 20 year old system is a toss up. To do it correctly means dismantling the system at pretty much every connector. In my experience, the power flushers that don't involve opening the system are at their best extracting money from the customer's pocket but little else. The 240's and 740's don't have an orifice tube so that doesn't need changing. A new accumulator/dryer is a good thing as it has a desiccant bag in it that helps keep the moisture from the rest of the system but, again, if money is tight, it can be left for now. Be sure that fluorescent dye is added so if the charge leaks down the leak can be identified.

 

Be sure to check your fittings.
I had a very tiny leak that took a week or two to diminish the A/C.
Nothing was obviously leaking.

Until I went to change the oil and bumped a fitting with my shoulder.
The steel hose snapped clean in two spraying coolant all over me.

None of the parts stores in town had the correct line and the only online seller wanted $200.
The plumber in me went "looks like a pipe".
So I found the right size compression union and it works better than the factory fittings.

 

Thank you everyone for all your replies!! I really do appreciate it!

Scott