I have an 81 Chevy C10 that I inherited from my dad. This truck has mostly sat for the last seven years, with me running it enough to keep it in condition. I've been using this truck mainly for the last year now and have replaced parts as needed. I would really like to get the AC working again. It hasn't been ran in about 7 years now. The heat works fine. The AC had a leak before and needed to be charged to work but the leak was never fixed. What kind of problems can running the AC cause now? Can I just charge it up? Should I at least get it evacuated and charged with a dye first?
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If it was never converted to use R134a instead of R12, you're not just going to be able to charge it up yourself, since something that old probably came from the factory with R12 refrigerant. You need a license to buy R12 in the United States, and it's not cheap. Since your system is compromised and needs attention anyway, you might as well do the R134a conversion if it hasn't already been done. Here is a thread from a C10 forum. Bare minimum for the conversion would be new service ports. At the top end, a new accumulator (receiver/drier), orifice tube, o-rings, compressor and pressure cutoff switch. There is probably a happy medium somewhere in the middle.
It's standard procedure to evacuate the system before charging. You probably don't have much to recover, but sucking the system down to a vacuum can tell you how bad your leak is (by how long the system remains holds the vacuum). Since your system has been open for so long, adding some oil when charging is probably a good idea. Especially if you're going to do it yourself, since you'll be running the compressor while charging through the lower pressure port.
You'll need to charge the system to find the leaks. A detector wand can pick up refrigerant as it escapes the system. Adding UV dye when charging works, too. You can find the leaks with a UV light.
Once you know where the leaks are you can evacuate the system again, fix them, and charge it again and see if you got them all.
You can probably do all the work yourself if you have access to the proper equipment to evacuate and charge (vacuum pump setup, gauges and hoses), but you really shouldn't be venting refrigerant to the atmosphere (especially not R12). If you charge it and it leaks out, I guess that's one thing, but a shop would recover it and dispose of it properly.
As far as things to watch out for with a system that has been open for years, as Brian Knoblauch is pointing out, the compressor could be a problem. Even if it's not seized, the seals could be shot from lack of lubrication. If you haven't pulled the relay/fuses for it though, it's been getting power whenever you've had the defroster on. It might not even be engaging due to low pressure, or maybe engaging and immediately shutting itself back off. If you're hoping to salvage it, pull the relay and / or fuse for it until it's gotten some lubricant (or don't use your defroster). I'd guess the accumulator / receiver / drier is shot. As for the rest of the system, if moisture has gotten into it, you could have leaks everywhere due to corrosion. I wouldn't be surprised if you had to replace the accumulator (receiver / dryer), orifice tube and possibly the compressor along with dealing with whatever hard lines or hoses are leaking. You might get off easy, but then again, you might not. Labor is not going to be cheap. You're probably going to be using aftermarket parts, since the originals were meant for R12 and they're long discontinued anyway. The good news is an assessment should not break the bank, it should be around a hundred bucks to vacuum and charge the system, maybe a little extra labor to check for the leaks. Then you just have to decide if the bill for fixing it is something you can bear.
There's a possibility that the compressor will have seized up from sitting so long. They really need to be run periodically. However, I've seen some that have sat a long time fire up again just fine.