Okay, here goes. I used to teach the R-12 systems at a trade school almost thirty years ago. Now with the weather warming up, I need to get the A/C in my '97 F250 working again.

It was working last spring (2016) when we took a trip but several days later, it quit. I did a bit of troubleshooting and the compressor will run when I have the low pressure switch jumped out.

Now, since the system doesn't have a sight glass like an old R-12 system, I'm at a loss here on what I should see with a set of gauges. I know it needs somewhere around 36 to 38 oz of R-134A but that's from empty/vacuumed down. I have not found a measurement for PAG oil to add from empty/vacuumed down, either.

I have not checked yet (weather too hot at the moment) to see if there is anything left in the system to work with. In the event there is, any pointers on topping off the system? PSI on the high side/low side when running? System pressure when off/not running? Pressure/temperature formula???

BTW, I only let the compressor run for a few seconds but I had my hand on it when it was turning. Felt smooth, not like it was self-destructing.

Any help would be greatly appreciated.

 

Anyone?

For the record, I did find out I needed 7oz of PAG-46 but that's if the system is flat. Still not sure where to go with this, since I don't want to kill the compressor by operating it with low or no oil if the system is flat when I refill it.

Guess I'll just get it serviced by a shop.

 

First, get a set of manifold gauges if you want to actually know what's going on in the system. Actual pressure readings are usually in a band depending on operating conditions and ambient. A fair rule of thumb is that the low side should be in the 30-45 psi range and that the high side should be in the range of the ambient temp in degrees F plus 75-125. Those are extremely crude estimates but will get you fair operation. I've attached charts from the 96 FSM, click on the links below if the charts don't display automatically.

One can usually get into the right ballpark by monitoring the evaporator inlet and outlet temps with a fingertip while adding refrigerant. When the system has a charge that's close enough (usually), the outlet temp is just above the freezing point (usually 30-45° F), assuming that the system is set up correctly and is operating normally.

If oil level is a concern and you didn't plan on flushing and vacuuming, you can buy little cans that have 2 oz of oil and a small shot of refrigerant to "top off" system oil level. It's a crap shoot though, since there's no way of knowing how much oil is remaining in the system.

Obviously, in a perfect world where cost is no object, the system would be leak-test, repaired, flushed, and everything re-charged by weight using accurate shop equipment. We know, of course, that cost is usually an object.

 

Thank you for the reply!

Like I said in an earlier post, I do know R-12 systems but R-134A wasn't being used much, if at all at that time. I was just going to get one of those "Shot Cans" with the gauge and go for it, until I read about the additives they put in those cans. Damned near as nasty as the "Hot Shot" that could be put in either R-12 or R-134A, messing up both systems.

Further investigation turned up one "Shot Can" that was pure R-134A, no sealants or additives. Still, I'm hesitant, knowing you could create a bomb by overfilling a system. That's why I was asking for some specs, which you so graciously provided. Thank you for that.

I'm going to pick up a manifold gauge set, the R-134A sniffer and the adapter to attach a 12oz can to the manifold. Somewhere I spied refrigerant with dye in it so that might let me see where the leak is.

It could be it just needs to be topped up (slight leak somewhere) since it was very efficient when I bought the truck in 2010 but it seems that in hindsight, the efficiency has been dropping off over the years.

I really don't want to take it to a shop because they will want to charge it fully, find the leak, vacuum/recover the system, fix the leak ($$$) and recharge it. More than my pocketbook can afford.