I hang out in the 73-79 trucks but my son has a 91 explorer and he is wanting to get the a/c working. We charged it in the spring and found out that the hose from the box on the firewall to the condenser and mounts to the compressor was leaking. e replaced it and charged it and it worked for a short time, but you had to hit the clutch on the compressor with a hammer while running and it would come on and work. Well did this for about a day but the compressor would not come on by itself. Ok we went the junk yard and got a 95 r134a compressor and installed it and the first half of a can the compressor came right on. Put 3 12 oz cans in it and nice cold air. Well the compressor looked bran new except the pulley was rusted, not bad but I am sure the hood had been open for a time and we have had some rain. When it first started after charging it would spin and shoot of some rust dust when you turn it on and off, and makes a 1/4 of a second hiss like a air brake. Anyone have any ideas that may help as to what may be leaking , no signs of any leaks and that is the only hiss it makes. Any ideas would sure help. I can replace the dryer and the condenser new for what some shops want to leak test it with dye in it. Well he is on a tight budget but I would really like to get it going for him. Thanks

 

You can purchase a can of leak detector at Walmart for about $13. If the "hiss" you are hearing is what I think it is, it is normal to do this when shutting down. All my Fords, and my 08 Freightliner do the same thing, and I never loose any freon.

 

There's no leak but your clutch is just about done for.
You chould have adjusted the clutch gap on the old compressor and saved some problems, junk yard compressors are seldom worth installing.

You don't mention pulling a vacuum on the system before charging. If you didn't, this can cause the High Side pressure to be way too high and cause the clutch to slip and eventually burn up. It will really decrease the performance, too.

Being a 91, that system uses R12. Did you convert it to R134a or just charge with R134a after swapping the parts? How did you determine how much oil was in the system?