Head pressure on A/C

[Bob Yates]

My 86 has been converted to use 134a and I can't find what the head pressure should be. My shop manual only has specs for R12 systems. And all the late after market manuals say take it to a shop in the A/C sections. I had to drain the system when I tore the engine down awhile back . I took a 4 hour vacuum of the system and add 134a . It's not working like it did last summer so I need to start looking at things. Head pressire is one of them ...Thanks for any help...Bob Yates

 

[milehigreg]

R134A Vs R12

134A and R12 have very similar properties but they are not exactly the same. At the low end the refrigerant pressure at 52 degrees is 48.8 PSI for R12 and 47.7 PSI for 134A. At the high end at 110 degrees 136.4 PSI for R12 and 146.3 PSI for 134A. To get the same 110 degrees and the same heat transfer from the condenser the 134A has to be compressed to a higher pressure. If the compressor isn't desgined for that, system performance must decrease. It won't work as well. This is just an example. If you want to tell me what the book says for condenser pressure orcompressor discharge pressure and temp I can probably figure out what the 134A pressure should be to match.

 

[tyrel]

Here is a great site for a/c questions, and the resident techs are very knowledgable about the conversion:

http://www.aircondition.com/wwwboard/

 

[G Winter]

R134A needs only 80% as much as R12 installed. If you put to much in it will not cool like it should.

 

[bradfordsvettes]

g winter is correct Steve

 

[G Winter]

I often put in freon according to the sweat on the evaporator outlet line .
when that is really sweating it seems like it is real close. Also going by temp drop from the vents works. I don't remember for sure but it seems like 40 to 50 deg temp drop from ambiant is about right .

 

[Bob Yates]

much thanks to all

I had to much 134a... I let some out and drop the temp inside to 38 degrees

 

[Bob Yates]

P.s.

My compressor is not cycling now ... It runs all the time...Should I let more out ?????

 

[G Winter]

With the temp drop you have I would say your real close but not sure which way you need to go from here.

 

[Bob Yates]

I replaced the recycling switch and it still doesn't recycle... I guess I'll let more of the charge out and see what happens. It did get down to 34 degrees today so I must be on the right path if I don't trip over my dumbness I just might fix this. And I didn't need Mr. Goodwrench