R134a charge for '75 GMC Truck
Posted: Sun Jun 25, 2017 9:44 am
Years ago I converted my '75 GMC truck from the old A6 system to a Seltec compressor, parallel flow condenser, and Ford blue orifice that I purchased from Arizona Mobile Air. I also added a high pressure cutoff switch and had a local shop replace the hoses. It worked well for about 3 years, but after the first year developed a leak that required adding a little R134a every few months. The final failure was a complete lockup of the Seltec compressor, which is when I disconnected and capped the hoses and pulled the compressor off.
I now have time to get the system going again and I plan to use a new Sanden compressor, new (stock) accumulator, and new Blue orifice, and completely disassemble and flush the system and replace all the O-rings before I have it charged. Now I realize that I don't remember how to figure out how much R134a should go in the system. The upgraded items are a Sanden 508 compressor and a large parallel flow condenser, but the evaporator and accumulator will be stock. This is a single cab truck, so you are only cooling a fairly small cubic area.
GM originally called for 3 lbs, 12 oz (3.75 lbs) of R-12 in this system, and I thought that the first time I fixed the system I told them to put in about 3 lbs of R134a, and it seemed to work well. However, when I compare that to the newer systems it seems like a lot of refrigerant. For example, if I purchased a complete Sure-Fit system from Vintage Air, which also uses a Sanden compressor, it recommends less than 2 lbs of R134a.
The recommendation I remember from years ago was to start out with an R134a charge that was about 75% of the R-12 charge (about 2.8 lbs), and I also found a refrigeration table on line that recommends 2.8 lbs of R134a for a R-12 system with 3.75 lbs. Any good idea of where to start out with R134a charge, and what pressures to look for?
Thanks,
Bruce
I now have time to get the system going again and I plan to use a new Sanden compressor, new (stock) accumulator, and new Blue orifice, and completely disassemble and flush the system and replace all the O-rings before I have it charged. Now I realize that I don't remember how to figure out how much R134a should go in the system. The upgraded items are a Sanden 508 compressor and a large parallel flow condenser, but the evaporator and accumulator will be stock. This is a single cab truck, so you are only cooling a fairly small cubic area.
GM originally called for 3 lbs, 12 oz (3.75 lbs) of R-12 in this system, and I thought that the first time I fixed the system I told them to put in about 3 lbs of R134a, and it seemed to work well. However, when I compare that to the newer systems it seems like a lot of refrigerant. For example, if I purchased a complete Sure-Fit system from Vintage Air, which also uses a Sanden compressor, it recommends less than 2 lbs of R134a.
The recommendation I remember from years ago was to start out with an R134a charge that was about 75% of the R-12 charge (about 2.8 lbs), and I also found a refrigeration table on line that recommends 2.8 lbs of R134a for a R-12 system with 3.75 lbs. Any good idea of where to start out with R134a charge, and what pressures to look for?
Thanks,
Bruce