A/C vent temps?

[Mscromer]

Just got my ac system up and running and was curious what kind of vent temps people are seeing on systems converted to 134A. The best I’ve seen on my car was 57 center vent temp with outside temp of 90. If your still running R12 want are your vent temps for comparison.

[Tom]

I posted my saga with r134a a/c in this thread if you are looking for bedtime reading. But the bottom line is I was able to get the temps down to the point where it cycles on and off as a function of the freezeover switch -- i.e., mid-30's even on super hot days. It was kind of a quest. Lots of tips in that thread, and happy to help here if you wanted to dig in....

[dr bob]

Just got my ac system up and running and was curious what kind of vent temps people are seeing on systems converted to 134A. The best I’ve seen on my car was 57 center vent temp with outside temp of 90. If your still running R12 want are your vent temps for comparison.

I've seen temps like this primarily in cars that weren't evacuated correctly. Did you follow the steps to attach a vacuum pump and pull all the air out? Leaving air in the system clogs the heat exchangers (condenser and evaporator) and fools the expansion valve. It compresses OK but never condenses to liquid, so there's none of the benefit we get from boiling refrigerant with cabin heat. It doesn't take much air.

You have a sight glass on the drier. If you've pushed the right amount of refrigerant into the system, you should see very few bubbles in that sight glass at normal test condition: 2500 RPM, fan speed set on 2, test at the center vent (closest to the evaporator), and use a real thermometer rather than an IR type. Sometimes when we see a lot of bubbles, we are tempted to add more refrigerant. OK but there are some sharp limits on high-side pressure. At the same time, the low-side (compressor suctions) pressure is related to cabin temp and how well the expansion valve is working. Look at the equivalent temp on the low-side gauge, and keep it below about 30 or so for best performance. High side limit is mid 300's PSI or so. With cooling fans working correctly, of course.

------

I struggle to appreciate why we still have R12 in cars. It's $$$$ to buy, and if you are DIY'ing the work it's not that big a deal converting to R134a. Just the difference in refrigerant cost is enough to pay for hoses rebuilt, a drier, o-rings and an expansion valve.

[Mscromer]

Just got my ac system up and running and was curious what kind of vent temps people are seeing on systems converted to 134A. The best I’ve seen on my car was 57 center vent temp with outside temp of 90. If your still running R12 want are your vent temps for comparison.

I've seen temps like this primarily in cars that weren't evacuated correctly. Did you follow the steps to attach a vacuum pump and pull all the air out? Leaving air in the system clogs the heat exchangers (condenser and evaporator) and fools the expansion valve. It compresses OK but never condenses to liquid, so there's none of the benefit we get from boiling refrigerant with cabin heat. It doesn't take much air.

You have a sight glass on the drier. If you've pushed the right amount of refrigerant into the system, you should see very few bubbles in that sight glass at normal test condition: 2500 RPM, fan speed set on 2, test at the center vent (closest to the evaporator), and use a real thermometer rather than an IR type. Sometimes when we see a lot of bubbles, we are tempted to add more refrigerant. OK but there are some sharp limits on high-side pressure. At the same time, the low-side (compressor suctions) pressure is related to cabin temp and how well the expansion valve is working. Look at the equivalent temp on the low-side gauge, and keep it below about 30 or so for best performance. High side limit is mid 300's PSI or so. With cooling fans working correctly, of course.

------

I struggle to appreciate why we still have R12 in cars. It's $$$$ to buy, and if you are DIY'ing the work it's not that big a deal converting to R134a. Just the difference in refrigerant cost is enough to pay for hoses rebuilt, a drier, o-rings and an expansion valve.

Thanks for that info. I did pull a vacuum but I think I’m going to have it evacuated and pull a vacuum for a longer period of time. I think I pulled it for an hour last time. I’ll pull it all day this time and shoot for the below 30 on the low side.

[shft22]

Perfect timing for this; I am about to recharge my system.

The new-to-me 951 had previously been converted to R134a, but when I checked the system, it had absolutely no pressure. I vacuumed the system, and 3 days later it still had vacuum.

When I get back this weekend, if it still has vacuum, I am planning to put in 3 oz of oil (I don't know what they did previously), pull vacuum again for an hour or so, and charge the system. My understanding is 30oz of R134a should be what is needed.

What do you guys think of my plan?

[fasterfaster]

Perfect timing for this; I am about to recharge my system.

The new-to-me 951 had previously been converted to R134a, but when I checked the system, it had absolutely no pressure. I vacuumed the system, and 3 days later it still had vacuum.

When I get back this weekend, if it still has vacuum, I am planning to put in 3 oz of oil (I don't know what they did previously), pull vacuum again for an hour or so, and charge the system. My understanding is 30oz of R134a should be what is needed.

What do you guys think of my plan?

Converting from the factory R12 spec of 950 grams to R134a (80-85% by weight) gives 760 to 800 grams (26.8 to 28 oz). Safest way to confirm is to measure your high side pressures while operating (full cold, revving above idle). You should see:
Ambient Temp / High Side Pressure
65 - 70F / 135 - 160psi
75 - 80F / 150 - 190psi
85 - 90F / 190 - 230psi

You just want to make sure you're in the range, and especially that you don't overcharge it.