A/C -- How High Should the High Side Be?
 

I have a 1982 911SC with the stock two-condensor setup (one under the nose, the other under the "whale tail" on the deck lid). The system is stock except for the new Unicla compressor I installed last year. I am aware of the notoriously leaky nature of the system, with the pre-pricked rubber hose running the length of the car from the engine to the condensor in the nose. But last year, when I did the compressor, I leak-tested the system with nitrogen and it held pressure for two days, so I didn't change the hose; I installed the new compressor and new receiver, evacuated the system, charged it up to spec with R12, and it cooled like a champ. Again, this was last year.

I started using the A/C yesterday, as the temperature has hit 90, and the cooling is piss-poor. Because I never changed the hoses, I assumed that the freon charge might be low, and was about to shoot a can of freon into it, but the readings on the gauges slowly creep up to 50 low and 350 high (obviously standing still, with the deck lid down). Does that seem awfully high? Does it sound like a plugged expansion valve?

High side pressures will climb rapidly without airflow over the condensors. Could you hear the front fan running(in the trunk near the battery.What about the sight glass?Lots of bubbles indicate low charge levels as well. The sight glass is not reliable indication with R134a but fairly good with R12.

That is very high, don't run it very long at those pressures. Should be around 190-200 at 90F. I would think overcharge at first but given it operated well last year, that isn't the problem. An expansion valve stuck open or poor airflow over the condensor will result in high pressures.

"Could you hear the front fan running?" "Poor airflow over the condesor." That nailed it. The condensor fan is seized. Bingo. Thanks guys.

Good rule of thumb for R12 high side OPERATING pressure.

1) start @100 psi
2) add the ambient temp; in this case 90
3) multiply @20% of 1&2 then add result for high humidity
This would equal @228 psi on high side

Much more than that (like 250 psi on high side) means that the system is a) Overcharged b)not enough liquid c) condenser fins clogged or very dirty d) Condenser fan not working propery or or impeded air flow e) possible restriction in the high side

Jim Sims, the board's resident a/c guru, once posted these numbers:

Typical high side pressures (without adjustment for altitude):

For R-12
80 F ambient 170 to 210 psi
85 F ambient 180 to 220 psi
90 F ambient 190 to 230 psi
95 F ambient 205 to 250 psi
100 F ambient 220 to 270 psi

For R-134a
70 to 80 F ambient 115 to 200 psi
80 to 90 F ambient 140 to 235 psi
90 to 100 F ambient 165 to 270 psi
100 to 110 F ambient 210 to 310 psi


Typical low side pressures (not adjusted for altitude):

For R-12
32 F Evaporator temp 30 psi
36 F Evaporator temp 35 psi
42 F Evaporator temp 40 psi
48 F Evaporator temp 45 psi
53 F Evaporator temp 50 psi

For 134a
33-50 F Evaporator temp 16 to 29 psi
33-60 F Evaporator temp 19 to 39 psi
40-65 F Evaporator temp 25 to 43 psi
48-65 F Evaporator temp 37 to 51 psi

Great. Thanks. Once I get my hands on a new fan motor and pull that damed squirrel cage off my old one, I'll be all set.