The Stovebolt Site Forums The Shop Area General Truck Talk Calling R12 experts, pressures question

My truck has an OldAire AC system from the 90's, still using R-12 freon. About 85-90 degrees in the shop today, the system blows 47 degree air. I measured the high and low pressures, low about 12-15 psi and the high about 225-240 psi depending on rpm. I can live with the 47 degree air but the pressures seem off, the low too low and the high too high. There are no bubbles in the sight glass. Any ideas if this is a problem waiting to happen or if I just drive it? I have several road trips planned this summer and hopefully trouble free. Thanks for your time.

Mark

It appears as though the system has an internal refrigerant restriction. The refrigerant changes from a liquid to a gas at the metering device, that could be a thermostatic expansion valve(TXV) or a fixed orifice. As the refrigerant changes state it also changes in pressure at the metering device, in your case it is too great of a pressure drop, I would guess that some debris is plugging the screen if it's a fixed orifice or if it is a TXV system, that device may have failed. With the low pressure in the evaporator, the system will create ice on the coil, the ice will further lower the low pressure and raise the high pressure, at some point you will get no airflow from the vents due to solid ice on the evap coil.

Thanks, a restriction makes sense. There is an adjustable thermostat in the coils that cycles the compressor. I will make sure it is not on full cold to keep the system from freezing up. Does it seem that its OK to run it this way?

Mark

An adjustment at the thermostat could help keep ice from forming, in theory you would like the compressor to run as long a cycle as possible for longer compressor and clutch life. Short cycling will wear out the clutch pretty fast and it also doesn't allow oil to return to the compressor as well as a long cycle. If you are planning on using the ac it might be a good idea to take it to a shop that does a lot of that type of work.

"Normal" pressures depend on ambient air temperature, and humidity. The hotter and more humid the air is, the less efficient the cooling will be. R-12 usually reaches 32 degrees F. at around 28-30 PSI suction pressure. Most systems were designed to cycle the compressor off just as the evaporator coils would begin to ice over, then kick back on at around 35-45 PSI. If the compressor failed to cycle, the evaporator would freeze over. Watch the suction pressure as the compressor trips off- - - - -if it gets below 20-25 PSI, something's wrong with the clutch control circuit. I started doing automotive A/C work in the early 1960's.
Jerry