67 A/C Charging Pressures Help!!

[Al R.]

Trying to charge the A/C on my 67 300-327. Compressor has just been installed and system held a vacuum of 29 in for39 hrs. Shop temp is 80*. I have a box fan in front of radiator Low pressure 26 psi high is 265psi. Air feels cold at vents. When I cut off the car, pressure on the low side went to 120psi. After 10 mn. with fan stlll blowing on the rad., they dropped to 95 low and 100psi on the high side. Unable to find my infrared therm at this time--still looking. I was trying to add R12 by weight on scales, but it only appeared to take a little over 1 1/2 lbs. BTW compressor had a full 6 oz of oil per Domenic. I have a 67 chassis service manual and in looking at it, the high pressure seemed too high for the shop temp of 80*. Manual shows 175-195psi and POA pressure of 29.5-30.5. Does it appear I don't have enough R12 in the system or possibly too much? Also concerned about the amount of low pressure when the car was cut off. Is this normal upon shut down?

[Al R.]

Re: 67 A/C Charging Pressures Help!!

Added more R12 to clear UP bubbles pressures dropped to 17psi low & 225 high. with car cut off pressures went to 70 low and 120 high after 5 min with the fan n fon ofrdiatr

[Edward J.]

Re: 67 A/C Charging Pressures Help!!

Al, once the car is shut off the pressures equalize which is normal, Your pressures sound normal to me. pressures change with outside temp. the hotter the higher the high side will be. low side generally is 25-35 PSI ,if the system is cold you should be okay. take the car for a 5 minute run and let the air flow through the condenser to help the cold air flow.

[Larry M.]

Re: 67 A/C Charging Pressures Help!!

Agree with Ed. Take the car out and drive it and see how it performs. The temperature of the air at the vents should be around 40-45 F with the fan on MED or HI. Low pressure side measured at the POA valve should be around 30-32 psig, and the high side should be around 200 psig (or so) at 80 F ambient temperature.

When you shut the AC down, the pressures will equalize, and at 80 F, they should both be around 85 psig. If the under-hood temperature is 120 F (engine not yet cooled down), the high-side and low-side pressures will both be around 160 psig. Again, this is with the compressor OFF.

The total R12 capacity of your factory system is around 3 lbs-4oz total. There should not be any bubbles visible in the drier sight glass.

Did you replace the drier?? Did you check and clean the expansion valve screen?? If not, some residual drier absorbent (molecular sieves dust) could be plugging the expansion valve inlet screen causing slightly higher than normal HI-SIDE pressure. Anything below about 230 psig at 80 F should be okay. If outside temps get up to 100F, the HI-SIDE discharge pressure could get up to 260-280 psig.

If DOM rebuilt your compressor, then it should not be a problem.

Performance Specs for your system as a function of ambient air temperature are in the 1967 Chevrolet Service Manual Section 1A-33.

Larry

EDIT: Where are you measuring the LO-SIDE pressure?? At the POA valve or at the compressor inlet. The 1967 factory manifold does not have a connection at the compressor inlet, but replacement manifolds often have them. The pressure at the POA valve should always stay at 30 PSIG or higher (and never lower). However, the pressure measured at the compressor inlet could get much lower than this. That is the reason for the POV………….to keep the evaporator condensing pressure up to prevent evaporator freeze up. It is basically a back-pressure regulator or controller.

[Al R.]

Re: 67 A/C Charging Pressures Help!!

Thanks Larry and Edward for the replies. I found my infrared checker and the outlet temp on the passenger side is around 34-36*. After driving the car for approx. 30 min, my wife informed me cold water was dripping on her feet (condensate from the box inside--chrome ducts also condensating). I did have to add a few ounces of R12 to clear up the sight glass upon returning home. The high pressure will approach near 300* with the car idling with the hood up and an engine coolant around 210-220*. BTW, plenty of water was draining out of the AC condensate drain pipe. Very humid on Sunday..