328 AC Pressure Readings – actual empirical data

My AC is blowing cold now and since I haven’t seen a post that discusses line pressures I thought that I would share some empirical data:

First some background:
’87 328
1 AC evacuation/recharge had been done on the car since new -- in 2001 (R12)
When I acquired the car in ’07 the AC blew cold
In April ’09 it didn’t blow as cold (subjective I know)
I was told by an HVAC guy that with the car turned off the low side pressure should be between 50-70psi depending on ambient temp. He also said that once the compressor kicks in the pressure should not drop less than 30%. These are ball park numbers and should not be used as an absolute indication of anything.
For reference: on pg 20 of the 328 manual the system will hold 2.2 lbs of refrigerant (R12). Careful not to over charge.

Hooking up the lines. I didn’t want to jack up the car & take off the rear wheel well to access the compressor fittings. With a lot of skepticism I tried to hook up the low pressure line from the top. It turned out to be very easy. I had it on in less than 30 seconds. First scope out the fitting with a mirror, then facing the rear of the car maneuver your right hand around, back, and under the airbox. The low pressure fitting is the lower one. Take off the cap and attach the low pressure line. Forget about right angle fittings and taking off the wheel well unless you’re a heavyweight. It was very very easy.

Once the line was connected I immediately saw 66 psi (okay I have line pressure, that’s good)
Ambient temp outside was 95 deg (yes a hot day)
Started the car and nothing changed (so far so good)
Kicked on the compressor and the pressure dropped to 33 psi (a little low based on the 30% rule)

Note that to get an accurate representation of what was going on I should have attached the high pressure line but I didn’t so I don’t have any high pressure data. If there was a significant issue with the high pressure side then I’d be in big trouble anyway since this is typically caused by a compressor failure, high side restriction, stuck expansion value, or plugged orifice, all of which would require opening up the system to repair. So I crossed my fingers.

My next step was to add material. It took 9 minutes to add 3 oz of R12. This brought up the low side pressure to 45 psi. The AC was now blowing cold. Considering that it was 95 deg outside I figured 45 might be low so I gave it another 15 minutes (it took 3 more oz) and the pressure increased to 47 psi. Once I turned the compressor off the pressure jumped to 68 psi which was 2 psi higher than when I started.

It’s now cold. Still pretty weak (relatively) due to the lack of air flow but cold nonetheless.

To summarize the low pressure readings with an ambient temp of 95 deg F were:
Compressor off: 66 psi
Compressor on: 33 psi -- (slight cooling)
Added 6 oz R12
Compressor on: 47 psi -- (cold)
Compressor off: 68 psi

For the actual temp at the vent I’m skeptical of my own data. I saw 55 deg F which is a delta of 40 deg from 95 deg ambient. For reference on the same day my SUV vent temp was 53 deg, and my home AC system was pumping out 60 deg. I figured all of them would be cooler, and yes I did check the calibration of my $5 thermometer. Ice water measured 33 deg.