Got things back together this weekend (new/used compressor, new receiver/drier, flush, R134a o-rings, PAG oil, new high-side switch). It seems to be working well, but I wasn't sure what pressure I should be shooting for in the charge. I used approximately 2-1/2 12-oz. cans plus a 3 oz can of "performance enhancer". At a high idle, pressure was about 260/40. Temp was about 85 around the car.

 

That low side pressure is too high. Is that the cycle off point, or does it just go there and sit? If it's cycling off, try adjusting the low pressure cut off switch. If you take the plug off, you'll see a single flat-bladed screw. IIRC, turning it coutnerclockwise (loosen) lowers the cut off pressure. I have my Olds set at 19.

If that's simply as low as it will go at high idle, then you've got a slight issue. IIRC, R12 should cycle at about 25psi.

 

It's not cycling off. I may not have gotten the low-side pressure correct (that may have been what it was when the can was charging it). 30 psi may be closer to what it is steady state.

I was under the impression R134a runs at a higher pressure than R12. My main concern was over/under charging, though. Should I be concerned about not cycling for the low side pressure?

 

If it's hot enough out, it may NOT cycle off. The low side pressure is usually a good indicator of evaporator temperature. R-134A does function at higher pressures, but that doesn't mean quite what you think it does. For a given temperature change, the pressure will change more with R134A than R12. I had a temperature chart when I charged mine that indicated that 19psi on the evaporator side was functionally similar to 25psi for R12. It actually appears that for R134A, the high side will be higher pressure and the low side will be lower pressure for the same temperatures. There's a point at which R12 and R134A have the same pressure for a given temperature and IIRC it's somewhere around 50* F. I'm still looking for a good table on the internet.

 

The 260 high side pressure seems decent. What is your high idle anyway? I really think that something ain't quite right with your low side pressure.

 

I had it going at about 1200 when I was doing all this.

I'll hook the gages back up and see what it's really doing.

 

30 ozs (more or less) is about right for charge amount for our year of car.

IMHO, 30 -40 psi on the low side and 220 -260 high side pressures on a converted 134a system is about normal.
What is your discharge temp out of the vent closest to the evaporator at max a/c and low blower speed ?
If it is in the 30's, don't worry, be happy

The rule of thumb (tho I've never met the man) is interior temperature 30* cooler than outside temperature.

On big trucks, a low side of 30-40 is acceptable, over 40 and the question of a stuck expansion valve is raised.

 

Well, if you can get the gauge to read a solid 30, you can probably be happy with that. I still think 40 is too high.

 

99,
Yes, a solid, system stabilized, 40 psi low side pressure is probably too high.

But, if it is below 40 and the discharge temp is in the 30's, then I wouldn't be concerned.

When I converted to R-134a in my Z, my pressures were a steady 35 psi low side and 220 on the high side @ 800 rpm. The discharge air temp was about 38*

 

I haven't stuck a thermometer in there yet, but it keeps the interior cool (black) under high-noon heat.

The pressures I got at about 90 degrees outside, 900 RPMs, were 275/40. Perhaps overcharged?

 

It may be a little over charged, but only by a few ounces.

My system, when it was R-12, held 38 ozs. When I converted, I charged it with 30 ozs of R-134a (80 %).

You may want to vent a little bit and see if it improves the cooling.

 

Pressures at 900 RPM only tell you that you're not spinning the compressor fast enough.

Kick the idle up to 1500 (or hold with a pedal) and watch the gauge. If it's 275psi on the high side, at 900 rpm, I bet you'll find it goes well over 300psi at 1500rpm. High side pressure is supposed to be a good indication of charge.

I'm still trying to get the hang of it, but the AC manual I have says that any R134A system that reaches 340psi on the high side should be shut down before damage occurs. In fact I think the factory high pressure switches cut off at that (of course my olds doesn't have one, which is the car I'm doing all the AC work ON).

I'm currently having a bitch of a time figuring out if my Olds is overcharged myself. I can only get 55* vent temps in 88* weather, and my low side stays around 35psi (high side 275psi). I'm thinking of removing a little more refrigerant to see if that helps the cooling performance--my guess is that it's going to take some trial and error to get a good "feel". I have also done some readings on my Firehawk (factory R134A car) and gotten 30psi and 250psi for the low/high side readings, but with much colder vent temps. The bigger condenser must have some role to play in that though...

 

It hit about 300 when I rev'd it up. Low went to about 45. I haven't heard it cycle off yet, hopefully that new high-side switch works the way it's supposed to.

I'll get a temp gage in the outlet, and try bleeding a little off soon. I'm betting I'm below 45 degrees at least, but you know how that goes. The receiver gets coated with moisture, even at our 20-30% humidity (wow, it's up to 45% right now!).

 

You want the high side press to be approx 2.2 times the temp 6" in front of the condensor at a high (like 1500 RPM) idle.
Your system will run below the high press switch set point but compressor life will degrade significantly.

If you want to really dial-in the optimum charge; do the following.
Charge/discharge system to a point where you know it is undercharged.
Check duct temp with eng at a high idle and A/C set to pull in fresh air (as opposed to recirculating). Fan on high.
Begin adding the 134A in small increments while checking the duct temp frequently. The duct temp should start coming down.
Continue adding the 134A until the duct temp just starts to rise.
Stop there, your done.
It's very important none of the A/C control settings are changed during the charging. You want the 134 charge level to be the only thing that changes. If your out in the sun and it gets cloudy, or if the sun is starting to set, or the wind picks up those things will effect your readings.