I've recently replaced compressor, accumulator and orfice. System was flushed, evacuated 12 oz oil aded. I converted to R-134. I charged with aprox. 30 oz of 134. System works, compressor cycles etc. but the vent temp is only getting to about 57 degrees at cruising speed. The FSM calls for 26-35 psi on the low side, of course of r-12. I'm at 38psi. I'm thinking maybe I am a bit overcharged for r-134. Can anybody tell me what they are charging to (low side psi) to get cold air? Thanks

 

Typically 134 runs a little higher pressure than 12. I'm not sure what temps you'll see on a c4, but i've dealt with many a/c systems, and your'e not going to get the air as cold in an original 12. The main difference is the condenser on a 134 sys. is larger than a 12. You'll also want the orrifice tube from a 134(usually the red one). Be sure you pull a good vacum, and change ALL the o-rings to the green ones.

 

I did the conversion on an '87 I had. I used the Mickey Mouse conversion kit, and it worked pretty good. I did an extra 'flush', to be sure, and didn't change any hardware. If I had, I don't think it would have worked much better.

 

The low side pressure on r134a will be about 3# less than the r12
pressure. That will be about 32-35#. The high side is higher with
r134a but the low side is a little lower. benny

 

Operating pressures are a function of air temp and humidity so any chart can be a bit deceiving. R134 freezes at 28 psi; R12 at 30 psi. It also has a smaller molecule, so it's easy to cram in too much, but 30 ozs isn't a bad place to start when the R-12 fill is 36 ozs. And without getting too technical, the key to either gas is to keep it boiling at around freezing and after it's done that, add in some sensible heat which will keep the Evaporator from turning into a big hunk of ice (and the gas you've just made from turning back into a liquid and ruining your compressor). The low pressure switch helps it do that. It steps in and cuts power to the compressor right at the threshold for making ice and for R12, it's set to 25 psi. For R134, 22.5 psi works a lot better. Adjust the switch by disconnecting the harness and turning the screw between the terminals counterclockwise to lower this threshold; clockwise to raise it. To find out where it's at now, put the system on Max and yank the blower connector. That'll drop the low side and you can note on your gage where it cuts out. Adjust and retest from there. By the sounds of things (since you say it's cycling), you should pick up a few degrees by making sure it's running a little longer; you'll probably be turning the screw counterclockwise and I'd start with a 1/2 turn at a time.

All of this assumes of that your not staring at ice on the lines now and that you properly evacuated all of the air out of the system before you started putting in gas. When it's working right, the compressor shouldn't be cycling (if it's 70 or better at the Condensor) and the air out of the vents will be hitting somewhere around 45 to 50 degrees - until it gets to about 95 degrees at the condensor. At that point, you're probably looking at a 58 to 60 degree vent temp, but 60 degrees at 100 is a heck of lot better than 100 degrees at 100.

Finally, if your Vette is '90 and up, consider getting the ECM reprogrammed to turn on the fans when the Pressure Sensor hits about 180 to 200 psi. For R12, they're not going to turn on until 220 to 240 psi. R134 can get a bit out of hand, particularly at higher ambients, if you don't keep it cool from the outset and that means you'll never get it metering into the Evaporator at the ideal pressure/temp. I know there are different orifices out there that may or may not overcome this, but since the OEM's accomplished the same thing by turning the fans on sooner, why not follow their lead? If you're '89 or below, you don't have this option, so another gizmo, such as the variable orifice, might be your only option.