Automotive Air Conditioning Information Forum

After evacuating my new system on an old 1974 car (new everything except evaporator which was flushed and tested fine), I ran into trouble charging the system. The system held full vacuum flawlessly. I attached the R134a to my manifold gauges, purged the air out of the yellow line, closed the can and started the car with the A/C on. I started to open the low side valve and the pressure shot up to 115, the can wasn't emptying; at which point I stopped everything. I opened the high side valve and it read 125. I got under the car and noticed a few drops of oil dripping form the rear portion of the new compressor. Any help would be appreciated. I am a well intentioned novice and did my homework on all I have done so far, but my diagnostic experience for what I describe is near zero. Thanks again - Scott in Fort Lauderdale

Scott, A few questions. What make and model of vehicle are you working on, and does it have factory A/C?
You said that as you began charging it (through the low side), the low pressure shot up to 115, but the can wasn't emptying. Since you then immediately shut everything down, how do you know the can wasn't emptying?
It's normal for the low-side pressure to rise like that because you're charging refrigerant from a can that has approximately the same pressure you saw. After you finish charging it, expect the low side to drop into the normal range with the system running at around 1,800 RPM, windows open, and blower on high.
After shut-down and the system equalizes, the low and high sides will be nearly the same, as you saw. Of course, you haven't finished charging it yet.
Lastly, what kind of oil did you put in it and how much? R-134a requires PAG oil, not the mineral oil (and R-12) that was in it originally.

scottjeffrey wrote: I attached the R134a to my manifold gauges, purged the air out of the yellow line, closed the can and started the car with the A/C on.

Hmmm - here's what I do (wear safety glasses):

After evacuation step and checking to ensure that vacuum holds, I attach the refrigerant can to my manifold gauges, purge the air out of the yellow line, close the can, HAVE THE ENGINE AND AC OFF, and open the low pressure valve and allow the system vacuum to pull in the refrigerant. The can be right-side up or down because the compressor is not running.

After all refrigerant that can be pulled in, the can will warm to ambient. That's when to turn on the engine and AC, have the can upright, and warm/shake with your hands. Close low side service valve and add new can, purge the yellow line again, then open up the low side service valve, etc. OK to use warm (not hot) water bath to help force refrigerant into the system.

If you do correctly, figure on getting 11 oz. actually into the system from each can. To get a real low side reading, the low side service valve must be closed. Never open the high side service valve except during the evacuation step.