Wish you had read the FSM before getting this far into it......
Lets assume that you;re standing there with a clean, empty system.
Vac system to max (29hg) and hold that for an hr pump running. D/C pump and observe the vac....make sure it stays where it is. Assuming the new orifice is correct, (blue tubes are most popular or smart tubes) then add OIL charge w/134a FIRST to lubricate the dry system, engine NOT running. Actually, you should have poured an oz in the compressor before assembly and rotated it by hand....
Ok, got 6-8 oz of oil in and now time to start the engine and connect the 1st can of gas. NOW, with the gas connected, jump the LOW pressure cut-out switch so the compressor will run and take the charge. IF you fail to do this the gas will not circulate, the system will not release the circuit and nothing will happen.
The system for opt 68 elect a/c is powered all the time and like everything else on a Corvette, its managed by completing ground paths inside the BCM or control head, whichever the case may be.
Low press switch jumped, gas ON and discharging the compressor should now run to accept the gas. 134a charge will be about 90% of what the R-12 was, so get the FSM out and calculate how much you will need, approx 2 lbs give or take a few oz.
BUT< always use the gauges to know when its full and when its time to stop adding gas charge. Even IF you are 1/2 lb short of the calculated gas charge, STOP when the pressures reach a specific point. 134a is VERY fickle and can work well or not at all with only 2 oz difference. The key is the low pressure. I do not recall exactly what that should be, but its well documented and can be found if you do a google search and do some homework on conversions. You have to stop when the high pressure reaches a certain point, but if the LOW is not high enough its not going to work well...usually this means there is something wrong at the condensor. Add gas slowly and give it time to react before going too far.
The proper amount of oil is important...too much effects the 1 or 2 oz of gas charge.....too little and you risk seizing the compressor.
The system should be sterile before charging. New, clean dryer, tube, flushed lines and compressor, flushed condensor. Even NEW condensors require flushing because they have debris inside that settles in the bottom coils and plugs off rows so you loose heat exchange...and cooling efficiency suffers.
The secret to making a conversion work well is in the condensor and how well you can draw the heat off the thing...mine was all new...everything under the hood. Like a new car, except done by me and not the factory. The heat from my condensor is so great that it drives the radiator temp upward within 3 or 4 minutes.
If my (big all alum radiator) has me running at 187 degrees.....turning on the a/c drives it up beyond 200. As long as you are dumping that condensor heat you are cooling the cabin. If your 134a charge is off and there is too much heating then it cannot cool. If the 134a is too low it cannot collect enough cabin heat to carry to the condensor. So, make certain that your cooling stack is clean,(would be if you installed a new condensor) no leaves or debris trapped in the air-box, and make sure the air-dam is good and in one piece. It ALL matters because these conversions are so fickle. Some will put frost on the windshield while others do good to blow 55 degree air from the vents.
Bottom line on conversions is that you have to follow the pressures more than you do the weights and other specs.
Jump the low pressure switch to make the compressor run but ONLY after you have installed a can of oil charge so there is enough lube in the system to allow it to run for a few minutes and charge. After you get 1 to 1.5 lbs of gas in, take the jumper off and reconnect the low press switch and it should be able to cycle on its own then.
Mine works well enough, BUT, since the street prices of R12 have dropped to around $35 a lb, and 134a is UP to around $15 and climbing, I'd go back. I WILL next time I have to open the system...
One more detail....
you may need t o adjust the low pressure cut out cycling switch since your 134a low side will be a few lbs different than the R12...I think its about 1/4 turn of the screw CC to prevent the switch from cycling the compressor too much.
