ON THE LFT FRT CORNER BEHIND THE GRILL IS THE PRESSURE SW, ANYTHING BELOW 30 PSI IT OPENS CIRCUIT TO PROTECT THE COMP,YOU SHOULD BE ABLE TO JUST PUT THE GAUGES ON BOTH HI AND LOW SIDE OPEN THE LOW SIDE VLV ONLY, AND LET IN ENOUGH REFRIGERANT TO GET THE PRESS UP TO 30-35 PSI AND THE COMP SHOULD KICK IN, AND THEN PUT IN THE CORRECT AMOUNT OF R134 , BE CARE FULL AND DO NOT FILL UNTILL THE SIGHT GLASS IS CLEAR , IT WILL BE WAY TOO MUCH, FILL TOO 80 % OF THE R12 FILL, OR YOUR PRESSURES WILL GO TOO HIGH AND AT ABOUT 340 PSI THE SW WILL OPEN AGAIN, JERRY