Hook gauges up and add freon with it off if it's so low the compressor won't engage. Then it will engage and you won't burn it up. Add 1 oz of oil to make up for losses- 150 pag (R134a), or 1 oz mineral for R12. Don't forget to bleed the lines of the gauges as you don't want any NCG's in the system(Non condensable gases- contaminations).