More refrigerant doesn't make it colder unless the system is low on charge. That's like saying two pounds of ice is colder than one pound. You have to go by the volume listed on the sticker under the hood. If you need more than what has gone in already, place the next can (upright), in a pot of hot water.
Since I don't know your level of expertise, I'm including these important details rather than ignoring them. Before adding refrigerant, the system needs to be pumped into a vacuum for at least a half hour. Any moisture in the system will boil at 77 degrees and get sucked out. The slightest amount of moisture will cause intermittent problems if it isn't removed. Always charge on the low side. If you bought one of those do-it-yourselfer cans, it will come with the correct fitting for the low side. Be sure to replace the plastic caps on both ports when you're done. It's common for those valves to leak. Their job is to hold the refrigerant in while you remove the hoses and install the caps.
Above all, wear gloves, safety glasses, and a face shield. Refrigerant is extremely dangerous to work with. It can cause frostbite and blindness.
Don't over-charge the system. More is not better. The refrigerant has to turn into a vapor in the evaporator in the dash. That's where it becomes cold. With an over-charged system, there's too much liquid in the evaporator so it vaporizes and gets cold under the hood, not in the heater box where the in-coming air is flowing. Also, if you hit some bumpy roads or go down a hill, that liquid refrigerant could spill into the inlet port of the new compressor. AC compressors can't tolerate pumping a liquid. They'll at least seize up, and potentially be damaged.
Monday, June 16th, 2014 AT 10:41 PM