I have a JD4440, that is switched over to 134A and tends to need one large can, every Spring.

Usually, the A/C works slightly (flowing little cool air), first thing in the Spring; then goes "dead" (no cool air, what-so-ever, out the cab vents) = this means the A/C system got so low on Freon, that it shuts off the electricity to the A/C clutch.

I get a large can of 134A freon (with its hose attached to can), at auto parts store; read directions on can {about shacking and twisting can}, then I have to open cab doors and run cab A/C controls on HIGH; then  "hot wire" 12 volts, to the tractor's A/C clutch wire (so it will want to "pull in" the new freon, when I squeeze the trigger, on new Freon can)

I just use a red 30" longer jumper wire (with alligator clips) to engine starter POS post, to "hot wire" that single wire going to tractor's A/C clutch = Watch the front of the A/C clutch (center part) and you will see it "Engage/Turn/Rotate" (with jumper attached). Then add new Freon, when A/C clutch is rotating/engaged. Note: once you get enough freon "into the system", then you could reattach factory 12 volt connector and it will provide necessary 12 volts to keep A/C clutch engaged and rotating.

It takes a whole large can, of Freon, just to barely get into the "Green" on the new Freon can's gauge = good enough for me, as I feel cool/cold air coming from tractor's air vents. I tend to get by the whole Summer and into Fall harvest; but this tractor doesn't get used much in the worst times, like August time frame. 

Worse case, I might have to add a "small can" in late Summer, or Harvest time, to get by until cold weather is constant. This is very rare, I ever need that additional small can, added to the A/C system.

This JD4440 is my sprayer tractor and grain cart tractor.