Go to your auto parts store, rent a manifold gauge set and make sure you grab the adapter for the canister that you'd be recharging with (likely self sealing) ask them to help you pick the right one. Do NOT buy anything with leak sealer in it. Just buy the cheap r134a without all the extra gunk. These trucks use an orafice tube setup that lies between the two pressure ports. Adding leak sealer can clog up the screen/ porous material and cause a whole other issue. Please note, that they will require a $150 deposit for the gauges (that youll get back) and the refrigerant is non-returnable. If you're worried, do the process in their parkinglot and buy the refrigerant if needed afterwards.
Ensure that the valves on the face of the gauge set are closed. Hook up the gauges to the high and low pressure ports, located in the rear passenger corner of the engine bay right next to one another. They are quick disconnects similar to an air hose. Then slowly turn the valves on top of these fittings until you see the gauge move. Each line correlates to their respective gauges.
take note of their readings. When the truck's ac hasn't been running, they will read the same.
Start the truck, turn the ac on, set it to it's coldest level. Check and watch the gauges.
When the compressor turns on, it will draw from the low side, compress it, and then release it to the high side. If the low side is drawn too low, the compressor will shut off. The high pressure will then drop and the system will start to equalize back out a bit. The compressor will then turn back on and repeat this cycle.
Depending on the temperature outside when you check, the low side is going to run around 35-45psi when the compressor is on and 220-240psi on the high side. (roughly. cant remember the exacts off the top of my head). When the low side drops to about 20psi, the compressor will shut off for low pressure. If you watch your low side nosedive when the compressor kicks on, followed by it shutting off when it drops to 20psi, it's just low on refrigerant.
If this is the case, you can connect a cannister of r134a to the central line. Open the valve on the can if it has one, then depress the purge valve on the line until the refrigerant sprays out. Then you can slowly open the low side and add refrigerant to the system. While this valve is open, the low side will spike to ~80psi as the can pressure is introduced. shut this valve off every 5 seconds or so and let the system stabilize.
wait for the compressor to cycle on if it has not already and watch it draw down the low side. Repeat the above process, replacing the can as needed until the compressor remains on the whole time. I added a little extra to empty my can and wound up roughly at 42psi @90 degrees while the compressor was on.
At this point, you should be able to get into the cab and feel it blowing cold the whole time. Head back out, slowly "loosen" the valves on the connections to the truck and remove the connectors. Close the valve to the can (if it has one) and while holding onto one of the gauge hoses, slowly open the corresponding valve to remove any remaining pressure in the tool. Disassemble as needed and boom. You're done.
