So, I've completely taken apart, flushed cleaned, new oil, new seals, new accumulator, and reassembled, and I'm ready to vacuum & charge the system.

My question is this: Charging instructions say to turn on a/c to max. The compressor won't be running when I start the charge, because the low-pressure cut-off switch will prevent it from running until pressure is sufficient. Am I to expect that as I begin adding the cans of 134a, the compressor will kick on?

Or should I jumper the low-pressure cut-off while charging?

 

Vacuum it first.

Then, the refrigerant will almost jump into that vacuum on its own. It helps if the refrigerant is warmed slightly first, perhaps 20-30 degrees over ambient.

The pressure of the cans is usually enough to activate the cycling switch. As the system draws in more gas (assuming you're not charging it as a liquid), the compressor-on time will gradually increase as the amount of refrigerant level increases.

 

thanks! I'm finding that I'm unable to vacuum the system myself. I have the vacuum, bar gauges, & bought hoses, but none of the fittings match up to the hoses, and I can't find any adapters to help (the vacuum pump & bar gauges were given to me by a refrigeration & bldg. a/c guy)

But I did find a local independent garage that said he will pull a vacuum for me for cheap...on MONDAY. ARGH! I'm so close!

So, Monday, he's gonna pull a vacuum, and I'm just going to have to use the janky fill/guage that came with the 134a. While it's not as "professional" as filling through the bar gauge, I think it will be fine.

 

I hope that works out for you. It surely isn't the best procedure.

Problems with this method could be:
Once the system is evacuated and all moisture boiled off, the usual step is to shut off the manifold valves, disconnect vac pump, and connect the refrigerant cylinder, then purging the hose to the manifold with refrigerant before opening manifold valves if using a 3-Hose manifold.
Or if using a 4-Hose manifold, isolate pump if need be, and open the refrigerant supply valve.
But in the evacuate and charge later idea, the gauge manifold's hoses will have to be disconnected. This disconnection can let in a small amount of air into the vacuum.
And if either the high-side or low-side service valves on the truck leak when under vacuum, you will have the possibility of some air in the system. This is not a problem in the usual method, because the service hoses are on both valves, and the depressors are holding them open.

Not saying it couldn't work out, it is just more trouble-prone, and will not really know the state of the vacuum when starting to charge.

If you are using 12 Oz. cans and a can-tapper and hose, remember after turning in the can-tapper valve to pierce the seal, to back out and open the valve ever so slightly to get a little hiss out the open end of the hose, to allow refrigerant to purge the air out of the hose before connecting to the low-side fitting on the truck. Once you hear some hiss, you can turn it off and connect it up.

If you are using one of those valve-gauge-and-hose on a can things, open up the can valve a tiny bit to purge the air out of the whole setup before attaching it to the truck.

A gallon plastic pail of very warm/hot water is a big help, it will sit up on the wheel well, and you can push the can down some into it, after you open up the valve and gas is flowing, to keep the can pressure up. To check how much is left in the can, swirl the can around once and lift out of water for a moment. If you hold the can level, you can feel down the side, it will be warm until you get down to the liquid level inside, there it will be suddenly cold.