AC compressor engages but no cool air.

The best way to know if you have the correct amount of refrigerant is to evacuate the system, pull a vacuum for the proper amount of time and then recharge with the specified amount for your truck. You can show adequate pressure and still not have enough refrigerant in the system. Also can have too much refrigerant and the pressure will show the same.

If you know you have the correct amount the the pump still doesn't pull the low side down, the pump is bad.

I've been through it enough times I carry a gauge set, fittings, spare expansion valve, drier and a vacuum pump I can run off the inverter. Every few years I'm out on the road and something happens. Glad to not rely on a truck stop.

At least it's getting to cooler weather.

 

Harbor freight sells a guage kit for cheap, check your pressures, is the suction line cool, the same, or hotter then everything else? Could be the compressor itself if everything else checks out

 

Sounds like the expansion valve. Needs to be replaced. Evacuated then charge again.

 

Just for curiosity, what make and model is the truck?

 

Should have a sticker somewhere. Most Freightliners take 3 lb 7oz. I always use 5 12oz. cans. Total 60oz. That’s 5 extra oz. I bleed the air from the line on each can before using. Always works just right. Good time to add some green ultraviolet dye. Available already mixed in a can refrigerant. One or two cans with it should be plenty. Better to buy a set of gauges. Get a small bottle, add it in through the hose. YouTuber “Chris Fix” has some great videos on A/C. Very thorough simply presented info.

 

I don't recommend the harbor freight gauges. They screw in far enough to bend the Schrader valves.

If you do decide to go that route. Only open them enough to get pressure readings. And slightly further. Don't open them all the way.

Different trucks take different amounts. Look around on the firewall or dryer. There should be a label that says what to use and how much it holds. Hopefully the label is still there and readable.

The low pressure switch requires 50 psi to kick the compressor on. A fully charged system will read around 120 psi depending on where you're at. That's in the dead of heat. Down by the southern border it's probably closer to 130 psi. The northern border probably around 100 psi.

Freon contracts in the cold and expands in the heat. So pressures will be different at different temps.

Your problem with measuring is you don't have a scale. You'll have to guess best the last can. I don't have a machine but I use 30 pound cylinders with a scale that measures ounces and pounds. I'm not a big fan of cans. I'm also certified so I can buy any quantity above 2 pounds.

The good news is freon has dropped in price. About 40% for me. It shouldn't cost so much to have a shop do the job. So you know it's done right. The bad news is they'll recycle the freon. Used recycle doesn't work anywhere as good as virgin freon.

 

Yep. Some lessons you have to pay for and that was one for me.