I have a build thread on here for my 03 Supercrew that originally came with a 4.6. I swapped in a 3.9 Cummins 4bt.

The 4.6 had a blown head gasket, but it ran when I bought it. The ac had a full charge when I broke the lines on the compressor. I have been driving it a very long time in the south MS heat and decided to hook the ac up. I had custom lines made and a custom fitting for the compressor to make it all work. Today, I rented a vacuum pump and it will pull just over 10 bar on the gauge, but goes back to zero very quickly once the pump is off. The turbo on the 4bt makes getting to the low pressure difficult, but not impossible. Since, I can't charge the system. How should I go about finding the leak. BTW, I put all new orings on the lines, but the kit did not include compressor orings. Can I pressurize the system somehow? If so, what fittings would you use. Any help much appreciated!

 

Assuming you are using a standard A/C service gauge manifold with 3 hoses, with the center hose to the vac pump. Before turning off the pump, close both manifold valves, to isolate the A/C system from the pump. Some vac pumps have a check-valve that prevents air entry through the pump and back into the system, many don't, mine doesn't. Sucking air through the pump backward may also put some pump oil into the A/C system, not good. I'm saying all this as a sanity check, in case your problem is this issue, and not an actual system leak.

Trying to find a leak in an evacuated system IS hard! You can evacuate it again, and introduce ~one can of R-134 to then pressurized it, keeping engine OFF. With some R-134 in it, you can then look for leaks. Doing this test charge method, very little refrigerant will be liquid in the system, so keep manifold set and charge can hooked up to keep the gas-state pressure up while you troubleshoot. Otherwise, pressure will bleed down quick, and make it hard to find.

This is different than a properly-charged system, which would have some refrigerant in liquid state. A system with refrigerant in liquid state with a leak, as pressure eases just a bit because of the leak, some of the liquid changes to gas, which occupies much more space. Equilibrium pressure is reached between liquid and gas, which is temperature dependent.
So without liquid, you need to keep adding some R-134 in small amounts to keep pressure up with a leak. Just leaving the can attached to the charge hose, with one or more manifold valves open will take care of that. There is no worry about over-pressurizing anything with the can. Keep engine OFF.

 

I thought about that and did close both sides of the gauges. I then removed the yellow hose (yes proper 3 hose setup). I held my thumb over the center hole to see if there was any vacuum left. It did not. The crazy thing is that with the air vacuum tool I could plug the exhaust hole with my thump and pressure the system to about 35 psi and it would hold? Not sure what's going on. The only two orings not in the kit were the ones on the compressor. As soon as I find those, i'll replace them and see if that's the ticket. Thanks again.

 

I discovered that the quick connects on my borrowed gauges were leaking. I also discovered that the vacuum pump does not have a check valve and it will cause a leak down as well.

I did get it to hold vacuum for well over an hour after the pump was turned off. I started to charge, but it wouldn't even take a whole can of Freon. I cannot get the ac clutch to engage. I have a "modified" wiring harness and I'm afraid I have cut one too many wires. I need to know where the red with yellow stripped wire is to go after it leaves the switch. It goes to a plug on the fire wall and from there I'm not sure where it should go. I have a wiring diagram book, but it doesn't tell where it goes from there. any help appreciated.