A/C Question/Help

[56 Hauler]

I am restoring a family owned and built 1965 Chevy truck with a 5.3 LS engine. The entire wiring harness for this vehicle was hacked up to almost nothing so I purchased a Painless Wiring harness #60617. Everything has gone together pretty effortlessly and with the help of many of the great people on this forum it fired up and has run great on the first try.
Now I've run into a problem I can't seem to figure out.
I installed a Classic Air "Perfect Fit in dash A/C unit complete with everything to have air conditioning, it works okay when hooked up per classic instructions but the Painless harness is designed to work with 2 electric fans and when the a/c system is hooked up as instructed and the a/c is turned on the fans do not come on and the high pressure switch shuts the compressor off. With my scanner hooked up it shows the a/c request turns on and off as it is supposed to but it never turns either the compressor or fans on. Trying to trouble shoot it I can't find any power out of either fan relay or a/c relay when turning the system on.
I've disconnected the H2o sensor and both fans turn on in the default mode so I'm really scratching my head, can any one help me ? Thanks

[Clyde65]

You need a tri-nary switch. also you should not be running the AC through the PCM. The AC system is a stand alone, the only interaction would be the connection to the compressor.

[ls1nova71]

Like Clyde said, it's way easier to just run the A/C as a stand alone system. You can run it through the PCM depending on what year of harness/PCM you're using, but it's not the noticeable difference people think it will be. What year are you working with?

[56 Hauler]

The engine is a 2004 5.3 but the harness requires the PCM to be flashed as a 1999-2000 Camaro.
The system can not be run by itself because it has no manual fan to keep air moving across the condenser at idle and the system over pressures it self and shuts the compressor off, so I need to run it thur the PCM. As it is explained to me when hooked up correctly when the a/c is turned on both fans should come on and run until approx. 40 mph then go off, also the fans work within an RPM window anything above 4300 the fans shut off and at full throttle it shuts the compressor off.

[ls1nova71]

Do you have the pressure sensor from the F-body? You'll need to wire it up exactly like the Camaro for it to work, or you just get a trinary switch (which I'm sure Classic Air sells) and wire one of the wires off of it to a relay to power up the fans when the head pressure gets high enough. That way running down the road when you don't need the fans on, because the head pressure will be low enough from the air from moving, the fans won't be on. Hooking them up to come on when ever the A/C is on will run the fans constantly which is just unnecessary run time on them.

[Clyde65]

LS1nova71 that is exactly how mine is set up, it works famously ( Old Air Prod)