For those with a Gen 4 Dodge Caravan (2001-2007)
Also applies to Gen 3 Chrysler minivans from 1996-2000.
(And most vehicles generally speaking)
If you do not have a Dodge Caravan this might be of no interest or value to you? You might want to skip (TL/DR),
Sometimes on these vehicles (actually any, but I've seen this a few times now on these Dodges) you can run into a situation where the basic AC diagnosis is not quite enough. You've replaced nearly everything, tested everything and still the AC doesn't want to work properly.
There are several modules that can have issues that can affect the AC other than just AC specific parts such as....
IPM (Integrated or Intelligent Power Module)
PCM (Powertrain Control Module)
BCM Body Control Module
Broken wires in the harness
Bad grounds
If you've checked literally everything and still the AC doesn't work, Send 12 volts directly to the connector to the AC clutch coil.
You can easily do this by jumpering pins 87 and 30 at the PDC (Power Distribution Center), AC Control Relay. Make sure the 20A fuse for the AC clutch is not blown. Make sure the ignition switch is on and the AC controls are on.
You can also connect 12v directly to the connector at the compressor but it's a lot easier to do this if the compressor is off the vehicle.
If the coil is working you should get a nice thunk when you jumper those relay pins telling you the clutch coil is responding.
If you don't then it's likely there is a problem with the clutch coil. But (and it's a big but), it could be an issue with a control module. I see this often in these older Dodge minivans with factory alarm systems and door remotes not working properly. Sometimes it's bad grounding or frayed wires, mice etc.
I tested some old compressor clutches I had laying around today and found that a working clutch coil (PA66) will show around 4 ohms of resistance across the coil. A bad one that wouldn't respond to 12v showed .11 ohms. If you have the inclination and tools, you can check the clutch coil to see if it's good. As it goes bad it will have less and less magnetic power to pull the clutch plate in to create enough friction to drive the AC compressor. When that happens it can get REALLY hot fast and burn up.
But over the years the chips that were flashed at the factory can experience data loss and corruption where ones and zeros get altered and so these modules no longer properly control the systems the way they were designed. New sensors may work, but even being properly fed to the modules, the modules may not process those signals and send the expected corresponding outputs to their destinations, such as turning on the AC clutch to engage it...or unlocking / locking doors etc.
Ideally, you should re-flash them every 20 years or so to refresh the instructions set. I'm working on gathering the code that goes into the various chips of my vehicles but it's daunting because every vehicle can have different options. I backup up the flash code on my vehicles and catalog it so I can reflash later or replace the flash chip in the vehicle and flash that with the OEM code but that covers a very small set of options configurations.
I have a decompiler but don't yet know how to use it effectively so if you have experience with decompiling machine language code drop me a message. I think they started encrypting the flashes with Gen 5 so this only works on Gen 4 and earlier. Unless you work for the CIA and have access to a 128 bit decryption computer.
The clutch coil has only two wires going to it. A grounding wire and a 12v+ hot wire. The grounding wire gets it's ground from the IPM so you will need to connect a ground directly to the Black wire at the coil connector. You can't depend on the ground wire at the connector to be connected to ground by default. In other words, the clutch coil has no chassis ground.
When you jumper that relay, you have effectively overridden all other controls and signals and sent a direct "ENGAGE" command directly to the clutch coil, bypassing the thermo switch on the expansion valve (2001 models), the high pressure switch in the liquid (high pressure line), the PCM, BCM and IPM. It's the same as connecting wires directly to the coil (as long as there is no defect in the wire between the PCM and the connector at the compressor.
If it doesn't respond you can be assured there is either a damaged wire somewhere, a bad connection somewhere, the battery you're using can't supply at least 2.2Amps and 12v or...... the AC clutch coil is bad and will need testing with a VOM.
As a last resort (to rule out even the wiring in the vehicle), you can connect your 12v power leads directly to the AC compressor coil. This energizes the coil and holds it in the engaged position.
That will tell you for sure whether you have a working compressor regardless of the Modules. if it works this way, you can start tracking down what part of the system is not sending the proper signals and where the fault is.