The thermal switch on the AC/Heater Box does not light up my test light however it reads voltage through the switch?

Without looking at the diagram, what it sounds like you're running into is a common, but confusing situation. Imagine you have a garden hose with 50 pounds of water pressure, and the hose is 99 percent restricted by your foot on it. The nozzle at the end is closed, so if you could put a pressure gauge there, you'd find 50 psi. Now open the nozzle and very little water would flow because your foot is on the hose. Also, you'd find very close to 0 psi just before the nozzle.

Voltage is electrical pressure. If you have very high resistance in the temperature switch, as in arced or pitted contacts, that excessive resistance won't have any effect while the circuit is unplugged / disconnected. You'll see the full 12 volts just like you'd see the full 50 psi with the nozzle closed. It's when you connect everything and current is trying to flow that most of the 12 volts gets "dropped" across the excessive resistance, leaving close to 0 volts to be measured after that resistance.

To say this a different way, the voltage readings are only valid when taken in a circuit that is fully connected and able to operate. By unplugging things, then taking voltage readings will work most of the time when you have a solid defect like a blown fuse, disconnected plug, and things like that. It's when things don't make sense that you have to revert to only working in the circuit when everything is connected or plugged in.

I've had a half dozen 1972 Darts and Demons, some with AC, but I never had to work on this system, so everything is from memory. Our online service manuals don't include wiring diagrams that far back. First, if the switch is a cycling switch, meaning the controlling device for the compressor, it should have continuity, (voltage on both terminals), at times, and open circuit when the evaporator gets cold enough. If you don't find that, use a jumper wire to bypass it. The compressor should cycle on. See how far this gets you, then, if necessary, I'll look for a diagram at home.

Some systems were controlled by a pressure switch. It is purely coincidence that with R-12, the pressure at rest is almost identical to its temperature. The low side is regulated to stay very close to 40 psi which equates to 40 degrees in the evaporator, right where the temperature sensor should be when the correct amount of refrigerant is in the system. You don't want the pressure / temperature colder than that because the main way AC systems provide comfort is by removing the humidity from the air. It condenses on the evaporator, then drips onto the ground. If the evaporator is allowed to get colder than 32 degrees, (32 psi), that condensate would freeze into a block of ice and block air flow.

I'm bringing this up because if the low-pressure switch does the regulating, any temperature sensor on the evaporator is just a backup, or safety cutout. The system should work properly with the sensor bypassed. On the other hand, if regulation is done by that temperature sensor, and you have it bypassed, the compressor will run continuously and make the evaporator too cold, so you can't leave it like that.

We can discuss this further once I look at a diagram. Let me know if this helps.

Thank you for posting the diagram, but be aware there were a lot of changes between the '71 and '72 models. My '72 Dart had a relay running the compressor coil. I don't see that in this diagram.

The switch you posted is indeed a cycling switch. The compressor clutch coil could draw a good five amps or more. That's a lot to ask of a set of switch contacts to handle. Regardless, you can jump those two wires together with a fairly heavy jumper wire to make the compressor run. Let me know. If you don't get to it soon, I'll be back tomorrow evening to see how you're doing.

Isn't it funny, for my '72 Challenger, the entire wiring diagram fits on two pages of service manuals. Today it takes two pages of diagrams to cover one taillight, and the cars are no more reliable.

Also be aware, for decades, Chrysler was the world's leader in innovations that benefited car owners. The list includes things like the first anti-lock brakes, (1969), first lock-up torque converter, (1977), first computer-controlled automatic transmission, (1989), first electronic voltage regulator, (1970), and first "AC generator", (1960. They copyrighted the term "alternator"). My reason for bringing this up is they were also the first to have a true electronic ignition system, but that showed up in 1972, but only on the Dodges. They used it on Plymouths and Chryslers in 1973. While this new diagram is not intuitively obvious, after following it for a couple of minutes, I'm pretty sure it is different from what I had. That's why I'm remembering it differently. The Scamp is the 2-door version of the Plymouth Valiant. I had a couple of Swingers and a Demon, the 2-door versions of the Dodge Dart.

The other thing that is noteworthy at the time was the Plymouths were the low-cost models with options you could add, including trim packages. The Dodges were higher cost because many of those options were included automatically, then, the options you could add to the Dodges were included automatically on the Chrysler models, plus the Chryslers offered some things you just couldn't get on the Plymouths and Dodges. Mostly that amounted to trim packages like leather seats or velour cloth.

Another exciting bit of trivia to add to your memory banks is starting with '72 models, every dark blue wire under the hood had 12 volts switched onto it with the ignition switch in the "run" position. That included feeding things like the alternator field, electric choke heater, ignition ballast resistor, and things like that. That held true through at least the 1989 models, except a few truck models used red wires for the switched 12 volt circuit under the hood. I see dark blue wires on your diagram, but not in that function. Regardless, I traced what I think is the current path to run the compressor clutch. I'm not sure what they're doing with the vacuum switch at the top left, but they did use them to turn the compressor off when vacuum was low. That occurs when passing a freight train, going up a steep hill, and you need every ounce of horsepower you can get to move the car. You can get along for those few seconds without the compressor.

Even if I'm interpreting that switch wrong, if you start at the lower left where it says, "Cavity "A" to air conditioning clutch", then work your way back to the "Temperature control switch", you know you gotta have continuity through that switch for current to flow to the clutch coil. Given the nifty photo you posted of your switch, and the way it's labeled here, it is the control unit that regulates system operation. It's not a safety cutout or anything like that. You can safely bypass it temporarily with a jumper wire, but that wire should be fat enough to handle at least five amps. A 14 gauge wire can do that.

You originally mentioned you found 12 volts on one side of that switch, but not the other side. Logic would dictate the switch is open, but check that again with everything connected. Now, if you find you have close to 0 volts on both wires, the high resistance is in the vacuum switch, or, less likely, the wire running back to it. What you would do is keep working your way back to the various test points until you find where you still have 12 volts, and the one next to it where it's missing. I know I'm not explaining that well, so I'll do a better job of it later if it comes to that.

Another fun fact to share with your friends and family is while the AC system was factory-installed, it looks like an aftermarket add-on system by the way it hangs down under the dash. What reminded me of that is when following the diagram back to the power source at the right side of the diagram, it is not coming from a fuse dedicated for the "AC" or "Heater" system. They're using an Accessory terminal in the fuse box. That terminal was often provided for owners who wanted to add a CB radio, tape player, or things like that, and they didn't want you to compromise the tension on the fuse holder terminals by stretching them with an extra wire stuffed in there. It also relieved them of some liability issues if something in the fuse box started a fire. For dealer-installed options, the cruise control would be a better example. Everything came in one package, including the wire harness to connect all the pieces, then the last step was to plug the 12-volt wire into that "Accessory" terminal. You didn't have to rewire the fuse box.

So, to boil down all this great and wondrous information, jump the two terminals on the "Temperature Control Switch", and the compressor should run, but the engine will have to be running so the intake vacuum turns on that vacuum switch. If that works, the next issue is finding a good replacement. I didn't see one listed on Rock Auto. AC in those early years was pretty rare, so finding a used switch in a salvage yard will be hard. I'm in Wisconsin where anything over ten years old is rusted away due to the unnecessary overuse of salt in winter. You'll have better luck down south.

Another resource to consider is a radiator repair shop or a shop that specializes in AC repair. The people there will likely know of some other brand or model that used a similar switch. Don't overlook the dealers too. Every single one of them that has been in business since the '70s or earlier has a lot of "new-old-stock, (NOS) parts covered in dust on the back shelf. You never know what treasures you're going to find there. As a really last resort, I would entertain a notion to take the old switch apart and see if the contacts could be sanded. If they're arced and pitted from repeatedly switching that relatively high current on and off, shining up the contacts will work. Thermal switches operate by means of a bimetallic strip that bends with changes in temperature. Often those strips become brittle or corroded and will lose their spring tension first, then even fall part. There's no fixing that.

Let me know what you find. Meanwhile, I'll keep working on ideas of what might work if you can't find a new switch.

Dandy. We have the nation's second largest old car show, after Carlilse, PA, 55 miles from me. About five or six years ago they had one of the turbine cars on display.

Let me know when you make some progress.