The temp, oil pressure and gas gauges in many of our old Mopars work by allowing current to flow through gauge into the sending unit (which does NOT send anything) into a ground. Technically the 'sending unit' is just a switch that allows current to flow to a ground, but I'll call it a sending unit since everybody else does. I cannot speak for older A Bodies (pre 67), but I am familiar with 67-early 70s. Here is how the temp gauge works on my 69 Barracuda with ralley dash (and gas and oil pressure are pretty much the same):
Power, in the form of 5.5 volts DC, flows from the Instrument Voltage Regulator (IVR) into one of the two pegs on the back of the temp gauge. On my car the IVR is inside the gas gauge. The current then flows through the bi-metal coil attached to the needle then to the other peg on the back of the temp gauge. Then a single wire goes to the temp gauge sending unit that is screwed into the engine (or intake manifold). The temp gauge sending unit is actually a resistor switch that allows varying amounts of current to flow through it depending on the sensed engine temperature. That is how the temp gauge is wired; now let's look at how it works.
While the ignition switch is in the ON position, the 5.5 V current mentioned above will always be flowing through the gauge to the sending unit to the temp sending unit. If the engine is cold, the resistor in the sending unit will allow NO current to flow through it to the grounded engine. Since the current stops at the temp gauge sending unit, that means there is not a completed circuit, which means current will not be flowing through the bi-metal coil in the temp gauge. That means the coil will not heat up, and it will not rotate and move the temp needle. As the engine starts to warm up, the resistor in the temp gauge sending unit will gradually start letting current flow through the temp gauge sending unit to the grounded engine (a complete circuit). The warmer the engine gets, the more current will flow through the temp gauge sending unit. As current flows through the entire temp gauge circuit, the bi-metal coil heats up, rotates and causes the needle to move. More engine heat means more current flowing through the temp gauge sending unit which means more current flowing through the bi-metal coil, which means the coil will get warmer, which means it will move more, which means the needle will move more towards the "H". If the engine overheats, the resistor in the temp gauge sending unit will open all the way allowing the full 5.5 volts to flow through it and the temp gauge will peg to full Hot.
So how do we test the temp gauge? If the gauge is not installed, or if the instrument panel is not installed in the car, all you need to do is place two 1.5 volt batteries end to end so as to get 3.0 volts. Then run a wire from the negative side of the batteries to one of the posts on the back of the temp gauge and a wire from the positive side of the batteries to the other post. 3.0 volts is about half of 5.5 volts, so the needle should go to about halfway. If you get no movement, change the wires around.
If the instrument panel is installed in the car it is almost as simple as taking the wire off of the temp gauge sending unit and touching it the grounded engine with the ignition switch on. HOWEVER I do recommend taking The following precaution. I was assured by a man who repairs gauges that temporary direct grounding to test a gauge will not damage it, but he suggested taking a precaution when testing at home. You have seen those circuit testers that look like a small screwdriver with a light in the handle, right? The filament in the bulb in the handle is very thin and will not allow a lot of current to flow through it. So have an assistant sit in the car and turn the ignition switch to the ON position. Then take the wire off of the temp gauge sending unit. Then attach the wire coming from the top of the 'light in the handle tester' to the wire you just pulled off of the temp gauge sending unit and then touch the pointed metal tip of the 'light in the handle tester' to the engine, thereby completing the circuit and allowing current to flow through the gauge. The light bulb filament will allow enough current to flow through it to operate the gauge, but not enough to damage the gauge. Your assistant should see the gauge gradually move up. Once you have seen the needle move about 2/3 to 3/4 of the way, stop the test. There is no need, in my opinion, to peg the gauge all the way to the "H". I have tested a lot of gauges this way.
The gas and oil pressure gauges work similarly and are tested similarly.
