67 Dart 273 is 100% correct when he tells you about the resistance needed for needle deflection. There is another way to test the gauges by using a few flashlight batteries. I have done this and it works fine. How to test the gauges is at the end. First is a brief explanation of why the test works. The oil pressure and temp gauges each have 2 pins on the back. The gas gauge has 3 pins. The needles in the instruments are deflected when current flows through a bi-metal coil they are attached to (like in a home thermostat) and makes it twist a little as it heats up (more current flowing) or cools down (less current flowing). In the car, the instrument voltage regulator drops 12 volts down to about 5 volts, and that 5 volts is what flows through the instruments. Take the temp gauge for example. One pin on the back has 5 volts coming into the gauge, and the other pin has a wire going out to the temp sending switch that is screwed into the intake manifold (and, consequently grounded to the engine). When the engine is cold, the resistor inside the temp sending switch does NOT allow any current to flow through it and into the ground. Since there is not a complete circuit, that means that current is not actually flowing through the instrument. So the needle does not move and stays on COLD. When the motor starts to warm up, the resistor inside the temp sending switch starts to allow some current to flow through it and into the ground. Now there is a complete circuit, current is flowing through the instrument, that current will heat up the bi-metal coil, and the needle will move. The hotter the motor gets, the more current the resistor will allow to flow to ground, the more current flows through the coil, and the more the needle moves. Same idea with the oil pressure and gas gauges. They may have different types of sending switches, but they still simply regulate the amount of current that can flow to ground. That is why the gas tank has to be grounded. To test oil pressure and temp gauges, place two 1.5 volt D cell batteries end to end to get about 3 volts. Use jumper wires to touch the pins on the back side of the gauge. 3 volts will give you about ½ deflection with the needle. This way you will know if the gauge is working. You may not know how accurate the gauge is, but you will know if they work. The gas gauge has 3 pins. You only have to touch two. They are marked, but I will have to check on the markings on Saturday morning to tell you which 2 to use. I’ll also post a few pictures from an article I wrote on this subject for Mopar Muscle Magazine.[/QUO
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