I would highly recommend getting a decent multimeter and finding a website or book on basic electronics. Here is a quick and simplified lesson on the very basic electronic theory of Ohms Law.
V/R = I
V x I = P
V is Voltage (sometimes called potential), measured in Volts. This is the force that pushes electricity.
R is Resistance (sometimes called load) measured in Ohms (which is symbolized with the Greek Omega symbol). This is the electrical property of whatever apparatus is "consuming" the electricity.
I is Current (sometimes called induction. I know, the initials are confusing. Just go with it) measured in Amps. This is the actual flow of electricity.
P is Power (sometimes called VoltAmps. That gets into more advanced theory though) measured in Watts. This is the work performed by an electrical circuit.
A voltage, such as in a battery, is applied to a load, such as in a lightbulb. The resistance of the load and the voltage of the battery will determine how much current will flow according to Ohms law. Additionally, the combination of voltage and resistance and subsequent current flow will create power, again accordingly with Ohms Law.
As voltage increases, current flow and subsequently power also increase. As resistance increases, current flow and subsequently power decrease.
Before you measure the resistance of a load, you would first set your multimeter to the Ohms setting. A decent meter will have a "continuity" toner. Continuity simply means that there is a continuous path for electricity to flow. If you touch the two leads of the tester together, you complete the electrical path in the meter and it will sound a tone or beep for as long as you hold the leads together. The display should also read zero. Zero Ohms means no resistance, which is a short circuit. (Looking at the Ohms law equations, zero resistance results in infinite current flow. That's usually a bad thing. Think sparks and fireworks). With the leads separated, there will be no tone and the meter will display something like "inf" meaning infinite resistance and no continuity. This is known as an open circuit. By doing this quick check you ensure the meter is working and doesn't have a dead battery or something.
Then to test the load, you touch the metal leads of the tester to the metal wires or connectors of what you want to test. Typically there would be a red "plus" or "hot" wire bringing voltage and current to the device from the battery and a black "minus" or "cold" or "common" (so many names!) wire to return the current to the battery thereby completing the circuit. In a car there can be multiple circuits in one wire harness so many colors are used. Typically the colors will be marked in the wiring diagrams in the fsm.
For the resistance test plus and minus doesn't really matter. What does matter is that you have the load disconnected from the voltage source. You do not want to apply your ohm meter to a voltage, that can fry it. It's also important that the leads not touch eachother or any other metallic objects like the frame. It's fine to hold the leads and wires together with your fingers, just don't let them touch anything else metallic as that can give you a false reading. Also, make sure all contact surfaces are clean. Dirt and grime are more resistive than conductive.
As long as each lead of the tester is touching each wire of the circuit the meter will show the resistance of the circuit. Again, zero Ohms is a short and infinite Ohms is an open circuit. Both indicate a bad device. The fsm should give you the acceptable resistance range for the device. As long as your measured resistance is within that range the device is good.
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