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Automotive Electricity Basics - a Rundown


One area that many do-it-yourself mechanics are often weak in is with electrical issues. A basic understanding of electricity and electrical systems, as they apply to automotive, is an important first step towards learning to diagnose and fix electric problems in a car.

In most automotive applications, power flows in one direction, or from "positive" to "negative." Because vehicles run on 12-volt direct current (DC), most of the electricity in them runs in a one-way path. There are exceptions, of course, but for the most part, everything on a passenger vehicle will have a positive (inlet) and negative (outlet) side for power flow.

There are literally hundreds of electrical parts on the average passenger vehicle. Those involved in electrical flow do different jobs, such as keeping power flowing the way it should, lowering or raising the amount of power flowing, controlling when power flows and when it does not, and more. For the most part, these types of power control are controlled by semiconductors (transistors, resistors, diodes, etc). Another common item that controls electrical flow are switches, which turn power on and off either automatically (when certain criteria are met) or when commanded to.

In any given circuit of electricity, there are amounts of power flowing - usually referred to as the "load." Electrical load is measured according to Ohm's Law. Three basic things make up power flow: amps, voltage, and resistance. Ohm's formula uses any two of these three things to find out the other. When you use an automotive multimeter to measure power flow in a vehicle's circuits, you're using Ohm's Law; which is why those meters are often referred to as "Ohm Meters."

In a nutshell, amps is a measure of the amount of power flowing, voltage is the potential power that can be used, and resistance is the difference between the two. Ohm's formula is Amps = Voltage divided by Resistance (A=V/R).

A passenger vehicle running a standard 12-volt system has a total potential of 12 volts. When power is sent through a circuit from the battery, that power has a maximum potential of 12V and will likely lose much of that potential as it flows through the circuit before returning to the negative (ground) side of the battery. This leads to one of the most useful tests that can be performed in automotive electrical diagnostics: voltage drop testing.

A given circuit will have a known expected voltage drop from the input (12V) to the output. If it's drop expectation is 6V or 50%, then the input end should receive X amount of power (say 12V) and the ultimate return end (ground) should have half that (6V). If it has more or less than that, there is a problem in the circuit. The problem will be dictated by the amount of power being returned. More means that something is not accepting power as it should. Less means that something is using up the power when it shouldn't. Measuring these differences through the circuit can pinpoint where the problem lies.

For example, if a windshield wiper motor is expected to use 3 volts of power when running on its highest setting, but is not running on that highest setting, there could be two issues. Either the wiper motor is defective or it is not receiving enough power to run on "high." A circuit test can show whether there is enough power entering the circuit and where it is being lost. If there is no loss until the motor itself, it's likely that the motor is defective. If there are no losses through the motor (or is less than expected), then the motor or its electrical connections are defective.

A simple voltage drop test can find many, many common automotive electrical problems.

This simplistic explanation of automotive electrical should help you in learning about how power flows through a vehicle's electrical circuits. It's a good beginning point towards learning more about automotive electrical.