Introduction to Vehicle Electrical Systems

In order to understand vehicle electrical systems, you first need to understand electricity. There are two forms of common electricity, alternating current (what you find in your house outlets), and direct current (what comes from batteries). Throughout this article I'll be relating electricity to water, as when talking about DC, both flow in a similar way. They both flow from an area of potential energy, and the flow of both can be harnessed to do work.

Think of the two terminals of a battery, positive (sometimes referred to herein as (+) or 12V power) and negative (sometimes referred to herein as (-), ground or earth) as two bodies of water. Think of positive as a lake up in some hills, and negative as the ocean. The water in the lake wants to flow to the ocean due to the difference in height (this is called gravitational potential energy). In a similar fashion, the energy at (+) wants to flow to (-) (this is called chemical potential energy, being driven by the chemicals in the battery).

When a path between the lake and the ocean is opened, a river is formed where water flows. This can be likened to a wire between the positive and negative terminals of a battery, where the electricity flows. Just like water, electricity will tend to follow the path of least resistance.

Some Quick Definitions

Conductive: Any material that electricity can pass through, typically metals.
Short: Abbreviated form of short-circuit, meaning that a path of lesser resistance has been opened, and so the flow of current isn't going to the intended destination.
Open (Circuit): A circuit that does not have a complete path from (+) to (-).
Closed (Circuit): A circuit that has a complete path from (+) to (-).

Units of Measure

If we wanted to measure the properties of that river, we could measure a few different things. We could measure how much water moves past a given point in a given timeframe (the flowrate), we could measure how much water is available in the lake (the potential energy), we could measure how obstructed the river is (the resistance to flow), and we could measure how wide the river is (the maximum flow rate). So, let's pair some of these measurements to units that we can use to evaluate them.

Amperage / Amps (A): Also called current, it's a measure of the flow rate of electricity, how much juice is going through the wire (how much water is moving through the river). This is the one that can hurt you if it's delivered with enough voltage to go through your body¹.
Voltage / Volts (V): A measure of the remaining potential energy in a battery or other power delivery system (how deep the lake is, how much water is left in it).
Ohms (Ω): Also called resistance, it's a measure of how much resistance to electrical flow there is between two given points (how many rocks / how much debris is in the river between two points). An important thing to note about resistance is that it has an inverse relationship with current: less resistance will cause current to go up.
Wire Gauge / Rating (mm²): A measure of the cross-sectional area of a wire. Combined with the material of the wire, this can be used to determine the maximum save amperage.

¹A note about vehicle electrical systems. Being 12V, they do not have enough voltage to pass the natural insulation of your skin when dry (damp skin has far less insulation). This contrasts to household electricity (120V) which is enough to pass through your body. This is why we don't stick things in electrical outlets, but you can typically touch both terminals of a car battery with no ill effects. This does not mean you are free to play with them, they are still dangerous and can be hazardous when shorted with metal. In addition, there are electrical components that are stepped up to significantly higher than 12V (such as ignition coils), so be aware and cautious when working with electricity.

Measurement

Each of these measurements has a dedicated device to measure them in high-sensitivity industrial settings, however for household use, all of the functionality is combined into a device known as a multimeter. They come in both analog and digital varieties.

Looking at the face of your multimeter, you'll see some symbols that you might recognize from the above list. You will likely also see multiple sections for volts, either marked ACV and DCV, or marked with the AC symbol and DC symbol . We care about the DC setting in this case, as our car battery outputs DC.

Your multimeter should have a red and a black lead. The red lead is for the positive side, and the black lead is for the negative side. Your multimeter may have multiple ports for the red lead. If it does, each port should be clearly marked with what settings it goes with. Failing to follow this may pop the fuse in your multimeter.

Measuring Amps: Measuring amps must be done in the path of least resistance (the river must flow through the multimeter). The positive (red) lead should be connected to the 12V power wire, and the negative (black) lead should be connected to the ground/destination wire. It can be difficult to measure current without having a connector that nicely accomodates it. Measuring amps can be useful to determine if something is drawing more power than it should be, or not enough.
Measuring Volts: Measuring volts is done between any live wire and ground (whether that be a ground wire, or a bolt or piece of metal ¹). The positive (red) lead should be connected to the 12V power wire, and the negative (black) lead should be connected to any ground. Measuring volts can be useful to determine if power is available where it should be, or if there is a short between the measuring point and the battery. If you reverse the leads, you'll likely get a negative number, you can liken this to measuring how much flow potential there is between the ocean and the lake (the lake is flowing to the ocean, so the potential is negative).
Measuring Ohms: Measuring ohms can be done between any two points that are connected via conductive material. Which lead goes where is not important. If you were counting rocks between two points in a river, which side you start at doesn't matter, you should get the same number.

¹The negative terminal of the battery should be grounded to the body of the car, meaning any exposed metal on the car should be usable as a ground point (coating and paint will block electricity).

Electrical Components

Fuses: A fuse is a piece of thin metal placed in the middle of a wire that is designed to melt at a very precise level of current. This breaks the circuit when it melts, ensuring that the total current of the attached wire does not exceed safe levels. If you pop a fuse, replace it with the same number fuse, otherwise you could damage wires and cause electrical fires.
Relays: Relays are a way to toggle high current wires using low current. Essentially, this lets devices such as the ECU (operating on low current) toggle things like the fuel pump (operating on high current) without having to route high current through the ECU. You can liken them to a manual dam in a river. The dam can be opened and closed by a smaller force than the river itself has.

Things to Avoid

Shorting Positive to Ground: You want to avoid the creation of any shorts between battery positive and battery negative with no load in the middle. Due to the nature of the relationship between current and resistance, a short from positive to negative through a low-resistance wire with no load will result in extreme amounts of current. Typically automotive wire cannot handle these levels, and such shorts result in melted wires and electrical fires if the source of the short is not fused. This includes bridging the two battery terminals with metal tools!
Cutting Unknown Wires: You should never cut unknown wires in a non-partout situation, as repairing them is exponentially more time-intensive than cutting them. This is especially true of large bundles. Always confirm a wire's identity at a connector prior to tapping or severring it.
Use of Improper Splicing Techniques: Under no circumstances should wire nuts be used in an in-service vehicle, nor should two wires be held together only by twisting or taping. Ideally all joints should be done with solder, however, butt connectors are acceptable in a pinch.
Not Documenting Modifications: Documenting your electrical modifications is a huge plus when you sell a vehicle. One of the handiest tools with which to do so is Diagrams.net, in which you can import screenshots of factory service manual diagrams and draw/sever lines therein.

Reading 3S Wiring Diagrams

Reading wiring diagrams may prove daunting for anyone who's never seen them before, but understanding the individual parts goes a long way towards grasping the entire diagrams.

The Numbers, What Do They Mean?!

Wiring Diagram Connector Identifier — This denotes the identifier of a given connector. It can be cross-referenced with the pin diagrams at the bottom of a given FSM page to see what the connector looks like.

Wiring Diagram Terminal Number / Direction — This denotes a terminal number and orientation. The wire on the open side of the carets will be the male pin, and the wire on the pointed side of the carets will be the female terminal. To find which terminal is associated with which number, find the connector diagram at the bottom of the FSM page.

Wiring Gauge — This denotes the gauge of the wire, as well as its color. B represents black, R represents red, L represents Blue, etc. A hyphenated color denotes the left color is the base color of the wire, and it has a stripe of the right color.

Putting That Into Practice

Putting this into practice, we can examine this wiring diagram snippet. We can see a 1.25 gauge black wire with a red stripe (on the bottom) has a pin that plugs into terminal 9 of connector C-34, and the associated terminal has the exact same style wire (on the top) coming out the other end.