Automotive Electrical 101 – Part 1

How do you determine how a electrical circuit works and how to test it? In the first section you’ll learn basic electrical theory and how to trace current flow in an automobiles electrical circuit. I will also explain wiring diagrams and how to read them.

There will be many up and coming articles about tracing and repairing electrical circuits within today’s vehicles so please make sure to check back for more.

Starting thought

Before attempting to diagnose or repair any type of electrical circuit you need to understand and know how electricity works in today’s automobiles.

I could talk all day long about electron flow theory, but with simple explanations of how, where, and why electricity operates, I will teach you what it takes to understand and diagnose simple automotive circuits. Here is a crash course on how it works.

Define Electricity:

Electricity is a controlled movement of electrons from one atom to another.

What does this mean and how does it relate to automotive technology?

What this means is, anything that is electrically operated on your vehicle operates by electrons moving through each circuit. When you have a power source like a car battery, you have a way to make these electrons unstable and wanting to move about. These unstable electrons are now your electrical source to power up objects that you can use in today’s cars. Electric motors are everywhere in your car. These motors operate your windows, sunroofs, seats, wipers, heater blower, cooling fans, and even the starter on your engine. These are only a few to mention.

I’m going to use these objects I’ve mentioned, and put together a circuit that is something you will see on today’s cars.

Basic electrical circuit

• An electrical power source
  (the car battery)
• A load device
  (something to use electricity to perform a working task ex; bulbs, motors, etc)
• A control device
  (this would be a switch or control unit that creates an open within the circuit)
• A path for the voltage and current to flow between these two objects
  (this would be the wiring to complete the circuit)
• A protection device
  (a fuse or circuit breaker, this would prevent an electrical meltdown if something failed in the circuit)
• Connectors
  (these allow sections of the circuit to be connected together and are easy places to test within the circuit when problems occur)

Electricity always leaves the battery positive side and flows to the load device which uses the voltage only, allowing the current or amperage to return back to the battery negative side completing the entire circuit. If there are any breaks or opens in the circuit, the voltage will only be present up to the break and not allow the circuit to operate. This is why it is important to have a good ground on electrical circuits. The ground paths use metal or frame areas through out your vehicle and return back to the battery.(We refer to this as chassis ground)

Plan ahead

Before doing any type of on car diagnosis, you should have a plan of attack. This means map out your electrical testing steps on a wiring diagram. Just like taking a road trip or vacation of some type, you probably wouldn’t just take off on a long trip without knowing where you are going ahead of time.

Every component in a car has an electrical diagram to show the circuit layout. Over the next few electrical articles I’m going to show different circuits, explain how they work, and list all the symbols used in these circuits. Here is a horn circuit, I’ll walk you through where the power comes from all the way through the circuit wiring, horn, and back to the battery ground.

Typical wiring diagram

Electrical circuit breakdown

The first thing to always remember is the power source always starts at the top of the page and flows down. Here you can see the text Hot At All Times on the left. This means no matter which position the key is in, off, position 1, 2, or cranking there will always be battery power present to this circuit.

Just below that you will see Fuse 13 /20A. This means the protection device in this circuit is a 20 amp fuse in position 13 of the Under hood Fuse/Relay box. (Note the dashed line around the fuse box, this represents a partial view of the box and there is more to it)

In the center is the Horn Relay. The relay is a electromagnetic switch that controls the power to the horns. The coil is on the left of the relay and the switch is on the right. When the coil is energized, it’s magnetic field pulls the switch and sends power to the horns. Notice the box around it is solid. This means it’s a full view of this component. Just above the relay there is a dot where the two wires split from the original wire, this is a splice in the wiring that is a soldered or permanent connection, this doesn’t come apart like a connector. The relay is located inside the Relay Control Module, which is inside the Under hood Fuse Box.

Note the 3 wires coming out of the fuse box and going down. They have letter and number designations next to them. (I9, M5, M6) This is for reference when trying to find their location on the car. The I and M are different connectors, and the numbers represent where the wire terminals are in the connector. (terminal 5 for one wire and terminal 6 for the other)

On the left of the wires are designated color codes for the wires. GRN/YEL is a green wire with a yellow stripe. BLU/RED is a blue wire with a red stripe. In the middle of the GRN/YEL wire is a C401 /18. This “C” always represents or stands for connector. The 18 represents the terminal inside this connector. C405 is another connector and E1, F1, and G1 are the terminals inside the connector.

This section shows the GRN/YEL wire going through 2 components. The first is connector/terminal B1 of the partially viewed Cable Reel. The cable reel is a special coil of wire that rotates within the steering wheel. The wire continues out of connector/terminal C1 on to the partially viewed steering wheel.

The other two BLU/RED wires continue on down the diagram to the next component.

Once inside the steering wheel, the GRN/YEL wire changes color to a BLK wire. (black) This wire continues on to the horn switch (full view) which is inside the partially viewed steering wheel. At the bottom of the steering wheel there is a dot with 3 small lines attached horizontally, this is the symbol for a ground.

On the right, the two BLU/RED wires continue on to the horns. One low tone horn and one high tone horn that have separate grounds.

Now that I’ve described each symbol and component on this diagram, I’m going to explain how the voltage and current flows through the circuit.

Understanding what electricity is

All automotive electrical circuits have 3 things that are constantly talked about, voltage, current, and resistance. These 3 things are what it takes to operate a circuit. The direct relationship between voltage, current, and resistance is most commonly referred to as ohms law. (more on ohms law later)

Voltage is referred to as EMF, the Electromotive Force that supplies electrical energy to the circuit. Coming from the battery it’s the electrical pressure that pushes current through the circuit.

Current is the electrical flow within the circuit. What is flowing? Electrons!! Lets go back to our definition of electricity; a controlled movement of electrons from one atom to another. Electrons are the current flow. All matter is composed of atoms which are full of electrons that can be stable or unstable depending on their environment. In this case, our battery is an electrochemical device that is designed to make electrons unstable. (more about battery’s later)

Resistance is a restriction to the current flow. Using the horn diagram, the horns are the load device in this circuit. They restrict the current flow. As voltage and current are traveling down this circuit to the load device, they are searching for a path to ground. The horns operate by using the voltage to perform a task. A horn is basically a loud speaker. When activated, the horn will use the voltage and allow the current to flow back to the battery creating a constant flow of electrons in a complete loop or path called an electrical circuit.

Wiring Diagram Review

In reference to the horn circuit diagram, here is how the circuit works. The battery power supply is hot at all times through the under hood fuse box and fuse 13 to the horn relay. The power supply feeds both the coil and the switch inside the relay. Here is where I make you think. The coil power on a working circuit stops here because in order to energize this coil to make it a magnet for pulling the switch, we need to ground the other side of the relay coil. If you follow the GRN/YEL wire through the rest of the circuit, the horn switch that you push to honk the horn completes the path to ground. Now with the relay energized, the switch is pulled closed to send power down to the horns that are already grounded, waiting to sound off. This is how the horn circuit works.

Key Electrical Terms

(Key terms in bold throughout this post)

Note:

Now that your through with Automotive Electrical 101-Part 1, you can refer back to System 101 for more automotive topics

Coming Soon

• Automotive Electrical 101-Part 2 (Digital multimeter usage)
• Automotive Electrical 101-Part 3 (Testing inoperative and shorted circuits)
• Automotive Electrical 101-Part 4 (Measuring voltage, current and resistance in a circuit)