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HomeTips & GuidesAuto Electrical Wiring Diagram: How Basic and Complex Car Circuits Appear on Paper

Auto Electrical Wiring Diagram: How Basic and Complex Car Circuits Appear on Paper

The auto electrical wiring diagram is a map that enables you to understand and navigate vehicle electrical systems and their components.

But cars have incredibly complex wiring systems, so understanding them is difficult. 

Standardized color codes and symbols help make the diagrams easier to understand, but beginners can still find it difficult to follow. 

So let’s look at the simplest car circuit and gradually advance to the complex systems in modern vehicles.

Table of Contents

Basic Auto Electrical Circuit

All circuits must have a high and low potential difference to give the electric current energy to flow through the load. 

And they usually complete a circle from the power source and back, hence the name circuit.

In vehicles, the battery voltage runs the circuit, creating a high potential difference from the battery positive. 

The chassis forms the low or zero potential difference by connecting to the battery negative.

Think of it as a waterfall, which needs a high and a low point for the water to fall or flow. 

These high and low points represent the potential difference or voltage. And water is the electric current.

But the alternator replaces the battery power when the engine is on, generating electrical current to power all car accessories and charge the battery.

Basic Auto Electrical Wiring Diagram

This wiring diagram usually indicates the battery as the power source for simplicity. So let’s ignore the alternator for now.

A red wire connects the positive battery terminal to the load input, and a black wire links the load output to the vehicle’s chassis. 

Another black wire connects the negative battery terminal to the chassis, completing the circuit.

All critical car electrical components have dedicated ground wires to ensure proper grounding and smooth operation.

A headlight wiring diagram

A headlight wiring diagram

But unlike the ground connections, the positive red wire cannot connect to the load directly. 

The circuit needs control and protection elements. Let’s consider the car head unit circuit.

The power line from the battery first goes through a fuse to protect the wires and elements after that point from faults like overloading and short circuits.

From there, an ignition switch gives control to turn the unit on or off. So turn the car key in the ignition switch for the stereo to turn on.

After that, the wire goes to the load (the head unit). You can add another fuse between the ignition switch and the head unit, but it is unnecessary. 

The remaining part is linking the stereo’s ground to the car chassis.

Relay Auto Electrical Circuit

Not all vehicle circuits are this simple. High-power devices like headlights and starter motors need relay switches for control. 

Relays are electromagnetic switches that control high-load circuits using low-amp circuits.

A vehicle’s relay switch

A vehicle’s relay switch

Without relays, vehicles would need bulky headlight switches on the dashboard instead of the tiny combo switches on the steering column. 

Running a high electrical current through these small switches can burn them and cause a fire in the worst-case scenario.

Relay Numbering and Operation

Traditionally, relays had terminals numbered using double digits. But modern ones have single-digit labels. 

And these numbers have meanings. The two-digit numbers correspond to the single-digit numbering in the following way.

  • 30 → 1
  • 86 → 2
  • 85 → 4
  • 87 → 5

1/30 is the voltage input for the load circuit, while 5/87 is the output. On the other hand, 2/86 is the voltage input for the control circuit, while 4/85 is its output (ground).

A relay diagram

A relay diagram

Let’s use the headlight circuit as an example. When you turn on the headlight switch, power flows from the battery to a fuse in the fusebox, then into the switch and relay’s control circuit via terminal 2/86. 

This low current energizes an electromagnet that pushes an armature to close the load circuit.

Power from the same fuse above finds a path via terminal 1/30 into the relay and out of terminal 5/87 to power the lights.

When you turn the switch off, the electromagnet gets de-energized and opens the load circuit, disconnecting the power flow.

A simple relay circuit diagram

A simple relay circuit diagram

Control Module Auto Electrical Circuit

Modern vehicles have complex electronics, and individually wiring them requires multiple wire harnesses that can weigh down the vehicle.

 So car manufacturers have switched to using control modules.

Also known as computers or control units, these devices come in different types to handle various car components. These include:

  • ECU for controlling internal combustion engines
  • TCU for transmission control
  • BCM for controlling the power windows, mirrors, immobilizers, central locking, etc.
  • EBCM (Electronic Brake Control Module)
  • CEM (Central Electronic Module) for coordinating the functions of all other modules
A car’s ECU

A car’s ECU

These modules are expensive to fix and make vehicles more complicated. But their benefits outweigh these disadvantages because they:

  • Reduce the required wiring
  • Make cars cleaner, more fuel efficient, and safer
  • Enable the introduction of multiple sensors and electronic systems, such as driver aids
  • Allow quick troubleshooting from the computer error codes

The devices communicate using a CAN (Controller Area Network) system via twisted pair wires.

An automatic transmission and ABS control module

An automatic transmission and ABS control module

Control Module Wiring Diagram

Control modules eliminate direct contact between the switch and the load. 

For instance, body control modules disconnect the power window switch from the power-window motor.

So when you press the switch, it sends a signal to the computer, and the computer will only power the window motor if this signal meets certain conditions.

 For instance, a quick press-and-release instructs the computer to run the motor until the window is fully up or down.

In the circuit, the switches connect to the control module directly without a fuse in between. 

The module then draws power from the battery (with a fuse in between) to power the switches and loads. 

And it connects to the loads, such as power windows and mirror motors.

A body control module

A body control module

The downside is you can’t see the internal computer operations of these systems when reading their wiring diagrams. 

So if a component malfunctions, check the load device and the wiring to and from the control unit. 

The module is usually the culprit if these wires and loads pass tests and the fault persists.

But you also need to understand the control module’s operating parameters.

 For instance, the AC/heater control module might not activate the AC if the ECM detects the engine is running too hot.

Auto Electrical Wiring Diagram Legend

Remember the keys on maps? Legends are equivalent to these map keys in auto electrical-wiring diagrams because they give symbol references.

 Although standardized, these symbols can vary depending on the manufacturer.

Electronic circuit symbols

Electronic circuit symbols

Generally, these documents contain the following information.

  • Wiring color code charts
  • Control module codes
  • Symbol meanings
  • System group codes
  • Component abbreviations
  • Special notes

So even expert electricians can get auto electrical wiring diagrams wrong without these legends.

A color-coded wiring harness

A color-coded wiring harness

How To Read Wiring Manuals/Diagrams

Digital wiring diagrams are easier to read because you can scroll to different parts and zoom in to view tiny details. 

And in most cases, they indicate the component names, eliminating the need for a legend in some areas. 

Usually, the power lines are at the upper sections, while the ground is on the lower part of the page.

But traditional wiring diagrams are more challenging to read because they come in paper form. And car circuits cover broad spaces. 

So manufacturers had to fit them on multiple pages and indicate a page number at the end of one diagram to show the page to find the continuation.

Wrap Up

Auto electrical wiring diagrams can be complex in modern vehicles, but understanding the basics makes it easier to follow them.

 And their control modules make the wiring less clustered. 

But always remember to use the wiring legend to ensure you read the diagram accurately. And contact us if you encounter challenges.

 We’ll be happy to help.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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