How To Read Car Wiring Diagrams (Short Beginners Version)


A car wiring diagram can look intimidating, but once you understand a few basics you’ll see they’re actually very simple.

A car wiring diagram is a map. To read it, identify the circuit in question and starting at its power source, follow it to ground. Use the legend to understand what each symbol on the circuit means.

I’m an auto technician for over twenty years, I’ve always loved the electrical side of auto repair. After reading this post you’ll understand how to read a basic wiring diagram, which as you know is key to finding electrical problems quickly.

Car Horn Circuit

Understanding A Basic Circuit

Here I’ll explain the basic principle behind a circuit. This stuff is easy and if you’re already familiar, you can skip it.

A wiring circuit is so called because the wiring must make a complete circle in order for voltage to flow. A break or restriction in the circle will causes a intermittent or permeant fault.

Battery terminals
Ground path back to battery negative marked in black

Power leaves the positive (Red plus sign) side of the car battery through the power cable and is always actively looking for the shortest possible return path to the negative (minus sign on the battery casing) side of the car battery.

The path back to the negative side of the battery after the load is known as the ground path. The load is what ever the consumer is, in the case of the above diagram it’s the light.

Basic wiring circuit diagram

Obviously there’ll be more complex looking circuits, which will have relay’s and control units, but remember, they all operate under the same basic idea.

Power leaves the battery positive and looks for the shortest path to the ground side of the circuit.

Basic power circuit
Ground symbol indicates chassis connection

A typical basic circuit consists of five important parts:

  1. Power supply (Positive from battery)
  2. Fuse (Protects the circuit from overload)
  3. Switch (Manual or controlled)
  4. Load (Light bulb, motor etc.)
  5. Ground (Return path to negative side battery)

What’s Power?

Power is battery voltage and in any circuit the path to the load from battery positive may be described as the power side of the circuit.

What’s Ground?

As you know, voltage loves to travel through any metal, and not just the metal within wires. A ground is therefore any metal part of the chassis or engine that’s connected to battery negative.

Simple wiring circuit
Ground path highlighted in blue

The return path after the load is known as the ground side of a circuit. And typically isn’t drawn on a diagram as a wire going back to the negative side of the battery, instead a ground symbol is used.

Ground symbols diagram

What’s A Relay?

Relay

Relay’s haven’t changed very much over the years, they’re in old cars and new ones, a good idea never gets old.

The function of a relay is to control a high amp circuit like a starter motor or head lights using a low amp switch circuit.

Running high amps through a small switch would cause the switch to burn out and fail, possibly starting a fire.

Relays are common in circuits and also housed within control units. When they’re integral to the control unit, the diagram will often refer to it, but it won’t be a serviceable relay.

Traditionally, relay terminals were numbered using double digits, but latest versions use single digits, I’ve marked both on the diagram below.

Wiring relay circuit diagram

How’s it work?

A relay is a electromagnetic switch, it has two separate circuits, a Control circuit and a Load circuit. A switch is either manually operated or a control unit sends power through the 2/86 terminal which passes to ground through terminal 4/85.

This causes the coil of the relay to become magnetic, which pulls the movable armature within the relay, closed. When closed (open in the above diagram), it allows power travel from the battery to the light. (Through the 30 and 87 pins)

Relay testing

When the switch is turned off, (battery disconnected) the coil is no longer magnetic and the spring loaded movable armature returns to open (default position).

Pro tip: When fault finding circuits, a quality DVOM is mission critical. Cheap volt meters are OK for finding power and grounds, but modern vehicles will require accurate resistance readings to correctly diagnose a faulty circuit or component.

An incorrect meter reading can cause a ton of trouble. If you’re buying a volt meter buy something like the Klein MM400, it’s perfect for the beginner or veteran and is conveniently sold and delivered by Amazon.com.

Basic starter circuit

The starter circuit relay in the pic above operates in an identical fashion. By turning the ignition switch to start, voltage flows through pin 86 and grounds at 85. This magnetizes the coil which in turn causes the armature (pin 30 to 87) to close completing the load side circuit, and the engine cranks.

What’s A Control Unit?

ECU

You’re here to learn how to read a wiring diagram, and so you’ll most certainly encounter control modules (computers). Modern cars as you know are packed with control modules. Generically they’re also known as Control Units, CU, Controllers, Modules, CM, Electronic control unit and Computers.

Different system control units will go by different names and each manufacturer will have their own acronym, here’s some of the more common names PCM – Power-train Control Module aka ECU and transmission control unit combined, ECU – Engine Control Unit, CEM – Central Electronic Module, EBCM – Electronic Brake Control Module, BCM – Body Control Module, etc.

I’m not going to go deep into the weeds here, but it will be useful to have an outline of how control units operate.

Pre-computer classic cars have a simple wiring circuit – for example, pressing a switch sends power through a wire to a window motor and the window moves.

Window control unit

Modern cars handle it a little differently – pressing a switch, sends a signal through a wire to a control unit (computer), which in turn sends power to the window motor.

The control unit or controller will only send power to the window motor if certain pre-programmed conditions are met. There may be conditions where the control module won’t send power to the window. For example, if it’s programmed to save power when the battery is low.

Of course the window may not move for other reasons, the control unit may be faulty, wiring issue, motor faulty etc.

So why did they go and make things more complicated and expensive to fix? Well, control units do offer significant advantages, some of which include:

  • Less wiring needed
  • Cars more fuel efficient
  • Cars are cleaner
  • Cars are safer
  • Allow for more electronic modules like infotainment systems and drivers aids
  • System fault codes can be read

All control units are connected to each other via a twisted pair of wires, the communication system is know as CAN (Controller Area Network).

Window control circuit

When reading wiring diagrams, a technician doesn’t get to see the internal diagrams for control units and so we don’t concern our selves with their workings.

Instead, we use the Sherlock Holmes approach – Check all wiring to the control unit and from it, if all check’s out and fault persists – A Faulty module is the only logical conclusion.

Of course it is easy to incorrectly interpret data, especially if the tester doesn’t understand the controllers parameters.

For example understanding that the climate control unit won’t turn on the aircon unit not because there’s problem with the a/c system but instead because the ECM detects the coolant system is running too hot.

If not understood correctly it is very easy to assume these a problem where no problem exist.

That’s why I advise all DIY vehicle owners to invest in a wiring diagram and a workshop manual. It will pay for itself several times over.

Control module starter circuit

Understand The Legend

Every diagram will have a legend, it’s the key to understanding the wiring diagram. It will typically show a set of symbols and a brief description.

wiring diagram legend

It’s not important to know these symbols by sight, you can reference the legend as you meet the various symbols along the circuits your reading. And anyway, you’ll find the symbols vary from one manufacturer to another.

Tip: Some diagrams are easier to understand than others, but having the wrong wiring diagram can catch out even the pros. To avoid frustration, be sure that your wiring diagram is correct for your vehicle.

Have your legend close to hand as you read the wiring diagram. Without knowing what each of the various symbols mean, you’ll quickly get bogged down.

Wiring symbols

The information in a legend may include:

  • Wiring color code
  • Symbol meanings
  • Module codes
  • System group codes
  • Component abbreviations
  • Any special notes

The legends are usually well thought out, logical and easy to follow.

Wiring diagram

Reading A Wiring Diagram

Wiring diagrams were traditionally printed in book form, diagrams are big as you know, to fit them all on one page would make them unreadable.

The solution – a number at the end of each circuit indicated the page on which the rest of the circuit diagram was continued.

This can be a little cumbersome especially when referencing lots of different circuits at once.

Other solutions include showing just one systems wiring circuit to a page, for example, just showing the wiring diagram for the headlights. This works pretty well and was carried over to the digital age.

Digital wiring diagrams are a lot more efficient and easier to use, so if possible, always opt for digital schematics.

Now you know what the legend is and have a brief understanding of what the various symbols mean, it’s time to read a wiring diagram.

Nearly all modern diagrams are laid out with the power at the top of the page/screen and the ground at the bottom. This is the natural flow, and it’s the best way to read them.

The diagram below is a basic car light circuit, at first sight it might look complicated, but as you understand the flow, it will become clear.

Remember, battery power (voltage) at the top of the page is trying to get to the ground at the bottom of the diagram.

Wiring diagram

Starting at the top of the included diagram, you can see power flows in two paths, (1) down to the light relay (left) and (2) to the central electronic module (CEM) which is a control unit.

Diagram drawn with ignition in position 0 – the “OFF” position.

Path (1) – The light relay receives the voltage but, since the armature is in the open/off position, it stops at this point.

Path (2) – The control module receives the voltage and that path ends.

The picture changes however when the ignition switch is in position two“On”.

Wiring diagram ignition on

The CEM module offers a ground at X when ignition is on. This as you know magnetizes the relay coil and causes the armature to close. The closed armature in turn offers a path for power to flow onwards to the switch.

The switch is now primed. Hitting the light switch now, allows voltage flow through the light switch relay coil and grounds through the CEM integrated ground path.

The light relay coil is as you know, now magnetized and so it pulls the armature of the relay closed allowing power flow from path 1 all the way through to ground at the bottom of the diagram, powering the lights as it does so. The circuit is now complete.

That’s it, you’ve read the diagram, some circuits will be more complex, but the more you practice, the better you’ll get.

You may also like these post:

Car fuse keeps blowing

OBD won’t connect to ECU

How to check fuel pump fuse

To see the wiring diagrams and special tools I use check out the Auto electrical repair tools page.

Related Questions

What’s the difference between a diagram and a schematic? A diagram is a detailed map of a system and a schematic is a more simplified representation.

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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