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 follow it to the ground, starting at its power source. Use the legend to understand what each symbol on the circuit means.
I have been an auto technician for over twenty years, and 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.
We’ll cover the following:
- Understanding a basic circuit
- Basic wiring circuit diagram
- What’s a relay?
- What’s a control unit?
- Understanding the legend
- Reading wiring diagram
You may find these posts useful too:
If you are wondering what ground is, check out this post “Is ground the same as negative?” (easy beginners guide with illustrations)
Check out this beginner guide to MAF sensor diagnosis using a DVOM (Voltmeter)
Check out this easy-peasy electrical short finding post with color illustrations. I wrote a ton of other common auto electrical problems and repairs, they are all written with the beginner in mind, and you can check them out right here in the auto electrical blog section.
Understanding A Basic Circuit
Here, I’ll explain the basic principle behind a circuit. This stuff is easy; you can skip it if you’re already familiar.
A wiring circuit is so-called because the wiring must make a complete circle for voltage to flow. A break or restriction in the circle will cause an intermittent or permanent fault.
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 whatever 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 with relays 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.
A typical basic circuit consists of five important parts:
- Power supply (Positive from the battery)
- Fuse (Protects the circuit from overload)
- Switch (Manual or controlled)
- Load (Light bulb, motor, etc.)
- Ground (Return path to negative side battery)
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.
As you know, voltage loves traveling through any metal, not just the metal within wires. Therefore, a ground is any metal part of the chassis or engine connected to the battery negative.
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.
You may find this post helpful – Is ground the same as negative?
What’s A Relay?
Relays haven’t changed much over the years; they’re in old and new cars; a good idea never gets old.
The function of a relay is to control a high amp circuit like a starter motor or headlights 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 are also housed within control units. When they’re integral to the control unit, the diagram often refers to it, but it won’t be a serviceable relay.
Traditionally, relay terminals were numbered using double digits, but the latest versions use single digits; I’ve marked both on the diagram below.
How’s it work?
A relay is an electromagnetic switch with 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 the ground through terminal 4/85.
This causes the relay’s coil to become magnetic, which pulls the movable armature within the relay closed. When closed (open in the above diagram), it allows power to travel from the battery to the light. (Through the 30 and 87 pins)
If you need help understanding DVOM (a multimeter), check out the how-to-use multimeter Kindle books on the Amazon link below.Amazon How To Use Multimeter
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 voltmeters are OK for finding power and grounds, but modern vehicles will require accurate resistance readings to diagnose a faulty circuit or component correctly.
An incorrect meter reading can cause a ton of trouble. If you’re buying a voltmeter, buy something like the Klein MM400, it’s perfect for the beginner or veteran and is conveniently sold and delivered by Amazon.com.
The starter circuit relay in the pic above operates identically. 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?
You’re here to learn how to read a wiring diagram, 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 units, and Computers.
Different system control units will go by different names, and each manufacturer will have its own acronym; here are 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 won’t 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.
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 a faulty wiring issue, the motor is 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 are more fuel efficient
- Cars are cleaner
- Cars are safer
- Allow for more electronic modules like infotainment systems, and driver aids
- System fault codes can be read
All control units are connected via a twisted pair of wires; the communication system is known as CAN (Controller Area Network).
I wrote a post about diagnosing a CAN issue; you may find it helpful – Scan tool won’t communicate
When reading wiring diagrams, a technician doesn’t get to see the internal diagrams for control units, so we don’t concern ourselves 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 the fault persists – A faulty module is the only logical conclusion.
Of course, it’s easy to misinterpret data, especially if the tester doesn’t understand the controller’s parameters.
For example, understanding that the climate control unit won’t turn on the aircon unit not because of a problem with the a/c system but because the ECM detects the coolant system is running too hot.
If not understood correctly, it is very easy to assume these problems where no problem exists.
That’s why I advise all DIY mechanics to invest in a wiring diagram and a workshop manual. It will pay for itself several times over.
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.
It’s not important to know these symbols by sight; you can reference the legend as you meet the various symbols along with the circuits you are 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, ensure your vehicle’s wiring diagram is correct.
Have your legend close to hand as you read the wiring diagram. You’ll quickly get bogged down without knowing what each symbol means.
The information in a legend may include the following:
- 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.
Reading A Wiring Diagram
Wiring diagrams were traditionally printed in book form; they are big, as you know, and fitting them all on one page would make them unreadable.
The solution – a number at the end of each circuit indicates the page on which the rest of the circuit can be found.
This can be a little cumbersome, especially when referencing many different circuits simultaneously.
Other solutions include showing just one system’s wiring circuit on 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 more efficient and easier to use, so if possible, always opt for digital schematics.
Now that you know the legend and understand the various symbols, 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 the best way to read them.
The diagram below is a basic car light circuit; it might look complicated at first sight, but it will become clear as you understand the flow.
Remember, battery power (voltage) at the top of the page is trying to get to the ground at the bottom of the 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.
The diagram shows the 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.”
The CEM module is programmed to offer a ground at X when the ignition is on. As you know, this 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 allows voltage to flow through the 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 the 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, and some circuits will be more complex, but the more you practice, the better you’ll get.
You may also find these posts helpful:
- Car battery drain – easy beginners guide with pictures
- How to find a wiring short – like a pro!
- Car fuse keeps blowing
- Battery sparked now won’t start – Yikes!
- Symptoms of a bad MAF sensor
- OBD won’t connect to ECU
- How to check fuel pump fuse
Check out the Auto electrical repair tools page to see all the tools I use.
Check out the Auto Electrical Repairs page for troubleshooting electrical gremlins – the easy way.
About the Author
John Cunningham is a Red Seal Qualified automotive motive technician with over twenty-five years of experience in the field. When he’s not writing about car repair, you’ll find him in his happy place – restoring classic cars.
You may find the following links helpful:
Visit our DIY Car Maintenance page and level up your car care skills!
What’s the difference between a diagram and a schematic? A diagram is a detailed system map, and a schematic is a more simplified representation.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.