Electrical problems are every car owner’s worst nightmares. So many wires and complicated components to inspect. Luckily for you, automotive wiring diagrams are there to help you speed up the whole process. Obviously, you’ll need to understand the different codes and symbols in order for diagrams to be really helpful. To assist you with that, we have created this quick guide on how to read car wiring diagrams to troubleshoot and fix simple electrical system problems in no time.
Most Common Symbols and their Meaning
In electrical diagrams, every straight black line represents a wire. Pretty simple right? On a diagram, there’s no visual difference between wire gauges and materials. All wires are then identified using a color code and a number but we’ll see more about that later in this article.
The symbol used to show that two wires are connected is a little black dot. These wires are included in the same system or, at least, use the same power source or ground connector.
In the case where wires run in the same harness or cross over each other without being connected, the symbol is the same but with a little bump added to it. When testing for continuity, these wires shouldn’t trigger your multimeter. If they do, the wires may be skinned somewhere and may cause a short-circuit in the system.
Battery / Power Source
This is the symbol for a standard 6 cells 12v car battery. Some diagrams may use a simplified version which is only two vertical bars instead of 6. On more complicated electrical systems or electronic modules, diagrams can quickly become filled with tons of lines and symbols making it hard to read for the user. Depending on the editors of the manual, this symbol could be used to indicate a 2 cells battery or simply to make the whole diagram a little more user-friendly.
Fuses serve as a mean of protection for the electrical system. In case something goes wrong and a wire is damaged and cause a short to ground condition, the fuse will instantly blow, preventing any more damage to the whole circuit. It’s no surprise that blown fuses are the cause of most electrical malfunctions. When trying to diagnose an electrical problem, always start by inspecting the fuses related to the faulty circuit and 9 out of 10 times, you’ll find the problem right away.
On electrical diagrams, fuses are often located on a different page than the circuit they protect. Most automotive repair manuals have a dedicated section reserved for all the fuses, relays and pretty much everything included in the fuse boxes called Power Distribution diagram to make things easier.
In addition, most diagrams will indicate if the fuses are “hot at all time” or not. This is used to let the reader know if the fuse is constantly powered by the battery of only when the ignition key is in the ON position. Never forget to check that out before testing a fuse or you could end up with a wrong diagnosis.
On a car, the ground is always the body of the vehicle. In fact, I should say that the ground is the negative post of the battery but since a wire goes straight from the post to the body of the car, every metal part touching the body is also considered as a ground.
Most important components all have their own dedicated ground wire to ensure that it’s grounded at all time. For example, the engine and the transmission both have one or more large braided ground wires connected to the body. If for some reason, one of the wire was damaged, the corresponding unit should still keep on working since the engine is bolted onto the transmission and vice-versa. On some occasions, though, the added resistance induced by the longer circuit may cause sensible electronic components to go haywire and cause various problems.
The alternator, for instance, usually gets his ground from the alternator bracket bolted onto the engine head. If the engine ground is broken, the alternator may not be able to produce enough current to power all the accessories at the same time and may cause worrying charging system problems.
There is all kind of switches on a vehicle but the most common one is definitely the toggle switch. A good example of this is a simple dome light switch. Push to one side to turn the component on and to the other side to turn it back off. These are widely used throughout the car because of their simplicity and relative reliability.
Push Button Switches
These are a little less common but still used a lot by car manufacturers. Think of the rear defrost and emergency flashers switches. The same motion is used to trigger it on and off.
This type of switch is used to either control more than one accessory at the same time or when there’s more than one possible selection. The Park/Neutral position switch is probably the best example. Only one input for multiple different possible outputs (P, D, etc.).
Multi-function switches like the headlight/flasher switch are somewhat different but still work based on the same principle. They are basically more than one selector switches included in the same unit.
Relays are basically remote controlled switches. They allow car manufacturers to install low voltage switches inside the cabin to control higher voltage components. They began to be widely used when manufacturers started to replace bulky headlight switches on the dashboard in favor of small combo switches on the steering columns.
Their working principle is quite simple. A low voltage switch is used to turn on and off a small electromagnet which in turn will activate a bigger switch to provide power to components such as headlights, cooling fans, fuel pumps, etc. Almost every major system and component on your vehicle are controlled using a relay.
Since the amount of power flowing through them is often higher than most other types of switch, the smaller internal components are often prone to failure. It’s not rare to see a fuel pump relay burn and stop clicking.
This one is self-explanatory. Everyone knows what a light bulb is and what their purpose is. But in an electrical diagram, light bulbs are everywhere. It’s important to understand that headlights and blinkers are not the only light bulbs on your car. In fact, newer vehicles use light bulbs for pretty much every electrical component inside the cabin to indicate if they are turned ON or OFF.
Nowadays, it’s also common to see light bulbs enclosed in door panels, floors, under the dash, inside the parking brake switch and even under the seats for the owner’s convenience.
Especially for light bulbs under seats and other locations where salt and water can easily reach, open circuit and bad connection problems often lead to lighting system failures. Being able to identify the light bulb symbol and locate them in an electrical circuit might be helpful to speed up the whole troubleshooting process.
Resistors are small electronic components used to apply a certain resistance to the flow of current in an electronic circuit. There’s not much to do with them as an auto mechanic since they can rarely be replaced by themselves. They are reliable and rarely come into problems. If you are unlucky enough to find a burnt resistor in the circuit you are inspecting, the problem most probably lies elsewhere. A blown resistor is often the result of a faulty module driving too much power inside the circuit or an internal short to ground condition. In both cases, a faulty resistor is usually the consequence of another problem and hardly ever the cause of it.
It still helps to know what resistors are, what they do and how to find them, if only to not worry too much about it. Simply learn what the symbol looks like so you know what is what when you come across it in a car wiring diagram.
It’s worth mentioning that different manuals may use two different symbols to represent resistors. Keep that in mind if you use more than one type of repair manuals or you might end up with false test results.
This other small electronic component has the property of only letting the current flow in one direction. It is either used to protect sensible low voltage elements from being damaged by an excess of voltage going through expensive modules and components or to redirect the current in a circuit like inside an alternator. If over-voltage occurs, a diode will react exactly as a fuse and will instantly blow. You’ll then need to find its location using your vehicle’s wiring diagram and replace it.
This symbol is a bit more tricky to describe since the “Motor” logo can refer to multiple different components. As a rule of thumb, one could say that they usually refer to the power consuming element in the system you are troubleshooting. For example, power window regulators are represented as motors on the power window diagram. Same thing for the sunroof motor, power door locks, the wiper motor, power seats and so on.
Solenoids are small electromagnetic switches exception made of the fact that these are actually moving back and forth when submitted to an electric current. They usually serve to open or close a fluid or air passage and they have numerous different uses in a vehicle. Injectors are the most well-known of the solenoids but you could also think of the starter solenoid and automatic transmission solenoids connected to the valve body. In fact, they are so similar to switches that their symbol is one half of the relay symbol. And for a good reason. The electromagnet making the relay click is also, theoretically speaking, a solenoid.
To help you quickly and efficiently identify a specific wire in a wire harness or a connector, all automotive wires are color coded. Colors often differ from one car manufacturer to another but the code used to identify them in car wiring diagrams is always the same.
The color will be noted using abbreviation beside each wire following the exact same pattern: Wire color/Stripe color. For example, a white wire with a black stripe on the side would be identified as WHT/BLK. A dark green wire a yellow stripe could be named DG/YE. One repair manual may decide to use a 3 letter color coding system while others may use 2 letter abbreviations only. In all cases, make sure to refer to the color code chart at either the beginning or the end of the manual for more information on how the different wire colors are classified.
Connector Numbers and Locations
For the same reason, color codes are used, connectors and wire are also identified using a number either corresponding to a physical location or a page in the manual. In both cases, the number will let you know where to go to quickly find where the connector is located on the vehicle.
This comes in pretty handy when you need to find an open loop or a short to ground condition in a specific wire. Instead of having to remove all the carpeting and trims to follow the wires all the way to where the problem is, you can simply identify all the connector first, find their location and only remove the trims necessary to gain access to them.
To sum it all up…
Learning to read car wiring diagrams is an awesome skill to add to your auto mechanic skillset. Being able to master this competency will not only help you find solutions to common electrical problems a lot faster but will also give you a big headstart compared to less qualified technicians. And it’s not that hard at all. Once you get the hang of the most basic notions, all you’ll need is a bit of practice and you should be able to fix most electrical system troubles like a pro.