How to read automotive wiring diagrams (2022)

Editor's note: This article was originally published Jan. 27, 2017. Some of the information may no longer be relevant, so please use it at your discretion.

Wiring diagrams and road maps have much in common. Road maps illustrate how to get from point “A” to point “B.” However, instead of connecting interstates, highways and roads, a wiring diagram shows major electrical systems, sub-systems and individual circuits, all inter-connected.

Another feature they have in common are layers of detail. For example, if you look at a road map of California, you won’t be able to locate a street address in Los Angeles. You might find a city or town, but you won’t find a specific address. In order to find the exact location of a particular residence or building, you would need a detailed street map or go online and use Google Maps or the GPS feature on a smartphone.

The same is true (to a lesser extent) of wiring diagrams. Vehicles made before the 1970s usually had their wiring diagrams contained on one or two pages in a service manual. By the 1980s, the complexity of automotive, on-board electronics changed, and most vehicle manuals had multiple pages of wiring diagrams to show all of a vehicle’s electrical system. In the 1990s, printed service manuals started to disappear and now manuals, and wiring diagrams are found on digital media or online.

There is one aspect of wiring diagrams that has unfortunately remained constant. They lack directions regarding how to actually read them. Similar to a map, wiring diagrams will have a legend where symbols and naming conventions are spelled out but no “how-to” instructions.

While online automotive service manuals are written with the “professional” technician in mind, every technician had to learn to read and interpret wiring diagrams at some point in their career. The design and layout of wiring diagrams do not accommodate intermediate, or entry-level technicians by starting with easy-to-understand circuits that become progressively more difficult to read and understand. This article will take a different approach and begin with simple circuits and wiring diagrams, then move on to diagrams with more complexity.

This step-by-step process not only makes learning to read a wiring diagram less painful, it promotes a greater understanding of how electrical circuits operate. Becoming more proficient at anything, including reading wiring diagrams, involves practice, and there are some challenging questions included for that purpose as well.

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3 things

The simplified wiring diagram of the battery, light bulb and wires is easy to understand. However, if this same circuit was more complicated and included several relays, multiple power sources and a computer controlling the entire circuit, the resulting wiring diagram would be far more challenging to read. A quick review of basic electrical circuits will make understanding how they are depicted in a wiring diagram easier to understand.

Every electrical circuit on an automobile has to have 3 things to operate:

  1. A power source
  2. Load device
  3. A ground return

The charging system and battery function as power sources and are extended throughout the entire car by way of numerous wires. Load devices are simply anything that does electrical work and can include lighting, starter motor, on-board computers, relays, power windows, keyless entry, and many other components. The ground return completes the electrical path from the battery positive terminal, to the load device, and back to the battery negative terminal. If any of the 3 things are missing, the circuit won’t operate, and wiring diagrams provides a “map” to assist in determining which of the three is not present.

In addition to the 3 things, load devices must be controlled. Some load devices are switched on or off by controlling their power source while others are controlled by switching the ground returns on or off. The most common scenario is using a vehicle’s electronic control unit, or ECU, to ground relays that in turn control load devices. The process of figuring out how a load device is controlled, and its power and ground sources can be determined by using a wiring diagram. To learn a logical process for reading complex wiring diagrams, we’ll start out with a simple fog light circuit.

Figure 1 is a simple wiring diagram showing a fog lighting circuit. The circuit consists of a battery, 20A fuse (used to protect the circuit), a switch (located on a dash panel), and two fog lights. Ground returns are shown by the ground symbol of a vertical line with three horizontal lines. Not all wiring diagrams show ground wires, and it is assumed the ground symbols indicate wires that are connected to the negative battery terminal. This diagram is unusual in that the presence of 12V is illustrated with the circuit in both "ON" and "OFF" states.

Red lines indicate the presence of 12V and black lines represent the ground side of the circuit that connects to the battery’s negative terminal. In the circuit "OFF" part of the diagram, 12V is shown to be present from the battery, through the fuse and to the open dash switch. The lower part of the diagram shows the dash switch closed, connecting the battery to the lights and turning them on. It also illustrates one aspect of Kirschoff’s Law that the load device(s) will use all the power (12V) in the circuit as the voltage at the negative battery terminal, and on the ground side of the fog lights, is close to 0.0V.

Unfortunately, actual wiring diagrams do not provide any of these advantages and late model automobile diagrams may not isolate circuits to this extent — more likely they will be part of the overall lighting system. Color, if used at all in a wiring diagram, is for the purpose of identifying individual wire colors, not to indicate power and ground sides of a circuit. In addition, wiring diagrams always default to show a load device in its "OFF" state, and technicians have to imagine the presents of power throughout the circuit with the load turned on and operating.

There is an inherent problem with the design of the fog lights circuit as shown in Figure 1. These particular fog lights require high amperage (8A each, or 16A total) from the battery to operate and this high electrical load has to travel through all the wires and dash panel switch to reach the lights. The wires, and especially the switch, would have to be heavy-duty to handle the high current. A simple solution is the addition of a 12V relay as shown in Figure 2.

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The relay takes the place of the heavy-duty switch and provides the high-amperage connection between the fog lights and the battery. The dash panel switch is still a part of the overall circuit, but now it only has to switch the low-amperage relay control coil (0.3 A) instead of the high-amperage fog lights. The dash panel switch, and the wires connecting it to the circuit, can be smaller because the relay is connecting the battery to the lights and not the switch.

The control coil inside the relay is an electro magnet, and when terminal 4 of the relay is connected to ground by the dash panel switch, the coil is energized and pulls down the high-amperage contacts within the relay connecting terminals 1 and 2. This diagram does show the circuit in the "OFF" position and is more typical of a real wiring diagram as the technician has to visualize where power is present in the circuit when the lights are on.

While Figure 2 illustrates the basic layout of how a relay is used to operate a high-amperage circuit, it has relevance to modern electronics used in today’s automobiles. Many automotive circuits are controlled by the vehicle’s PCM (power control module) that cannot directly control high current loads. The use of multiple relays solve this problem as the PCM only has to switch the low-amperage relays on and off.

The wiring diagram depicted in Figure 3 shows how the addition of a second relay to the fog lights circuit improving its functionality. Relay #1 provides power to relay #2, the same relay depicted in the previous diagram. Relay #1 is controlled by the ignition switch and only allows the fog lights to operate when the ignition switch is in the accessory or run positions. If the ignition key is in the lock or off positions, or removed from the ignition switch completely, no power is available at Relay #2. This prevents the fog lights from being left on inadvertently, even if the dash panel switch is left on. This diagram is more typical of wiring diagrams found in a service manual. Wires are identified by their color, but there is no color indicating where power is present; the circuit is shown in its OFF state and the relay terminals are identified by number.

The most effective way to learn how to read and use wiring diagrams is to practice. With that in mind, the following three practice questions will test your knowledge and ability read and interpret wiring diagrams. We’ll go through the first two questions together and leave the third one for you to answer.

Wiring Diagram Questions

Question 1:

This question refer to Figure 3. With the ignition switch in the “Acc” position, and the dash panel switch off, what terminal numbers on relays #1 and #2 will have 12V? Figure 3 is typical of wiring diagrams found in a service manual. Relays and switches are shown in their “open” position, and color is not used to indicate where power or grounds are present.

When reading any wiring diagram, start where a known source of power (12V) is located, usually at the battery positive terminal. Relay #1, terminal 3, is directly connected to the battery via the 20A fuse. Terminal 1 goes to the ignition switch, and the “Accy” position will also have 12V (RED wire to the ignition switch and the ORN wire between the switch and relay). Terminal 2 is a constant ground for the relay’s control coil. The relay is "ON", and terminals 3 is connected to 4 via the high-amperage contacts.

Relay #2 terminals with 12V are 1 (RED/WHT) and 3 (BRN) that receive power from terminal 4 on relay #1. Terminals 1 and 2 are connected via the relay’s low amperage control coil, therefore terminal 2 has power because the dash switch is open. If the dash switch was closed, terminal 2 would show 0V because it is connected to ground, and the relay would be “On.” Terminal 4 has no power because the relay is “Off.”

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Question 2:

Trace the path that provides power and ground to each cooling fan in the high-speed mode. Question 2 uses a wiring diagram that is more complex than the one used for the first question. Figure 4 is a typical automotive wiring diagram that shows a radiator cooling fan circuit.

Two fuses (40 and 10A) power the circuit and are directly connected to the vehicle’s battery (Hot at all times). There are three relays that connect power to the cooling fans and control low and high speeds.

The relays are controlled by the vehicle’s power control module, or PCM. The diagram also contains notes regarding labeling of components, their physical location and information on what other wiring diagrams are part of the overall circuit. The relay control coils look a little different that those in Figure 3. A resister is shown (zagged line) and is used to prevent voltage spikes from reaching the PCM when the relay is operated. Otherwise, the relays work the same as those in Figure 3

Note: This circuit operates on 12V. However, when the engine is running the operating voltage is 14V, or charging voltage provided by the alternator.

The three cooling fan relays determine the power and ground paths to the cooling fans. To run both cooling fans in the high-speed mode, the PCM grounds both terminals 42 and 33 (low and high-speed cooling fan relay controls). With PCM terminal number 33 grounded, the DK BLU wire becomes the ground for the cooling fan relay #3’s control coil at terminal B4. This turns the relay on because terminal C6 has power all the time from the 10A fuse.

The RED wire at terminal C4 of the relay is connected to the 40A cooling fan fuse, and with the relay on, connects to terminal B6 within the relay. The WHT wire from the relay (terminal B6) is connected to the right cooling fan and provides power. The right cooling fan has a constant ground on the BLK wire. With 14V (engine running) on the WHT, wire and a ground on the BLK wire, the right cooling fan runs at high-speed.

The left cooling fan receives power from the 40A fuse on the RED wire at cooling fan relay #1 (terminal B3). The PCM low speed cooling fan relay control (42) is grounded by the PCM providing a ground at terminal B1 (DK GRN) wire at cooling fan relay #1. On the same relay, terminal C3 gets power from the 10A fuse on the ORN wire.

With power at C3 and a ground at B1, the relay operates and connects relay terminals B3 to C1 providing power to the left cooling fan on the LT BLU wire. The GRY wire from the left cooling fan is a ground, but only when cooling fan relay #2 is turned on by the PCM high-speed relay control ground at relay terminal C10 on the DK BLU wire. Relay #2 connects the GRY wire from the left cooling fan to the BLK wire (no terminal number listed). The BLK wire provides the ground for the left cooling fan, and it runs at high-speed.

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We have walked through the answers and analysis to questions 1 and 2. Finding the answer to question 3 is up to you.

Question 3:

Trace the path that provides power to each cooling fan in the low-speed mode. Identify the wire colors, relays, and relay terminals that are powered during fan operation. Trace the ground return path for the relays and cooling fans— identify the wire colors and relay terminals used in the ground side of the circuit.

Answer to Question 3

To understand the low-speed fan operation, a quick review of electrical theory will help. In a parallel circuit (the most common type used in automobiles), all load devices operate on system voltage. For example, when the cooling fans operate in the high-speed mode, each has 14V from the 40A fuse.

A series circuit operates differently. With two load devices wired in series, they divide the available voltage between them. In the low-speed mode, the cooling fans are wired in series and each fan runs on 7V — one half of the system voltage of 14V.

During low-speed fan operation, the PCM low speed relay control is grounded turning on cooling fan relay #1. With a ground at relay terminal B1 (DK GRN wire) and power at C3, the relay’s control coil connects the high-amperage contacts (terminals B3 and C1). This connects power (14V) from the 40A fuse (RED wire) to the LT BLU wire going to the left cooling fan.

The GRY wire from the left cooling fan goes to terminal C8 of relay #2. Cooling fan relay #2 is not triggered by the PCM in the low speed mode, and the C8 to B9 relay connection is normally closed. The WHT wire at cooling fan relay #2 (B9) goes to the right cooling fan providing 7V (one half of 14V) to power the fan. Cooling fan relay #3 is not operated in low-speed fan operation.

The BLK wire from the right fan provides the ground return for both fans. Because the fans are wired in series, they divide system voltage (14V) equally between them and both operate on 7V, causing them to run at low speed.


How do you read an automotive wiring diagram? ›

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

How can I interpret the wiring plan easily? ›

First of all, there is a rule of thumb in standard wiring diagrams that you should read the diagram from left to right and from top-down. Exactly like reading a book! But sometimes, designers make some exceptions to have a better layout such as this page.

How do you read and interpret electrical diagrams? ›

The electrical schematics are read from left to right or from top to bottom. This is important to get right, as the signal direction indicates the flow of current in the circuit. It is then easy for a user to understand when there is a change in the course of the circuit.

How do you identify a wiring diagram? ›

A wiring diagram is a simple visual representation of the physical connections and physical layout of an electrical system or circuit. It shows how the electrical wires are interconnected and can also show where fixtures and components may be connected to the system.

What are the 3 types of wiring used in the automotive industry? ›

There are three basic _XL wires that may be used in the automotive segments today: TXL, GXL, and SXL. The copper strand counts are basically the same in each, but the main difference is the insulation thickness. TXL has the thinnest insulation and SXL has the thickest.

How do you read a wire code? ›

An electrical cable is classified by two numbers separated by a hyphen, such as 14-2. The first number denotes the conductor's gauge; the second denotes the number of conductors inside the cable. For instance, 14-2 has two 14-gauge conductors: a hot and a neutral.

What is the interpretation for wiring diagram? ›

A wiring diagram shows the relative layout of the components and the wire connections between them. This type of diagram shows the physical relation of all devices in the system, the conductor terminations between these devices, and are commonly used in motor control installations.

What are the 5 types of wiring diagram? ›

6.2: Types of Electrical Diagrams
  • Schematic Diagrams.
  • Wiring diagrams.
  • Block diagrams.
  • Pictorial diagrams.
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What are the 5 electrical symbols? ›

There are five commonly used symbols in Electrical – Switch, Wire, Contactor, Motor, Transformer. These symbols can be used in any electrical drawings. Switches are used for ON/OFF any control circuit.

What are the 6 most common symbols used for an electrical schematic diagram? ›

The Most Common Schematic Symbols Used in Electronics
  • Battery. The symbol for a battery is shown below. ...
  • Resistor. The schematic symbol of the resistor are drawn in two different ways. ...
  • Potentiometer. ...
  • Schematic Symbols of a Transistor. ...
  • Schematic Symbol for an Integrated Circuit. ...
  • Logic Gates. ...
  • Inductor. ...
  • Transformer.
8 Oct 2020

What does C mean on a wiring diagram? ›


What are the 4 Methods of wiring? ›

4 Basic Electrical Wiring Techniques
  • Casing and Capping Wiring.
  • Cleat wiring.
  • Batten Wiring.
  • Conduit Wiring.
4 Oct 2020

What are the three types of wiring diagrams? ›

Types of Electrical Diagrams or Schematics

There are three ways to show electrical circuits. They are wiring, schematic, and pictorial diagrams.

How do you identify a 3 way common wire? ›

Black wire: This is a hot wire that carries electricity from the power source to the first switch in a typical 3-way setup. It's also called the “common wire” or the “line wire.” Unless the breaker is off, this black wire is always hot.

What are the 3 most common electrical faults in automotive? ›

Top Five Car Electrical Issues
  • A bad alternator. One of the top electrical car issues is a bad alternator. ...
  • Dead bar battery. This is one of the most common car electrical issues drivers have. ...
  • Spark plug issues. ...
  • A blown fuse. ...
  • Battery cables need replaced.

What are the 3 colored wires called? ›

US AC power circuit wiring color codes

The protective ground is green or green with yellow stripe. The neutral is white, the hot (live or active) single phase wires are black , and red in the case of a second active. Three-phase lines are red, black, and blue.

What do the numbers mean on wire? ›

In AWG, the larger the number, the smaller the wire diameter and thickness. The largest standard size is 0000 AWG, and 40 AWG is the smallest standard size. It may also be called Brown & Sharpe wire gauge or simply the gauge of the wire. AWG is for single-strand, solid, round, electrically conductive wire.

What are the symbols of wire? ›

Wire is represented by a straight line joining two points. Hence, the correct option is D.
  • Is the symbol of. Medium. >
  • Is the symbol for. Easy. >
  • Why do we use symbols ? Medium. >
  • (symbol) is - Medium. >
  • A rheostat is used in an electrical circuit. Easy. >

What are the wire codes? ›

U.S. Electrical Wiring Color Codes
  • Phase 1 - Black.
  • Phase 2 - Red.
  • Phase 3 - Blue.
  • Neutral - White.
  • Ground - Green, Green with Yellow Stripe, or Bare Wire.

What are the rules of schematic diagram? ›

  • Schematic should label all pin numbers, part values, polarities, signal names, part model numbers, etc.
  • Every part should have a distinct name, e.g., R1, R2, C1, U1, U2, etc. ...
  • All wires and components are aligned horizontally and vertically.
  • Always use the standard symbol for a device.
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What do dots mean in a schematic? ›

When multiple connections exist, they are usually represented by drawing a dot where the two lines intersect. It is possible for many lines to converge to a single connection, but for clarity, most dots will only connect two lines.

Is schematic reading a skill? ›

Schematics are our map to designing, building, and troubleshooting circuits. Understanding how to read and follow schematics is an important skill for any electronics engineer.

What kind of information is desired on wiring diagrams? ›

A wiring diagram usually gives information about the relative position and arrangement of devices and terminals on the devices, to help in building or servicing the device.

What are the two basic types of wiring systems in automotive? ›

There are two basic types of insulated wire used in automotive applications: PVC and cross-linked polyethylene.

What does L1 stand for in a diagram? ›

Answer: Line voltage. L1 and L2 represent the line voltage side of the circuit.

What are the 4 basic electrical quantities? ›

Volts, amps, ohms, and watts are therefore the four fundamental units of electricity. Q.

What is the symbol for A coil? ›

A coil is commonly symbolized as a circle with letters and number inside. The letters often represent the type of device, such as M for motor starter or CR for control relay.

What are the 3 basic electrical quantities? ›

Ohm's law is the most important, basic law of electricity. It defines the relationship between the three fundamental electrical quantities: current, voltage, and resistance.

What circuit symbols mean? ›

Circuit symbols are used in circuit diagrams showing how a circuit is connected together. The actual layout of the components is usually quite different from the circuit diagram.

What does K mean in wiring? ›

Kirschner wires or K-wires or pins are sterilized, sharpened, smooth stainless steel pins.

What does R mean in wiring? ›

If you just have an R wire, it's responsible for powering your entire HVAC system (through the use of a transformer).

Is the C-wire just a ground? ›

A: C stands for common. The common wire is the ground or negative wire. To complete the circuit that provides power to the thermostat you need a positive and negative wire from the 24V transformer.

What is the most common wiring method? ›

The raceway and conductor method is one of the most common types of electrical wiring. It involves either a metallic or a nonmetallic conduit or tubing with multiple insulated phase.

What is the best wiring method? ›

One of the most important wiring methods applied today is the batten wiring. In this wiring, insulated wires will run through the wooden battens. The battens are fixed on walls and ceilings using either screws or plugs. The cables are fitted directly onto the battens.

What is the most common type of wiring? ›

The most common type of home electrical wiring is the NM cable, also known as the Romex cable, after the most popular electrical wiring brand name. The NM cables contain three or more individual conductors, wrapped together in a sheathing, which is a flexible plastic jacket.

How can you tell if a wire is load or line? ›

Load Wire - Generally connected to the top half of your switch. If the wire is coming from the top of the switch box, it is likely your load wire. Line Wire - Generally connected to the bottom half of your switch. In some cases, line wires are marked with “line”, “pwr”, or a lightning bolt symbol.

Which wire is neutral in a 3 wire? ›

White Wire: This is the NEUTRAL wire. This wire is connected on the Earth ground, however, provides the return path for the current that came from the hot wire. The prong of the neutral wire is longer compared to the hot wire to ensure that these two will not interchange.

Is common wire positive or negative? ›

So the negative connection of the supply in this case would be called "common" or 0V. 0V because all voltages within the circuit are measured from that common point, which also happens to be the supply's negative pole.

What do the numbers mean on wiring? ›

In AWG, the larger the number, the smaller the wire diameter and thickness. The largest standard size is 0000 AWG, and 40 AWG is the smallest standard size. It may also be called Brown & Sharpe wire gauge or simply the gauge of the wire. AWG is for single-strand, solid, round, electrically conductive wire.

What do the markings on wires mean? ›

The labels on the outer sheathing of NM cable indicate the size, or gauge, of the individual conducting wires, the wire material, the number of wires contained inside the cable, the maximum voltage rating, and whether there is a ground wire present. The wire size and number of wires are indicated with numbers.

What are the four types of wiring diagram? ›

6.2: Types of Electrical Diagrams
  • Schematic Diagrams.
  • Wiring diagrams.
  • Block diagrams.
  • Pictorial diagrams.
11 Sept 2021

What do dots mean on a wiring diagram? ›

When multiple connections exist, they are usually represented by drawing a dot where the two lines intersect. It is possible for many lines to converge to a single connection, but for clarity, most dots will only connect two lines.

What's the color code for the wires? ›

US AC power circuit wiring color codes

The protective ground is green or green with yellow stripe. The neutral is white, the hot (live or active) single phase wires are black , and red in the case of a second active. Three-phase lines are red, black, and blue.

What are the 3 types of wires? ›

The three types of wires used in household circuits are Earth wire, Live wire, and Neutral wire.

Which wire is hot and which is negative? ›

The positive wire, also commonly called the hot wire, will typically be black in color. It is the source of the electricity. The electrical current travels from the outlet or other power source on the positive wire, so if it is plugged in anywhere, it should be considered a live (and dangerous) wire.

How do you tell if a wire is positive or negative? ›

The Different Types of Wire Color Codes
  1. The positive current must be red.
  2. The negative current must be black.
  3. The ground wire, if present, must be white or grey.
19 Apr 2022

Is dotted wire positive or negative? ›

Usually* the wire with the white stripe or the dashed lines carries the "positive" (+) end, while the other, unmarked wire carries the "negative" (-) end.

What do electrical symbols mean? ›

Electrical symbols are a graphical representation of basic electrical and electronic devices or components. These Symbols are used in circuit and electrical diagrams to recognize a component. It is also called a schematic symbol.

Why are symbols used in wiring diagrams? ›

Thus in circuit diagrams and schematics, graphical symbols identify and represent electrical and electronic devices and show how they are electrically connected together while drawing lines between them represents the wires or component leads.


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