How to Read Automotive Electrical Schematics: An Introduction

Electrical schematics, or electrical line diagrams, are indispensable for anyone working in the automotive field. These diagrams are roadmaps that give the auto technician a visual representation of electrical circuitry.

Electrical schematics help the tech understand how electrical components interact with each other within a system. Electrical components are identified through standardized symbols, and these symbols can be arranged to show what components a device contains, how they are connected, and how power flows through them from the source (a battery or alternator) to the output of the device.

Electrical Schematics And Technician Safety

Understanding electrical schematics not only can guide a technician's understanding of the circuits involved, but it also indicates what devices are powered and the current that flows through them. This brings up the safety issue: technicians must understand what can be touched and what cannot be touched within a circuit, so as to avoid shock or possible electrocution.

This point is key: Some electrical components, such as capacitors, continue storing energy after a device is shut down and disconnected. An electrical schematic can be used to identify where capacitors are located so the technician can then avoid contact without proper safety equipment. A technician may need to discharge a capacitor before working on the circuit, so it is important to identify all capacitors and discharge them safely. Understanding electrical schematics is key to safety and success in the automotive field.

Electrical Symbols & Their Meanings

The electrical symbols used in a schematic and the format of the schematic itself, is governed by IEEE (Institute of Electrical and Electronics Engineers) and IEC (International Electrotechnical Commission) standards. Additionally, the Society of Automotive Engineers (SAE) has its own set of standards governing electrical systems in automotive industry.

Common symbols used in a wiring diagram include resistors, capacitors, switches, and power sources, which enable auto technicians to quickly understand complex systems. The primary objective of an electrical wiring diagram is to offer a clear visual representation of circuit structure.

The schematic documents the design and layout of the device, and is then used during the device's service life to guide repairs safely. Schematics act as a universal language among designer (the manufacturer) and repair technicians. This is critical during design phases, as it helps identify potential issues before they lead to costly errors or dangerous safety issues during implementation or repair procedures.

Common Electrical Schematic Symbols

In our previous article on commonly used electrical symbols, we covered more than a dozen must-know symbols, and supplied both visual depiction and function. Using those symbols, and introducing a few more, we'll walk you through an automotive electrical circuit to show how the symbols are drawn, what components they represent and how the power flows through the circuit to create the desired work.

Electrical Schematics, Wiring Diagrams & Pictorial Diagrams

There is a distinction between electrical schematics, electrical wiring diagrams, and pictorial diagrams: pictorial and wiring diagrams both describe the actual physical layout of the components, while electrical schematics allows circuit analysis without the distraction of physical layout. However, an automotive technician can use a schematic to understand circuit operation, even if the actual wiring is disorganized or has been modified during past repairs.

How the Three Documents Compare:

• Electrical Schematics: Show the correct components using standardized IEEE or IEC symbols. Layout may not match physical layout.
• Wiring Diagrams: Components drawn as a reasonable facsimiles of the physical parts of the device. The layout might be closer to the physical layout over the schematic, but that is not guaranteed.
• Pictorial Diagram: Similar to a wiring diagram but uses photos of the circuit actual components and may even approximate location of components relative to the installment.

Electrical schematics provide a permanent record of circuit design, invaluable for future maintenance, upgrades, or troubleshooting. Mastering the interpretation and creation of electrical schematic diagrams is a fundamental skill that underpins effective electrical diagnosis and repair.

Electrical Schematic Example

For this article we're going to share an electrical schematic and explain what it shows us. We'll then compare the schematic to real-life components through a pictorial diagram so the translation from line drawing to physical reality is clearly made.

For this example, we'll start with the basic two-speed windshield wiper-wash circuit design from the typical Pre-ECM¹ era General Motors (GM) passenger automobile. Variations of this circuit have been in use since at least the 1960's. We're going to use a simplified version of the system as used in the mid-production run for the C3 Chevrolet Corvette Stingray base-model. Keep in mind that this circuit was used on dozens of GM models with variations in design based on the model year and options selected.

Example: The Typical 1970's era Pre-ECM 2-speed GM Wipe-wash system

This electrical schematic is for a typical Pre-ECM base-model 2-speed windshield wiper system that uses an off-low-high slide switch combined with momentary-on push feature for washer activation.

How To Read An Electrical Schematic

This diagram visually represents the entire wiper system circuit and can be used as a guide to understand how the system works. The following steps will walk you through the system, component by component:

Step 1: Familiarize Yourself with Symbols

Electrical schematics use standard symbols for components like resistors, motors, and switches. Begin by studying the legend or key accompanying a schematic. Recognizing these symbols is essential for understanding circuit layout and components. For the above Schematic, here are the components:

Electrical Symbol to Component List

1. Battery: A 12 Volt car battery (direct current), or, if the engine is running, power for this circuit is generated by the alternator (not shown). This is the power source for the circuit.
2. Ignition Switch: In the Pre-ECM Chevrolet, the ignition switch is key operated. The activation of this switch provides power that is available to all ignition switched components.
3. 25 Amp Fuse: This fuse, found in the fuse clock (#8), protects the circuit from overheat/overload conditions.
4. Wiper-Motor: This two speed electrical motor has an internal resistor within the low-speed circuit (LOW), creating a slower speed setting. The high-speed circuit (HIGH) bypasses this resistor, and allows the wiper motor to turn unimpeded by any kind of resistance. The motor uses a shunt to create parallel fields of current, which is beneficial to a wiper system, as it provides smooth speed control under varying electrical loads demanded from the alternator.
5. Washer-Pump: This is a small DC motor that operates a pump. When the wiper switch is pushed in, the momentary-switch built into the Off-Low-High slider switch activates the pump, which in turn sprays windshield fluid through the washer nozzles.
6. Chassis Ground: The steel (or aluminum) chassis of the vehicle serves as a large grounding mechanism. The Negative from the battery and all electrical devices on the vehicle send their ground to the chassis.
7. Wiper-Wash Switch: a 2-speed sliding mechanism that has 3 positions: Off, Low, and High. This same sliding lever can be pressed in at any of the 3 positions to activate the washer pump (#5).
8. Firewall Connector/Fusebox: For this article, the Fusebox has been highly simplified with a simple placeholder, as we have moved the circuit fuse into the diagram for ease of display and explanation.
9. Conductor: #9 - Off-Low ²
10. Conductor: #10 - Off-Low-High ground ²
11. Conductor: #11 - Low-High ²
12. Ground Wire: The negative (-) wire (engine compartment).
13. Power Wire: The positive (+) power wire.

Step 2: Identify the Power Source

Locating the input for the power source is your starting point. In this schematic, it is represented by the battery. If the engine is running, the alternator supplies the power for system instead. Understanding power origin is crucial for following circuit flow and determining how electricity travels through the system.

Step 3: Identify Connections and Junctions

Connections and junctions indicate component interactions. Dots where lines intersect signal connections, while lines crossing without a dot indicate unconnected components. This is crucial for understanding circuit structure. Conductors represent the wires in a schematic. These are the circuit pathways that current flows through. Current flows from the power source through the circuit, and understanding this flow is important for installation and troubleshooting.

Step 4: Trace the Circuit Path

Use the electrical schematic diagram to trace current path from the power source identified in Step 2 (the Battery), follow the current flow through the components. Keep this rule in mind: Current flows from positive to negative, electrons flow from negative to positive. For DC circuit analysis, trace from positive to negative.

Wiper Schematic - Circuit Tracing Hints:

1. Starting with the power source (#1), we follow the circuit pathway (#13) to the ignition switch (#2).
2. When activated, the ignition switch allows power to flow through the 25 AMP fuse (#2 - a circuit protection mechanism) and on along #13 (power conductor) to where it forms at junction (a dot on the wires) with the power input lug on the wiper motor (#10).
3. At this lug (#10), the connector forms a junction between the power input wire and the washer pump lead. The washer pump will run when the wiper switch is pushed in, as this creates the grounding the circuit needs for current to flow. The grounding goes from the metal switch frame to the chassis ground.
4. Depending on the wiper switch position selected, it combines circuits (see the #9, #10 and #11 speed combinations) to control the motor. There are three possible outputs: the motor will remain off, run at low-speed or run at high-speed. When pushed in, the washer pump will run in either low or high speed settings.
5. For any of the circuits to work, both the wiper switch and the wiper motor must remain grounded to the chassis at all times (#12).

With the ignition on, both the low and high speed switch settings should allow the wiper motor to run at the indicated speeds. This particular design of wiper motor has an internal resistor: The low-speed setting channels power through the resistor to run the motor at a slower speed, and the high-speed setting bypasses the resistor and allows the motor to run faster (unencumbered by a resistor load).

Electrical Schematics Vs. Pictorial Diagrams

Once you understand what the electrical schematic above looks like, now you can compare it to a pictorial diagram. What a schematic shows looks is usually significantly different than what the technician would see when looking at a vehicles electrical system. The physical system is what the pictorial diagram seeks to emulate.

The electrical schematic and the pictorial diagram appear very different but represent the same thing. This chart uses the same list of 13 parts accompanying the electrical schematic above, but the configuration is now more in tune with what the actual system looks like.

A Pictorial Diagram of a typical 1970's era Pre-ECM 2-speed Wipe-wash system using period-correct GM parts set up on a layout demonstration loom.

Electrical Component List For Pictorial Diagram

Using the same 4-step guide under "How to read an Electrical Schematic", examine and follow the pictorial diagram above. The parts list remains the same, but now shows the actual components and wiring in place of the schematic.

1. Battery/Alternator
2. Ignition Switch (represented by on/off switch in this diagram)
3. 25 Amp Fuse
4. Wiper-Motor (2-Speed)
5. Washer-Pump
6. Chassis Ground
7. Wiper-Wash Switch
8. Firewall Connector/Fusebox
9. Wiring (Off-Low) ²
10. Wiring (Off-Low-High ground) ²
11. Wiring (Low-High) ²
12. Ground Wire - The negative side of the battery.
13. Power Wire: The positive (+) power wire.

Why Was This Circuit Presented?

This circuit is a very basic circuit, and is the perfect example to use to familiarize an automotive technician student with the depiction and operation of simple 2-speed wipe-wash system. Since the 1970's, wiper systems grew in features, functions and complexity. Through the years, features like timed circuits (intermittent wipers), and automatic activation when rain is detected on the windshield are standard on many, if not most, automotive models, and the more sophisticated systems employ ECU's for precise control and communication with other modules.

We started with this basic circuit, so the new automotive student can understand the reliable predecessor to the modern ECU-controlled systems. We will explore intermittent timing and rain detection circuits in follow-up articles.

¹ Pre-ECM refers to the era in the automotive industry before computers and control modules were used in electrical systems. Most, if not all, late-model production automobiles use electronic control modules, also known as ECMs. Learn more about ECMs in our article on the Electude learning management system used in our automotive technology programs.

² These are the electrical conductors (color-coded wires in real life) that, depending on the wiper-switch's position, will connect a specific pathway to the wiper motor and/or washer pump motor.