How to read automotive wiring diagrams

Written by david sandoval
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Automotive wiring diagrams outline how the electrical system is laid out. Each symbol in the automotive wiring diagram represents a power source, device to be powered, switching mechanism, electrical conductor, or electrical ground.

Older automotive wiring diagrams utilise symbols that look somewhat like the devices they represent (such as headlamps). Newer automotive diagrams, however, utilise ANSI (American National Standards Institute) or IEC (International Electrotechnical Commission) standard electrical symbols.

Skill level:
Moderately Easy

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Things you need

  • Automotive wiring diagram
  • Electric symbols key (see references)
  • Flat surface (such as a table, or the hood of an automobile)

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  1. 1

    Open the wiring diagram on a flat surface. Trace the schematic lines going from the positive battery terminal to each device. Each device in the schematic connected by a line to the positive terminal on the battery is powered by that battery, unless otherwise noted.

  2. 2

    Trace the schematic lines going from the negative battery terminal to each device. These connections represent the current path back to the battery from each powered device, unless otherwise noted.

  3. 3

    Trace any devices not connected directly to the battery (such as switches or relays). These devices allow electrical current to flow; however these devices must be in the "on" position to allow the electrical current to pass through.

Tips and warnings

  • Some devices, such as alternators, generators, and voltage regulators are connected to both the positive and negative battery terminals. These devices are power sources in circuits parallel to the battery. This set-up allows the battery to be recharged and for the generator or alternator to carry most (or all) of the vehicle's electrical load.
  • A wiring diagram will only show what electric components are connected to each other, not the physical location of the wiring or electrical devices.

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