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What is an Accessory Relay?

Relays isolate starter motors, ECUs, ignition systems, and other electrical components from the rest of your vehicle's wiring harness. Where normal switches might melt under load, relays can transfer much larger loads without melting down.

How does an Accessory Relay Work?

Relays are a type of switch. When given a low draw electrical input, the relay's internal parts snap together, closing a circuit that requires a larger amount of electricity to power a specific component. When you add high draw accessories like onboard air systems, powerful stereo amplifiers, or power inverters you must incorporate a relay in order to avoid damaging the rest of your vehicle’s electrical system should something malfunction. Many vehicles are equipped with relays to help power factory supplied accessories, too.

How is an Accessory Relay Made?

Inside a relay you’ll find two metal tangs. These metal tangs are designed to snap together when a low voltage electrical signal is applied to them. The metal tangs are large enough to safely transmit heavy electrical loads.

Why does an Accessory Relay Fail?
  • Relays have moving parts inside of them. All moving parts eventually wear out and break.
  • Like all electrical components, relays are vulnerable to heat, vibration, and moisture.
What are the Symptoms of Accessory Relay Failure?
  • Your vehicle's stereo may not work properly.
  • Accessories like onboard air systems and power inverters may fail to work properly.
  • Depending on the relay, your vehicle may fail to start.
  • Diagnosing a failing accessory relay is as simple as placing your fingertips on its housing, flipping its switch, and checking for the presence of a small click-like pulse or noise.
What are the Consequences of Accessory Relay Failure?
  • Your vehicle will not start, unless push or jump started, without a functioning starter relay.
  • Accessories like radios, onboard air systems, and power inverters will not function properly without healthy, functional relays to power them.
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If you have questions or concerns about accessory relays or any of your vehicle's components, come into your local Pep Boys where we can answer any question, help you find any part, or perform any vehicle service you might need.

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