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Your vehicle is equipped with relays to help power your starter, lights, and other parts of its electrical system. Not all vehicles have alternator relays, but if your vehicle’s battery is dead and you’re not sure why, a malfunctioning or broken alternator relay could be the cause.



What is an Alternator Relay?

Alternator relays isolate your charging system from the rest of your electrical system in case of shorts and other electrical malfunctions. If a short develops in the electrical system, it can cause damage to the alternator or the vehicle’s electronic control unit (ECU). In addition to fuses and fusible links, alternator relays are installed to add an extra layer of protection against common electrical malfunctions. On vehicles equipped with separate starting and charging systems, like RVs, an alternator relay allows the driver to choose which system charges while the engine is running. Depending on how the alternating relay schematic is designed, the alternator may not charge the house battery unless the starting battery has a full charge.

How do Alternator Relays Work?

Relays work by connecting high-draw electrical components, such as the starter or alternator, to low-draw switches. An alternator relay can be set up to divert power from a starting battery to another DC power source, sometimes called a house battery. This set up is common on RVs. On some cars and trucks, the alternator relay switches power to the alternator only when another circuit, like the ignition circuit, is live. You’ll know the alternator relay needs to be replaced when there’s zero continuity from the alternator’s charging wire to the positive terminal on the vehicle’s battery.

How are Alternator Relays Made?

Universal relays are often installed in place of factory relays bearing OEM part numbers. If you disassemble a typical relay, you’ll find two metal tangs inside. When an accessory switch electrifies these tangs, they snap together to complete a larger circuit. Relays are electromechanical devices that are not easily replicated by the home mechanic.

Why do Alternator Relays Fail?

  • Heat, moisture, and contaminants take their toll on automotive relays. It’s normal for relays to stop working as they age.
  • A malfunctioning alternator may produce too much voltage and melt the housing on your alternator relay.

What are the Symptoms of Alternator Relay Failure?

  • Vehicle jump-starts but immediately shuts off when the cables are removed.
  • Visual inspection reveals a melted relay housing.
  • Charging voltage does not increase when the engine is revved.

What are the Consequences of Alternator Relay Failure?

  • Charging system malfunctions may cause the battery to discharge completely and require replacement, depending on its age
  • The vehicle may not start back up after parking it.

If you have questions or concerns about alternator 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.