Automotive Relays - How They Work - Why You Need Them - Testing
Automotive Relays – How They Work – Why You Need Them – Testing

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Automotive Relays – How They Work – Why You Need Them – Testing

So, automotive relays are switches, controlled by electrical power. To clarify, the purpose of automotive relays, is to automate this power; to switch electrical circuits on and off, at particular times. However, the real benefit behind a relay is more than just automation. For example, they also provide the ability, to switch multiple circuits; including different voltage types, within the same relay, at the same time.

12V DC relay switches, are the best solution for full voltage applications. Because, they allow, a low current flow circuit to control, a high current flow circuit as in:

How Do Automotive Relays Work And Testing

So, if you were to open a relay, you would see an electromagnet coil, the switch, and a spring.

Looking at the diagram below; you can see the pinout of a typical 12V relay. 85 and 86 are the coil pins, while 30, 87, and 87a are the switch pins.

Automotive Relays
Automotive Relays

So, 87 and 87a are the two contacts, to which 30 will connect. If the coil is not activated, 30 will always be connected to 87a. You can think of this as, the switch in OFF. But, when current is applied to the coil; 30 is then connected to pin 87.

The great thing about automotive relays is that, you can set 87 and 87a to be either open or closed; depending on how you need the switch to work:

  • So, if you want a closed relay, you will want to wire to 87a.
  • If you want a normally open relay, you will wire to 87.

So, Realizing that 85 and 86 are the coil pins; these pins will be transferring the current through their coil:

  • One for ground.
  • The other to an accessory.

87 and 87a will be, connected to your controlled accessories. Consequently, the ones that you wish to, turn on and off with your relay. 30 will then be the pin connected; to your battery power.


So, high current electrical circuit components, are expensive. But, low current components, are much cheaper. Consequently, The use of automotive relays means; you can limit the installation of high current circuitry; to those parts of the system to which it is essential. As a result, this helps drive costs down.

Relay Contact Terminology

So, the table below, shows the codes used in automotive relays:

  • 85 Relay Coil Negative
  • 86 Relay Coil Positive
  • 87 Common Contact
  • 87a Normally Closed Contact (NC)
  • 87b Normally Open Contact (NO)
  • 88 Common Contact 2
  • 88a Normally Closed Contact 2
  • 88b Normally Open Contact 2
  • 30 Feed/ Line In Positive

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