How to Wire a 12-Volt Relay for a Fan

Written by k.k. lowell
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Relays are used in many places in cars and trucks. Switching power to auxiliary lights is an example of this use. Using a relay allows you to run smaller wires through the vehicle, which is a consideration in today's smaller cars. Relays should be used to switch the power to any circuit that draws more than a couple of amps of power.

Skill level:

Things you need

  • Signal stat 192 relay or equivalent
  • Solderless spade connectors (female side)
  • Primary wire
  • Inline fuse holder
  • Solderless ring connectors
  • Solderless butt connectors
  • SPST toggle switch with blade terminals

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  1. 1

    Disconnect the negative battery cable at the vehicle battery.

  2. 2

    Mount the relay using a self-tapping screw in a protected location near the battery.

  3. 3

    Install a solderless ring connector on one lead of the inline fuse holder, and attach the ring to the bolt on the positive battery terminal clamp.

  4. 4

    Using primary wire and a solderless butt connector, extend the other lead of the inline fuse holder as necessary to reach the relay.

  5. 5

    Use a solderless female spade connector, and attach the lead from the inline fuse holder to terminal 87 on the relay.

  6. 6

    Connect a lead from pin 87a to pin 86 using a short piece of wire with female solderless spade connectors.

  7. 7

    Connect the input wire of the 12-volt fan to pin 85 on the relay with a female spade connector. If the fan has two wires, the second wire will connect to ground. Do this by installing a solderless ring connector on the wire and screwing the connector to a metal piece of the vehicle with a self-tapping screw.

  8. 8

    Install a toggle switch in a convenient location in the vehicle. You may have to drill a ½-inch hole to mount the switch, or use a bracket and install it with self-tapping screws.

  9. 9

    Run a wire from pin 30 on the relay to either terminal on the toggle switch. Use solderless female spade connectors to connect both switch and relay.

  10. 10

    Make a wire lead with a ring terminal on one end and a female spade connector on the other. Attach the spade terminal to the second switch terminal and the ring terminal to a good ground under the dash using a self-tapping screw.

  11. 11

    Reconnect the negative battery terminal clamp. Test your relay circuit by turning on the toggle switch. This grounds the coil in the relay, closing the contacts and switching on your relay-controlled device.

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