A relay is nothing more than a remote switch that uses an electromagnet to close a set of contact points. When the relay magnet is supplied with voltage, the points close and battery voltage is routed through the main circuit. Relays are often used in circuits to reduce the current that flows through the primary control switch. A relatively low amperage switch, timer, or sensor can be used to turn a much higher capacity relay on and off. Another primary use for relays is when upgrading to halogen headlights on an older car. The current draw for halogens is much more than the OEM headlight switches were designed to carry, thereby putting additional strain on the headlight switch. This can cause premature failure of the switch. Another required use for relays is when you’re installing an electric cooling fan. If you wire direct, without a relay, all of the additional stress from the fan will be placed on the switch, leading again to early failure.

Relay 2

The picture above shows the wiring sequence for a typical relay. Here, we’re using a Bosch micro-relay. Your relay may differ a bit in appearance and terminal numbers. Just make the correct changes to the connections.

Here’s a guide to assist in wiring your relay:

  1. Disconnect the battery.
  2. Install the relay in an accessible location. Keep in mind that if the relay is going to be located in an area susceptible to moisture, you’ll need to utilize waterproof connections.
  3. Utilizing 12-14ga wire and a ring-connector, connect one end to the +12v battery terminal. Hook the other end of the wire with a spade connector to terminal #3 on the relay. An inline fuse should be located between these connections. For good wiring practice, use red wire.
  4. Install a spade connector on the POWER wire coming from the accessory.
  5. Using  black wire (16-24ga),  crimp a  spade connector on one end. Connect that to relay terminal #2.
  6. Install a ring-connector on the other end of the small black wire and connect it to a good ground, located close to the relay.
  7. Using a longer piece of wire, crimp a spade connector on one end and connect it to relay terminal #1.
  8. The other end of this wire should be connected to a good source for switched power.
  9. If the accessory that you’re adding requires a switch, put the switch in the wire that goes from terminal #1 to the source of switched power.
  10. When you have everything hooked up but terminal #5 (the power to your accessories), reconnect the battery. Turn on the ignition and test terminal #5 with a multimeter. It should read +12v or so. When a relay is powered up you’ll hear a small click… this is normal. You can now turn off the ignition and make your final connection. Turn the ignition back on and check your work.
  11. Your accessory will also have a black wire that needs to be grounded before it will work.  Again, connect the ground close to the accessory.

The following diagram depicts the connections for most relays:

Relay 3

Relays and their connections can vary. Determine which relay that you’re using, and wire it accordingly. Capp’s Hot Rods can supply relays, switches and quality wiring. Give us a call for all of your wiring needs.


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With over 40 years of combined restoration experience, Capp’s Hot Rods rebuilds and restores Model As, Deuces, ’33s and up, classic cars of all makes and models, and muscle cars.  Located in Woodland Hills, California, Capp’s specializes in complete wiring services, from repairs to complete harness installations.  Visit their website or call 818-974-7530 for more information, or to schedule an appointment for your classic car.  You can also find the Capp’s team at the Pomona Swap Meet on Road 22, Spaces 22, 24 and 26.