How to Put a Relay Switch Inside a Car

Relays transfer high amperage through heavier-gauge wire from the battery to their respective components. The reason the relays are frequently located under the hood is because it is undesirable to have this much amperage located under the dash, where they can create a fire hazard. It is much safer to put a relay switch inside the car, keeping the relay under the hood. The computer actuates a low-amperage switch inside the vehicle, which in turn activates the relay.

Difficulty: Moderately Challenging

Instructions

Things You'll Need:

• Wire-crimping tool
• Assorted wire connectors
• Roll of 10-gauge wire
• Roll of 16-gauge wire
• Wire cutters
• Inside switch rated at 30 amps
• Drill
• Phillips screwdriver
• Set of wrenches
• 4-terminal 30 amp relay
• Voltmeter

1. Locate the relay close to the fuse and relay box on the driver's side fenderwell. Use the drill to make a hole for the screw to hold the relay to the fenderwell or other secure location. Attach the relay to the fenderwell with the screw.

2. Run one wire from the relay to the battery positive post or to the main lead in at the fuse block. Use the 10 mm wire and the appropriate ends to attach the wire. Crimp the ends with the wire-crimping tool.

3. Run another 10-gauge wire from the relay to the object receiving the power. Again, use the appropriate ends crimped tight with the crimping tool. Now there should be two 10-gauge wires attached to one side of the relay terminals. One of the wires is continuously supplying power to the relay, and the other wire runs from the relay to the object.

4. Install the switch to actuate the relay inside the vehicle. Locate a convenient spot to locate the switch. Make sure that the switch is clear of the knees if it is located low on the dash. Use the drill to make the holes for mounting the switch. Insert the screws and use the Phillips screwdriver to tighten the screws.

5. Supply power to the switch by running a 16-gauge wire to the fuse block. Attach the wire to an open non-fused terminal that has continuous power. If you want power only with the key on, choose an open non-fused terminal that has power only with the key on. Use the voltmeter to check for the circuit in question.

6. Run a 16-gauge wire from the switch to the relay through the firewall. Use a blue blade terminal--crimp it and push it on the other terminal on the switch. Put the same terminal on the other end of the wire and attach it to the relay's third terminal.

7. Cut a 16-gauge wire to the appropriate length. Attach a blue blade terminal to one end of the wire. Run the wire from the last terminal on the relay to a good ground. Use the appropriate terminal for the ground to the frame or engine.

Tips & Warnings

• If it is difficult to locate a terminal to attach to, the wire can be run through the firewall to the relay. Cut the wire from the battery to the relay and use a yellow butt terminal to insert the 16-gauge wire along with the 10-gauge wire in the connector and crimp it. Insert the other end in the connector and crimp it as well. Install a blue blade terminal on the switch end and push it onto one of the terminals.

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