how to wire a relay to a starter motor

3 min read 05-02-2025

Wiring a relay to your starter motor is a common modification for high-current applications or to protect your vehicle's ignition switch. This article will guide you through the process, focusing on safety and clarity. Remember, working with automotive electrical systems involves high voltages and currents; incorrect wiring can lead to damage or injury. Always disconnect the battery's negative terminal before beginning any work.

Understanding Relays and Starter Motors

Before diving into the wiring, let's understand the components:

Starter Motors

Starter motors are powerful electric motors that crank your engine to start it. They draw significant current (hundreds of amps), placing a heavy load on the ignition switch.

Relays

A relay acts as a switch controlled by a low-current circuit. It allows a small electrical signal to control a much larger current flow. This protects the ignition switch from high amperage draw.

Parts You'll Need

Relay: Choose a relay with sufficient amperage rating to handle your starter motor's current draw. Check your starter motor's specifications for this information. An automotive-grade relay is essential.
Wiring Harness: Heavy-gauge wires (at least 8 AWG or larger, depending on the starter motor's current) are crucial to handle the high current. Use appropriately sized wire for the connections.
Wire Connectors/Crimps: Ensure secure connections to prevent loose wires and potential short circuits. Properly crimped connections are essential for safety and reliability.
Inline Fuse: A fuse protects your wiring from overcurrent situations, preventing damage and fire hazards.
Circuit Breaker (Optional): A circuit breaker provides an additional layer of protection, automatically cutting the power if there's an overload.
Soldering Iron and Solder (Optional): For more robust and durable connections.

Wiring Diagram and Steps

Here's a common wiring configuration. Always double-check your specific relay's pinout diagram.

Diagram:

Battery (+) ---[Fuse]--- Relay Coil (+)  --- Ignition Switch --- Relay Coil (-) --- Ground
                                     |
                                     V
                                 Relay Output (+) --- Starter Motor (+)
                                     |
                                     V
                                 Relay Output (-) --- Ground
                                     |
                                     V
                                 Starter Motor (-) --- Battery (-)

Steps:

Disconnect the Battery: Always disconnect the negative terminal of your battery before starting any electrical work.
Locate the Starter Motor: Identify the starter motor's positive (+) and negative (-) terminals. Consult your vehicle's service manual if needed.
Run Heavy Gauge Wire: Run a heavy-gauge wire from the battery's positive terminal to your inline fuse.
Install Fuse: Install the appropriately sized fuse in the line.
Connect to the Relay: Connect one end of the heavy-gauge wire to the relay's output terminal (usually marked 87 or similar).
Ground the Relay: Connect another heavy-gauge wire from the relay's output terminal (usually marked 85 or similar) to a clean, grounded point on the vehicle's chassis.
Wire the Relay Coil: Connect one wire from the ignition switch to the relay coil terminal (usually marked 86). Connect the other relay coil wire (usually marked 85) to a suitable ground point. Ensure the wire gauge is appropriate for the low-current coil circuit.
Connect to the Starter Motor: Run a heavy-gauge wire from the relay's output terminal (typically 87) to the starter motor's positive (+) terminal. Connect another heavy gauge wire from the starter motor's negative (-) terminal to the battery's negative terminal.
Reconnect the Battery: Carefully reconnect the battery's negative terminal.
Test: Turn the ignition key to the "start" position. The relay should click, and the starter motor should crank.

Troubleshooting

Relay Doesn't Click: Check the fuse, relay connections, and the wiring to the ignition switch. Ensure the ignition switch is functioning correctly.
Starter Motor Doesn't Crank: Check the battery voltage, starter motor connections, and the fuse. The starter motor itself could be faulty.
Overheating Wires: The wire gauge may be too small for the starter motor's current draw. Use heavier-gauge wiring.

Safety Precautions

Always disconnect the battery's negative terminal before working with any electrical components.
Use appropriately sized fuses and wiring to prevent overcurrent situations.
Ensure all connections are secure and corrosion-free.
Consult a qualified mechanic if you are uncomfortable performing this task.

This guide provides a general overview. Always consult your vehicle's service manual for specific wiring diagrams and recommendations. Incorrect wiring can lead to serious damage or injury. If you're unsure about any aspect of this process, seek help from a qualified automotive technician.