How To Test a Relay (and How Relays Work) - in 8 minutes

Introduction

This tutorial explains how relays work and provides step-by-step instructions on how to test them. Understanding relays is essential for diagnosing various electrical problems in your car. By the end of this guide, you'll know what relays do, why they're necessary, and how to determine if they are functioning correctly.

Step 1: Understand What a Relay Is

• A relay is an electrically operated switch that uses a small amount of current to control a larger amount of current.
• It allows a low-power circuit to control a high-power circuit, making it crucial for various automotive applications.

Step 2: Know Why We Need Relays

• Relays protect sensitive components from high current.
• They enable remote control of devices, such as lighting and motors, from a distance.
• Using relays increases the longevity of electrical components by reducing wear.

Step 3: Learn How Relays Work

• A relay consists of an electromagnet, a switch, and a spring.
• When current flows through the coil, it creates a magnetic field that moves the switch, either opening or closing the circuit.
• When power is removed, the spring returns the switch to its original position.

Step 4: Gather Your Tools

To test a relay, you will need:

• A multimeter (recommended: Fluke Multimeter)
• Basic hand tools (screwdriver, pliers)
• A test light (optional)

Step 5: Locate the Relay

• Identify the relay you want to test in your vehicle’s fuse box or relay panel.
• Refer to your vehicle's manual for the exact location and layout.

Step 6: Test the Relay

1. Set Up the Multimeter

   • Turn the multimeter to the resistance (ohm) setting.

2. Check the Coil Resistance

   • Disconnect the relay from its socket.
   • Measure the resistance across the coil terminals (often labeled 85 and 86).
   • A typical reading should fall within the range specified in your vehicle's manual (commonly between 60-120 ohms).

3. Test the Switch

   • With the relay still disconnected, measure the resistance across the switch terminals (often labeled 30 and 87).
   • There should be no continuity (infinite resistance) when the relay is not energized.
   • When the relay is energized (apply power to the coil), check again for continuity (should show close to 0 ohms).

Step 7: Fastest Way to Check a Relay

• If you need a quick check, swap the relay with another identical relay in your vehicle (if available).
• If the new relay works, the original is likely faulty.

Step 8: Check Relay Circuit Basics

• Ensure that the relay socket is not corroded or damaged.
• Verify that the power supply and ground connections to the relay are intact and functioning.

Conclusion

Testing a relay is a straightforward process that can save you time and money in diagnosing electrical issues in your vehicle. Remember to always prioritize safety when working with electrical components. If you determine that a relay is faulty, replacing it is usually a cost-effective and simple fix. For further learning, consider exploring more about electrical systems in vehicles or practicing with different relay types.