HOW TO TEST A RELAY (40 AMP)

Need to know if your 40A relay is working? This step-by-step guide will show you three quick ways to test it using either a car battery or a 9V battery. You’ll learn how to check for the “click,” identify pin terminals, and confirm whether the relay is still good.

Tools You'll Need:

  • 40A Relay

  • Multimeter (optional but helpful)

  • 9V Battery or Car Battery

  • 18-Gauge Wire (at least 1 foot)

  • Electrical connectors or tape

  • Flashlight

Getting Started

If you're wondering how to test a relay (40A), this guide offers the quickest and easiest method. Start by locating the relay. We will then walk you through three straightforward methods to test its functionality. This guide is also useful for confirming correct wiring of your relay and provides additional troubleshooting tips.

Take a look at the relay itself. Each terminal on the relay is noted with a small number that is cast into the plastic mold of the relay. The image below shows where each number is.

Step-by-Step: Car Battery Method

  1. Disconnect the vehicle’s battery by removing the negative terminal to avoid electrical interference.

  2. Cut two pieces of 18-gauge wire (6 inches each) and strip both ends.

  3. Locate pins 85 and 86 on the relay. Shine a flashlight if the numbers are hard to see.

  4. Connect one wire to pin 85 and the negative battery terminal.

  5. Connect the second wire to pin 86 and the positive battery terminal.

  6. The relay should click immediately. This means the internal switch has connected pins 30 and 87.

  7. If there is no click, double-check your wiring. If it still doesn’t work, the relay is likely bad.

Step-by-Step: 9-Volt Battery Method

  1. Locate pins 85 and 86 on the relay.

  2. Line up the 9V battery so that:

    • Positive touches pin 86

    • Negative touches pin 85

  3. Hold both contacts at the same time.

  4. You should hear or feel a click.

  5. No click? The relay coil may be damaged or open.

Optional: Multimeter Resistance Test

  1. Set your multimeter to Ohms.

  2. Connect probes to pin 85 and 86.

  3. A good relay will usually show 50 to 120 ohms.

  4. If you see "OL" or 0 ohms, the coil is bad.

FAQ: Relay Testing Questions

What does it mean if there’s no click?
The coil inside the relay might be damaged, or one of the contacts is broken.

Can I test a relay without a multimeter?
Yes, a simple 9V battery test works fine for basic diagnosis.

Should the relay make a sound?
Yes. A healthy relay will always make a clicking sound when activated.

Can I test a 5-pin relay with the same method?
Yes. The testing process is identical. The extra pin is just for normally closed (NC) functionality.

Next Steps

If your relay is bad, we offer replacement 40A relays and relay wiring kits built specifically for onboard air systems. Need help figuring out which one you need? Our team is available 24/7.

Viair 40 Amp 5 Pin Relay
$7.99
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