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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.
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 test using vehicle power. Disconnect the battery first to avoid electrical interference.
Disconnect the vehicle’s battery by removing the negative terminal to avoid electrical interference.
Cut two pieces of 18-gauge wire (6 inches each) and strip both ends.
Locate pins 85 and 86 on the relay. Shine a flashlight if the numbers are hard to see.
Connect one wire to pin 85 and the negative battery terminal.
Connect the second wire to pin 86 and the positive battery terminal.
The relay should click immediately. This means the internal switch has connected pins 30 and 87.
Double-check your wiring. If it still doesn’t work, the relay is likely bad.
A simple bench test — no vehicle required.
Locate pins 85 and 86 on the relay.
Press the battery against both pins at once so the coil can energize.
You should hear or feel a click.
The relay coil may be damaged or open.
Confirms the coil is intact without applying power.
Select the resistance (Ω) range on your meter.
Touch one probe to pin 85 and the other to pin 86.
A good relay will usually show 50 to 120 ohms.
If you see OL or 0 ohms, the coil is bad.
The coil inside the relay might be damaged, or one of the contacts is broken.
Yes, a simple 9V battery test works fine for basic diagnosis.
Yes. A healthy relay will always make a clicking sound when activated.
Yes. The testing process is identical. The extra pin is just for normally closed (NC) functionality.
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.
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