How to tell if a relay is bad on a car?

[LolaDella]

A faulty car relay is confirmed by a lack of audible click when powered, no continuity between pins 30 and 87 when activated, or a coil resistance outside the standard 50-200 ohm range. The most reliable diagnostic methods are swapping with a known-good relay or using a multimeter for electrical testing.

Common Symptoms of a Bad Relay The most direct signs are component failure and unusual relay behavior. If a device like the fuel pump, horn, or A/C compressor fails to activate, the relay is a primary suspect. Listen for the absence of a faint but audible "click" when the component is switched on—a silent relay often indicates a failed coil. Intermittent operation, where the component works sporadically, points to internal contact wear. Physically, inspect the relay for burn marks, corroded terminals, or a melted plastic casing, which are clear visual indicators of failure.

Step-by-Step Diagnostic Methods You can diagnose a bad relay without advanced tools using the swap test. Locate the fuse box and identify a relay with the same part number controlling a non-critical function. Swap the suspected relay with this known-good one. If the previously faulty component now works, the original relay is defective. This method is over 90% effective for confirming a total relay failure in field conditions.

For a definitive electrical diagnosis, a multimeter is required. First, test the coil resistance by measuring across terminals 85 and 86. A reading between 50 and 200 ohms is normal; an open circuit (infinite resistance) or a very low reading confirms a bad coil. Next, test the switch function. Apply 12 volts to terminals 85 and 86 to activate the relay. You should hear a click. Then, measure for continuity (near zero resistance) between terminals 30 and 87. No continuity with power applied means the internal switch is faulty.

Understanding Relay Terminal Functions Accurate testing requires knowing the four key terminals. Terminals 85 and 86 are for the control circuit (the coil). Terminal 30 is the common power input from the battery, and terminal 87 is the output to the component. When 12V is applied to the coil (85/86), it creates a magnetic field that closes the internal switch, connecting terminals 30 and 87 to deliver high-current power. The table below summarizes their roles:

Terminal	Function
85 & 86	Low-current control circuit (coil).
30	High-current power input from battery.
87	High-current output to the component.

If a relay fails any of these tests—swap, click, coil resistance, or switch continuity—replacement is necessary. Using a relay with an incorrect current rating can lead to premature failure, so always match the part number or specifications exactly.

[SabrinaRose]

As a mechanic, my first move is always the "click test." Turn the key to "on" or try the component. Put your fingers on the relay—you should feel a distinct click. No click? That's suspect number one. Next, I feel its temperature. If it's hot to the touch while the car is just sitting, something's wrong internally. Then, I pull it and look. Melted plastic, dark burn marks, or green corrosion on the pins are dead giveaways. These physical checks take 60 seconds and catch about 80% of bad relays before you even get a tool.

[LaSavannah]

I’m not a pro, but I’ve fixed my own car using the swap method. It’s foolproof. My horn wouldn’t work. I opened the fuse box under the hood and found the horn relay. Right next to it was an identical relay for the A/C. I just pulled both out and swapped their places. I turned the key and hit the horn—it worked perfectly. That meant the original horn relay was bad. I drove to the auto parts store, bought a new one using the number printed on the old relay, swapped it back in, and my A/C still worked fine too. It cost me $15 and 10 minutes. No fancy tools needed.

[SanGianna]

Think of a relay as a remote-controlled switch. The "remote" is the low-power circuit (pins 85/86). The "switch" is the high-power circuit (pins 30/87). When the remote fails, you get no click. When the switch fails, power can't flow. The multimeter tests these two parts separately. Testing resistance between 85 and 86 checks the remote's circuit. Applying 12V to 85/86 and listening for a click tests the remote's action. Checking for continuity between 30 and 87 after you hear the click tests if the switch inside closed properly. This logical, two-part breakdown makes diagnosis systematic.

[SanFelicity]

My approach is to match the symptom to the test. Start simple. Is the component completely dead? Do the swap test first—it’s the fastest way to confirm. Is the problem intermittent? That’s trickier. The relay might work sometimes. Here, you need the multimeter. Check the coil resistance; it should be stable. Tap the relay while testing—if the resistance value jumps, there’s an internal break. For a relay that clicks but delivers no power, focus on the switch side. Activate it and check continuity between pins 30 and 87. Remember, a relay can have a good coil (it clicks) but a burnt switch (no power output). Always test both functions to be sure before buying a new part.