how to test a relay for a car

[LePeyton]

The quickest way to test a car relay is by swapping it with a known good, identical relay from another non-critical circuit in your car, like the horn or A/C compressor. If the component works with the new relay, you've found the problem. For a more precise diagnostic, using a multimeter to check for continuity and proper voltage is the most reliable method. A relay is essentially an electronically operated switch; it uses a small current from the car's computer or a switch to activate an electromagnet, which then closes a circuit to carry a much larger current to a component like the fuel pump or radiator fan.

Essential Tools: You'll need a basic multimeter (set to the Ohms setting for resistance and DC Volts for power) and, optionally, a pair of jumper wires.

Step-by-Step Relay Test:

1. Locate and Identify: Find the relay, typically in the under-hood fuse box. Consult your owner's manual for its location and the diagram on the relay itself, which will show the layout of its terminals.
2. The Listening Test (In-Car): With the car's ignition on and the appropriate system activated (e.g., turn on the A/C to trigger the cooling fan relay), you might hear and feel a distinct "click" from the relay. This confirms the control side is working, but not necessarily the high-current switch side.
3. Bench Testing with a Multimeter:
   • Check the Coil (Terminals 85 & 86): Set your multimeter to Ohms (Ω). Place the probes on the two smaller terminals, often labeled 85 and 86. You should get a resistance reading typically between 50 and 120 ohms. A reading of infinite resistance (O.L. on the meter) means the coil is broken and the relay is dead.
   • Check the Switch (Terminals 30 & 87): Set the multimeter to continuity (the diode symbol that beeps). Place the probes on the two larger terminals, 30 and 87. There should be no continuity initially. Now, apply 12 volts from a car battery (using jumper wires) to terminals 85 and 86. You should hear a loud click, and the multimeter will now show continuity between 30 and 87.

Test Parameter	Good Relay Indication	Faulty Relay Indication
Coil Resistance (85-86)	50 - 120 Ω	0 Ω (shorted) or O.L. (open)
Switch Continuity (30-87)	No continuity without power	Continuous continuity (welded shut)
Switch Function	Continuity with 12V applied to coil	No continuity even with 12V applied
Voltage to Coil (in car)	12V when system is activated	0V (check fuse/switch)
Voltage to Terminal 30	Constant 12V (from battery)	0V (check main power fuse)

Always disconnect the relay from the car before bench testing. If the relay fails any of these tests, replacement is the only option.

[CarolineLee]

Honestly, the swap trick is what most mechanics do first. Pop the hood, find your fuse box, and look for another relay that looks exactly the same. Your horn relay is a perfect candidate. Pull it out, swap it with the one you're testing. If the dead part suddenly works, you just diagnosed the problem in 30 seconds flat. No tools needed. Just make sure the replacement relay is for a non-essential system so you're not disabling something important while you test.

[DelCassandra]

Think of a relay as a remote-controlled switch for heavy-duty items. The "remote" is a small electrical signal. To test it, you need to see if both the remote and the switch work. A multimeter is your best friend here. First, test for resistance across the small control pins; a reading around 70 ohms is healthy. Then, check the big power pins; they should be open until you send power to the control pins, when they should click closed. It’s a simple process of verifying two separate circuits inside one tiny box.

[VonReagan]

Living in an apartment, I don't have a garage full of tools. My method is simple. I carefully remove the relay and give it a gentle shake next to my ear. If I hear anything rattling inside—like a tiny piece of broken metal—it's definitely broken. If it's silent, I then take it to the local auto parts store. Many of them will test it for free with a specialized relay tester. It's a no-risk way to confirm your suspicions before you spend money on a new part. Saves time and guesswork.