How to test a car relay to see if it's good?

[IsabelaRose]

A functional car relay will produce an audible click when energized and show correct electrical readings. To definitively test a relay, you need a multimeter and a 12V power source. The process involves checking the coil's resistance (typically 50-200 ohms), verifying the switch's operation via continuity tests when powered, and using a substitution test for final confirmation. A systematic approach isolates the relay as the fault.

Functional Click Test (Energizing the Coil) Identify the relay's control circuit pins, usually labeled 85 and 86. Using jumper wires, apply 12V from your car battery directly to these pins. You should hear a distinct audible click as the relay's internal electromagnet activates and the switch contacts move. This is a quick, preliminary check. If there's no click, the coil is likely dead. However, a click alone doesn't guarantee the high-current switch contacts are good.

Coil Resistance Check (Using a Multimeter) Set your multimeter to the Ohms (Ω) setting. Place the probes on pins 85 and 86. A healthy relay coil will show a resistance reading. For standard automotive relays, this value typically falls between 50 and 200 ohms. This measurement confirms the integrity of the coil's windings.

• An "OL" (Open Loop) or infinite resistance reading indicates a broken coil wire inside the relay.
• A reading near 0 ohms suggests a shorted coil. Both conditions mean the relay is faulty and needs replacement.

Switch Contact Continuity Check (Under Load) This test verifies the main switching function. Identify the high-current circuit pins: 30 (common/input) and 87 (normally open/output). Set your multimeter to continuity or diode test mode (which beeps). With the 12V power still applied to pins 85 and 86 (the relay should be clicked), touch the probes to pins 30 and 87. The multimeter should beep, indicating continuity (low resistance), which means the switch is closed and current can flow.

• No beep when energized: The switch contacts are burned, stuck open, or failed.
• Beep when not energized (power off 85/86): The contacts are welded shut, a serious fault. Some relays have a fifth pin, 87a (normally closed), which should have continuity with pin 30 only when the relay is not powered.

Pin Number	Function	Test Condition & Expected Result
85 & 86	Coil (Control Circuit)	Resistance: 50-200 Ω
30	Common (Incoming Power)	Continuity to 87 when coil is powered.
87	Normally Open (Output)	Continuity to 30 when coil is powered.
87a	Normally Closed (if present)	Continuity to 30 only when coil is NOT powered.

Substitution Test (The Real-World Benchmark) The most reliable field test is swapping the suspect relay with a known-good relay of identical type from a non-critical circuit in your vehicle, such as the horn or a similar accessory relay. If the problematic component (e.g., fuel pump, headlight) works with the swapped relay, you've confirmed the original relay is bad. This method is favored by technicians for its speed and certainty.

Required Tools & Safety You will need a digital multimeter, a 12V DC power source (your car battery is ideal), and a pair of jumper wires. Always ensure the relay is removed from the vehicle for bench testing. Handle the jumper wires carefully to avoid short circuits on the battery terminals. Industry repair data shows that relay failure accounts for a significant percentage of electrical "no-operation" faults, and methodical testing quickly isolates the issue.

[DeAmir]

As a weekend DIYer, I keep a cheap multimeter in my garage. When my headlight acted up, I pulled the relay. No click when I hooked it to the battery—first bad sign. Then I metered pins 85 and 86: got an “OL” reading, meaning open circuit. That was it, a dead coil. Bought a new relay for a few bucks, plugged it in, and the light worked. The whole test took five minutes. It’s satisfying to pinpoint the problem instead of just guessing. The click test and a basic resistance check are usually all you need.

[OWalter]

In my shop, we see relays fail all the time. The contacts get pitted from arcing, or the coil just opens up. My process is muscle memory now. First, listen for the click with a power probe. No click? It's junk. If it clicks, I move to the meter. I expect to see a solid 60-100 ohms across the coil on most standard relays. Then the real test: checking continuity between 30 and 87 while the coil is powered. You'd be surprised how many relays click but don't pass current. I keep a tray of known-good relays for a quick swap test to be absolutely sure before telling a customer they need a new one. It’s a simple component, but diagnosing it correctly saves a lot of time chasing wiring ghosts.

[EvanMarie]

Think of the relay as a remote-controlled switch. The "remote" is the low-power coil circuit (pins 85/86). The "switch" is the high-power contact circuit (pins 30/87). Testing is just verifying both parts work.

Step 1: Does the remote work? Power the coil. Hear a click? Good.

Step 2: Is the remote's button not stuck? Measure coil resistance. 50-200 ohms is normal. Too high (open) or too low (short) means the remote is broken.

Step 3: Does the switch obey the remote? With power on the coil (click engaged), check if pins 30 and 87 are connected (continuity beep). If they are, the switch works. If not, the switch is broken.

This logical breakdown makes it easier than memorizing pin numbers. The substitution test is your final, foolproof check.

[OLorenzo]

Let's talk about why testing both states is crucial. A relay has two mechanical states: relaxed and energized. A full test validates both. The common failure of welded contacts is only caught if you test for incorrect continuity. That's why you must check pins 30 and 87 both with and without power on the coil.

With no power, there should be no continuity between 30 and 87 (for a standard 4-pin relay). This confirms the switch is open at rest. When you apply power to the coil, you must then get continuity. This two-step check proves the switch is moving correctly. If you only check it while powered and hear a beep, you might miss that the contacts are also welded shut—a dangerous failure where the circuit can't be turned off. Always verify the switch opens and closes on command. This thoroughness separates a proper diagnosis from a lucky guess. Using your car battery as the 12V source is perfect because it replicates the actual operating voltage.