How to Test if a Relay is Good or Bad?

[EzraFitz]

Methods to test if a relay is good or bad: 1. Use the ohmmeter function of a multimeter to measure the relay's coil. The coil should show a very small resistance value; if the resistance is infinite, it indicates an open circuit in the coil, meaning the relay is damaged. 2. Measure the normally closed (NC) and normally open (NO) contacts of the relay. The resistance values should be close to zero for the NC contacts (below 1 ohm) and infinite for the NO contacts. 3. Check if the relay can be manually operated by pushing the actuation part. If it can, keep the relay in the actuated state and then measure whether the NC and NO contacts have switched states. 4. Alternatively, directly apply the operating voltage to the coil to energize the relay, and then measure whether the states of the two types of contacts have changed.

[DiJasmine]

I've been driving for over 20 years, and here's the most reliable way to test a relay. First, unplug the relay and inspect it visually. If the casing is bulging or the pins are blackened/burnt, it's likely faulty. Then, set your multimeter to the resistance mode and measure the two coil terminals—if it shows infinite resistance, the coil is broken. Next, check the contact continuity; if the resistance between normally closed contacts exceeds 1 ohm, something's wrong. The most accurate method is a power test: connect a 12V battery to the coil terminals, listen closely for a crisp click, and observe the contact switching. If there's no sound during engagement or it's only partially engaging, just replace it—these small components aren't worth repairing compared to replacement.

[DeclanAnn]

Last time I modified my car audio system, I found the relay malfunctioning and came up with a three-step testing method. First, prepare a multimeter, remove the relay and measure the coil resistance, which should normally be between 50-150 ohms. The second step is to measure the static resistance of the contacts, using the buzzer mode is more convenient. Finally, apply 12V voltage to test the pull-in action, this step is the most critical: when the coil is energized, the multimeter should show the contacts changing from open circuit to closed circuit. Some high-end car relays have diode protection, and reversing the test leads will give inaccurate readings, so special attention should be paid to this. In fact, relays for headlights and air conditioning compressors in cars are the most prone to failure, with the symptom being the sudden stoppage of electrical components.

[VonWalker]

Last time my wipers suddenly stopped working, I tested the relay myself and it was quite simple. Borrowed a multimeter from a friend and located the fuse box according to the repair manual. Pulled out the relay and shook it first—if you hear parts rattling inside, it's definitely broken. Used the multimeter to test the two pins of the coil; normally, there should be around 80 ohms of resistance. Testing the contacts is even easier: use needle-nose pliers to connect pins 85 and 86 to the battery's positive and negative terminals, then connect pins 30 and 87 to the multimeter—the needle will swing when the relay engages. Fixed it myself in half an hour, saving 200 bucks in labor costs. These little relays are especially prone to failure in older cars, particularly those that have been waterlogged.

[VanAlessandra]

As a clumsy person, I rely on rudimentary methods to test relays. First, locate the fuse box position - manuals usually have diagrams. The most straightforward way is to pull out the suspect relay and swap it with an identical one for testing. For example, if the headlight relay is faulty, try replacing it with the wiper relay to see if the lights work. If you don't have a multimeter handy, ask the shop to test new relays when purchasing. Actually, relay failures are quite common, especially during peak usage seasons in winter and summer - sudden AC failure is often caused by them. For professional testing, it's best to visit a repair shop where they have specialized relay testers that provide instant results when plugged in.

[Santana]

Last time when repairing the car fridge, I found a relay malfunction. This method is particularly accurate: first use the multimeter's diode mode to test the coil's continuity, then switch to the 200-ohm range to measure the resistance value. For contact testing, use the voltage mode; measure the voltage drop across the contacts when powered—if it exceeds 0.5V, it indicates contact oxidation. For a more advanced approach, use an oscilloscope to observe the pull-in time; the action should normally complete within 10 milliseconds. The most dreaded issue with relays is contact sticking; during testing, you can repeatedly power on and off twenty times. Actually, the relay controlling the fuel pump in the car is the most critical—if it fails, the car will break down directly. It's recommended to check the contact condition every two years.