How to Check a Relay?
2 Answers
Automotive electromagnetic relays serve three main functions: 1) switching, 2) overload protection, and 3) fault protection. The methods for testing automotive relays include: 1. Static testing: Measure the coil resistance and normally closed contact resistance; 2. Dynamic testing: Apply power to the coil and measure the normally open contact resistance. The methods for determining relay condition are as follows: 1. Open-circuit testing: Use a multimeter in resistance measurement mode to check the relay. Set the multimeter to R×100Ω range and verify continuity between pins (85) and (86), and between pins (87) and (87a). The resistance between pins (87) and (30) should be ∞. If the results differ from these specifications, the relay is faulty. 2. Power application testing: If the above checks are normal, apply 12V power between pins (85) and (86), then check for continuity between pins (87) and (30) using a multimeter. If the results don't match these specifications or if the relay heats up after power application, it indicates the relay is damaged.
Last time my car's headlights suddenly stopped working, I tried troubleshooting the relay myself. First, I turned off the engine and disconnected the negative battery terminal for safety. Then I opened the hood and located the fuse box—usually in a front corner of the car—and flipped open its cover to reveal a cluster of small square relays. I gently pried out the headlight relay with a screwdriver and shook it near my ear to listen for sounds; if I heard loose internal components or rattling, it indicated the internal contacts might be burnt out. Next, I set my multimeter to the continuity test mode and checked the relay's terminal connections: normally, placing the probes should trigger a beeping sound. If there was no response, it confirmed the relay was dead, and simply replacing it with a new one of the same model would solve the problem. The key is not to delay, because a faulty relay can cause headlights, ignition, or even the AC to fail, increasing driving risks. Personally, I make it a habit to inspect them every six months, checking for oxidized or corroded terminals while cleaning off dust—this extends relay lifespan and prevents sudden breakdowns. The tools are basic anyway, so most car owners can attempt this themselves, saving both time and money.
