how do i test a car relay

That frustrating moment when you turn the key and nothing happens, or a critical component like your fuel pump or cooling fan refuses to work, can often be traced back to a small but mighty component: the relay. Tucked away in your engine bay or a fuse box, these electronic switches control high-power circuits with a low-power signal. Before you call a mechanic or replace expensive parts, learning how do i test a car relay is a simple and empowering skill that can save you time and money.

Gathering Your Simple Testing Tools

You don’t need a fancy garage to perform this check. The most reliable method requires just a basic multimeter, which measures electrical values, and a working 12-volt car battery. A wiring diagram for your specific vehicle can be incredibly helpful, but it’s not always essential for a basic test. Safety first: always ensure the car is off and the key is out of the ignition before you begin handling any electrical parts.

How Do I Test a Car Relay with a Multimeter

Start by locating the relay, often found in the under-hood fuse box. Gently pull it out. Your first test is for resistance across the coil, which is the relay’s internal electromagnet. Look at the diagram on the relay’s side; it will show you which two pins connect to the coil (typically numbered 85 and 86). Set your multimeter to the resistance (Ohms) setting and touch the probes to these pins. You should get a reading, usually between 50 and 120 ohms. A reading of zero or infinite resistance means the coil is damaged and the relay needs replacing.

The Bench Test: Simulating Real-World Operation

This next step confirms the relay can physically switch on. You’ll need your car battery for this. Connect a small wire from the battery’s positive terminal to pin 86, and another wire from the battery’s negative terminal to pin 85. You should hear and feel a distinct click as the relay energizes. Now, set your multimeter to test for continuity (it will beep). Place one probe on pin 30 (the power input) and the other on pin 87 (the output to the component). With the relay energized, the multimeter should beep, confirming the internal switch has closed and current can flow.

When It’s Time for a Replacement

If your relay fails either of these tests—showing no resistance on the coil or failing to click and show continuity—it’s definitively faulty. The good news is that relays are inexpensive and easy to swap. Simply purchase an identical replacement and push it firmly into the socket. This simple act of testing and replacing can often bring a seemingly major electrical problem to a quick and satisfying resolution.

By taking a few minutes to test a suspect relay, you move from guessing to knowing. This small act of diagnosis empowers you to fix your car with confidence and get back on the road without an unnecessary trip to the repair shop.