how to check car relays

[VonAlan]

The most reliable way to check a car relay is by swapping it with a known-good identical relay from another, non-critical circuit in your vehicle. This method is quick and doesn't require special tools. If the problem moves with the relay, you've found the culprit. For a more technical diagnosis, a multimeter is your best tool to test for continuity and coil resistance.

A relay is an electronically operated switch. A small current from a switch (like your power window button) flows through the relay's internal coil, creating a magnetic field that pulls a contact closed, allowing a much larger current to flow to the component (like the window motor).

Here is a step-by-step guide for testing with a multimeter:

1. Locate and Remove the Relay: Consult your owner's manual for the relay's location, typically in the under-hood fuse box. Gently pull it straight out.
2. Visual Inspection: Check for signs of melting, cracking, or corrosion on the plastic case and metal terminals.
3. Identify the Terminals: Look for a diagram on the relay itself. It will show four or five terminals, usually labeled: 85 and 86 (the low-current coil circuit), 30 (power input from the battery), 87 (power output to the component), and sometimes 87a (used in changover relays).
4. Test the Coil (Terminals 85 & 86): Set your multimeter to resistance (Ohms, Ω). Touch the probes to terminals 85 and 86. A good relay coil will show a resistance reading, typically between 50 and 120 ohms. An infinite reading (OL) means the coil is open and the relay is bad. A zero reading indicates a short.
5. Test the Switch (Terminals 30 & 87): Set the multimeter to continuity (the diode symbol). With the relay de-energized (no power), there should be no continuity between terminals 30 and 87. Now, apply a small 12V power source (a car battery is perfect) to terminals 85 (negative) and 86 (positive). You should hear a distinct click. While power is applied, check for continuity between 30 and 87; it should now be present.

| Common 5-Pin Relay Test Data (Typical 12V Automotive Relay) | | :--- | :--- | | Coil Resistance (between 85 & 86) | 70-90 Ω | | Voltage to Energize Coil | 9-14V DC | | Continuity (30 to 87) when energized | Present (near 0 Ω) | | Continuity (30 to 87a) when de-energized | Present (near 0 Ω) | | Maximum Current Rating (for terminals 30/87) | 20-40 Amps |

If your relay fails any of these tests, it needs to be replaced. Always match the new relay's part number and current rating to the original.

[DelEva]

Just find the same relay somewhere else in your fuse box—like the one for the horn. Swap them. If your broken part starts working and the horn stops, you found the bad relay. It’s the easiest trick. No tools needed, just a bit of careful pulling. If swapping doesn’t fix it, the problem is likely elsewhere, like a fuse or the component itself.

[OAngelo]

Think of a relay as a faithful assistant. You give it a small, quiet command, and it does the heavy lifting for you. When it fails, that command doesn't get through. The most common symptom is complete silence—you turn the key or push a button, and nothing happens. No click, no hum. Before you assume the worst about a major part like the starter or fuel pump, always check its little helper, the relay. A simple swap test can save you a lot of diagnostic time and money.

[SanLorenzo]

My go-to method is the tap test. Locate the relay you suspect—your manual has a diagram. With the car on and the problem circuit activated (like the A/C), give the relay a firm but gentle tap with the handle of a screwdriver. If the component suddenly kicks to life, even for a second, the relay's internal contacts are likely worn or corroded. It's a temporary fix, but it confirms the diagnosis. This works great for intermittent issues that are hard to catch. Then, head to the parts store for a replacement.

[LaAngelina]

For a definitive check, use a multimeter. Safety first: disconnect the negative battery cable. Pull the relay out. Set your meter to measure Ohms. Touch the probes to the two smaller terminals; you should get a reading, say 80 ohms. If it says "OL," the coil is broken. Next, check for continuity between the two larger terminals without any power; there should be none. Then, carefully use jumper wires from your battery to the two small terminals. You'll hear a click. Now check the large terminals—there should be continuity. If it fails either test, replace it.