How To Check A Relay In A Car

How to Check a Car Relay: A Comprehensive Guide

Relays are small but crucial components in your car's electrical system, acting as switches that protect circuits and components from high current. A faulty relay can lead to a variety of problems, from non-functioning headlights to a dead engine. This guide will walk you through how to check a car relay, troubleshooting common issues and ensuring you can get back on the road. Knowing how to test a relay can save you time and money on costly repairs.

Understanding Car Relays: What They Do and Why They Fail

Before diving into testing, let's understand the basics. Relays are electromagnetic switches. They receive a low-voltage signal from the car's computer or another switch, and then use that signal to close a much higher-current circuit, activating components like headlights, fuel pumps, or power windows. They fail due to several reasons, including:

• Overheating: Prolonged use or short circuits can overheat the relay, damaging its internal components.
• Corrosion: Moisture and dirt can cause corrosion on the relay's contacts, leading to poor connection and failure.
• Worn-out contacts: Over time, the contacts inside the relay wear down, losing their ability to make a solid connection.
• Manufacturing defects: Sometimes, relays are simply faulty from the factory.

Diagnosing a relay issue often involves tracing electrical problems back to the source, often using a multimeter.

Tools You'll Need

To check a car relay effectively, you'll need a few essential tools:

• Multimeter: This is the most important tool. It allows you to check for voltage and continuity.
• Relay Diagram (Wiring Diagram): Your car's repair manual or online resources will have a wiring diagram showing the relay's location and connections. This is essential for accurate testing.
• Screwdriver: You'll likely need a screwdriver to access the relay.
• Needle-nose pliers: These can be helpful for removing the relay from its socket.

Steps to Check a Car Relay

Here's a step-by-step guide on how to test a car relay using a multimeter:

1. Locate the Relay: Consult your car's service manual to identify the relay's location. Relays are usually found in fuse boxes under the hood or inside the cabin. They often have a specific number or label that matches the wiring diagram.

2. Visually Inspect the Relay: Before testing, visually inspect the relay for any obvious signs of damage, such as burns, corrosion, or loose connections.

3. Remove the Relay: Carefully remove the relay from its socket. Pay attention to its orientation; it usually has a specific way it needs to be inserted back.

4. Test for Continuity (Coil):

• Set your multimeter to the continuity setting (usually represented by a diode symbol).
• Touch the multimeter probes to the relay's coil terminals (usually marked with 85 and 86).
• A continuous beep indicates a good coil. No beep suggests a faulty coil, requiring relay replacement.

5. Test for Continuity (Contacts):

• Set the multimeter to the continuity setting again.
• Touch the probes to the relay's main contacts (usually marked with 30 and 87). The relay should be unpowered for this test.
• You should not hear a continuous beep. A beep indicates a short circuit. No beep is expected in the unpowered state. If the relay is powered, there should be continuity.

6. Test for Switching Action (Optional):

• This test requires applying power to the coil. It verifies the relay properly switches. Proceed with caution, as applying incorrect power can damage components. Consult your vehicle's wiring diagram for appropriate power sources.
• Apply a 12V power source to the coil terminals (85 and 86). You may need to use jumper wires and your battery.
• With the power applied, check for continuity between terminals 30 and 87. You should now hear a beep, indicating that the relay is switching properly.

7. Reinstall the Relay: If the relay passes all tests, reinstall it. If it fails, replace it with a new one of the same type and rating.

Troubleshooting Common Problems

If your tests reveal a faulty relay, replacing it is usually straightforward. However, remember that the faulty relay might be a symptom of a larger problem within the car's electrical system. Always investigate potential underlying issues to prevent future problems.

By carefully following these steps and using the appropriate tools, you can effectively check a car relay and diagnose electrical problems, saving yourself time, money, and potential frustration. Remember safety first – always disconnect the battery's negative terminal before working on the car's electrical system.