how to test automotive relay

3 min read 30-12-2024

Meta Description: Learn how to test a car relay effectively with simple tools. This comprehensive guide covers visual inspection, multimeter testing, and troubleshooting common relay problems, ensuring you can diagnose and fix electrical issues in your vehicle. Save time and money by mastering this essential automotive skill!

Introduction

Automotive relays are small but crucial electrical switches that protect circuits and components in your car. A faulty relay can cause a range of problems, from malfunctioning headlights to a non-functional power window. Knowing how to test an automotive relay is a valuable skill for any car owner, saving you potential repair costs. This guide will walk you through various methods to determine if a relay is functioning correctly.

What is an Automotive Relay?

Before diving into testing, let's understand what a relay is. An automotive relay is an electromagnetic switch that uses a low-voltage signal to control a higher-voltage circuit. This protects the low-voltage control circuit from the potentially higher current demands of the controlled component. Think of it as a protective intermediary.

Tools You'll Need

To test a relay effectively, you'll need these basic tools:

Multimeter: This essential tool measures voltage, current, and resistance. A digital multimeter (DMM) is recommended for accuracy.
Test Light: While not strictly necessary, a test light can help visualize circuit continuity.
Wiring Diagram: Locate your vehicle's wiring diagram to identify the relay's terminals. This will often be found in your owner's manual or online.
Screwdriver: For removing the relay from its socket.

Visual Inspection: The First Step

Before using your multimeter, perform a visual inspection of the relay. Look for:

Physical Damage: Check for cracks, burns, or obvious signs of overheating on the relay's casing.
Loose Connections: Examine the relay's terminals for any loose wires or corrosion. Corrosion can impede electrical conductivity.
Bent Pins: Ensure all the pins are straight and properly seated in their sockets. Bent pins can prevent proper contact.

Testing the Relay with a Multimeter

This is the most reliable method to determine the relay's functionality. Here's how to do it:

1. Understanding Relay Terminals

Most automotive relays have 4 or 5 terminals:

85 and 86: These are the control circuit terminals. A low voltage signal is applied here to activate the relay.
30: This is the power input terminal. It receives power from the battery or fuse box.
87: This is the normally open (NO) output terminal. Power flows through this terminal when the relay is activated.
87a (if present): This is the normally closed (NC) output terminal. Power flows through this only when the relay is not activated.

2. Continuity Test (Coil)

Set your multimeter to the "continuity" or "diode test" setting. You should hear a beep or see a reading if there's a complete circuit.
Test between terminals 85 and 86. You should get continuity indicating that the relay's coil is intact. A lack of continuity suggests a faulty coil.

3. Continuity Test (Contacts)

With the relay UNPOWERED, test between terminals 30 and 87. You should not get continuity. If you do, the relay contacts are likely stuck closed.
Now, apply 12V (or the appropriate voltage for your relay) to terminals 85 and 86. You can typically do this by connecting the positive lead of your battery to 85 and the negative lead to 86. This activates the relay's coil.
With the relay energized, test again between terminals 30 and 87. You should get continuity. If not, the contacts may be faulty.

4. Resistance Test

An alternative method involves measuring the resistance between terminals. Consult your relay's specifications for the expected resistance values. Significant deviations suggest a problem.

Troubleshooting Common Relay Problems

Relay clicks but doesn't work: This often indicates a problem in the circuit the relay controls, not necessarily a faulty relay itself. Check the wiring and the component the relay switches.
Relay doesn't click at all: This typically means the relay coil is faulty or there's no power reaching the coil. Check the fuse and wiring leading to the relay.
Relay continuously clicks: This can indicate a short circuit in the controlled circuit. Isolate the problem in the circuit connected to the relay's output.

Replacing a Faulty Relay

Once you've confirmed a relay is bad, replace it with an identical one. Be sure to carefully note the relay's location and orientation before removal. Many auto parts stores can help identify the correct relay for your vehicle.

Conclusion

Testing an automotive relay doesn't require specialized tools or extensive knowledge. With a multimeter and a little patience, you can effectively diagnose and troubleshoot many electrical issues in your vehicle. Remember to always prioritize safety and consult your vehicle's wiring diagram when working with electrical components. By mastering this simple testing procedure, you can save yourself time, money, and the frustration of dealing with intermittent electrical problems.