How to Test a Relay with a Multimeter: Easy Guide

Testing a relay with a multimeter is a straightforward process that helps you diagnose electrical issues in your car or other equipment. Understanding how a relay works and knowing the basic steps to test its components can save you time and money on repairs. This guide will walk you through everything you need to know to confidently test a relay using just a multimeter and some basic knowledge.

Simply put, testing a relay involves checking the continuity of its coil and the resistance of its contacts to ensure they are functioning as intended. This diagnostic step is crucial for pinpointing electrical faults before replacing components unnecessarily.

Key Takeaways

• Testing a relay with a multimeter involves examining both the coil and the contacts.
• A functioning relay coil will show a specific resistance, while a faulty one will show open or short circuits.
• Continuity checks on the contacts, both energized and de-energized, reveal if they are switching properly.
• Understanding relay pinouts is essential for accurate testing.
• This process helps differentiate between a bad relay and other electrical system problems.

What is a Relay and How Does It Work?

A relay is essentially an electrically operated switch. It uses a small current to control a much larger current, acting as a remote control for electrical circuits. This is particularly useful in automotive applications, where a low-current signal from a switch (like a dashboard button) can activate a high-current device (like a starter motor or fuel pump).

At its core, a relay consists of a coil of wire wrapped around an iron core, and a set of electrical contacts. When current flows through the coil, it generates a magnetic field. This magnetic field pulls an armature, which in turn moves the electrical contacts, either closing an open circuit or opening a closed circuit.

• Coil: The electromagnet that creates the magnetic field when energized.
• Armature: The movable part that is pulled by the magnetic field.
• Contacts: The switch points that complete or break the circuit.
• Spring: Returns the armature to its resting position when the coil is de-energized.

Relays are categorized by their type, such as SPST (Single Pole, Single Throw), SPDT (Single Pole, Double Throw), DPST (Double Pole, Single Throw), and DPDT (Double Pole, Double Throw). The most common in automotive use are SPST and SPDT relays.

Understanding Relay Pinouts and Terminals

Before you can test a relay, you need to know which terminals are which. Relays typically have four or five pins, each serving a specific function. Identifying these pins is the first critical step in the testing process.

Most automotive relays have standard markings on their casing that indicate the function of each pin.

The common terminals are the coil terminals (often labeled 85 and 86, or 30 and 87a in some European configurations) and the contact terminals (often labeled 30, 87, and sometimes 87a). Terminal 30 is the common input for the power that will be switched. Terminal 87 is the output that receives power when the relay is energized, and 87a is the output that receives power when the relay is de-energized (in a SPDT relay).

Common Terminal Labels & Function
85 & 86: Coil terminals. One is for power, the other for ground.
30: Common terminal. Power input for the contacts.
87: Normally Open (NO) contact output. Receives power when the coil is energized.
87a: Normally Closed (NC) contact output. Receives power when the coil is de-energized (only on SPDT relays).

Always refer to the specific relay’s diagram or consult your vehicle’s service manual for precise pin assignments. Incorrect identification can lead to misdiagnosis or damage to the electrical system.

What You’ll Need to Test a Relay

To effectively test a relay, you’ll need a few essential tools. The primary tool is a multimeter capable of measuring resistance (ohms) and continuity (often indicated by a beep). Ensure your multimeter is set to the appropriate range for the resistance you expect to measure, typically in ohms (Ω).

Beyond the multimeter, a basic understanding of your vehicle’s electrical system or the system the relay is part of is helpful. Sometimes, a small jumper wire or a 12-volt power source (like a car battery) might be needed for specific tests to simulate energizing the coil, though many tests can be done with just the multimeter and the relay removed from the circuit.

• Digital Multimeter: Essential for measuring resistance and continuity.
• Relay Diagram: Crucial for identifying terminal functions.
• Service Manual: Provides specific specifications for your vehicle or equipment.
• Jumper Wires (optional): For applying power to the coil if needed.
• Safety Glasses: Always recommended when working with electrical systems.

Having these items readily available will make the testing process smoother and more accurate. Remember to always disconnect the battery before working on any electrical components to prevent short circuits or shocks.

How to Test the Relay Coil with a Multimeter

The first step in testing a relay is to examine its coil. The coil is an electromagnet, and like any electromagnet, it has a specific resistance. A healthy coil will have a resistance within a specified range, which you can often find in a service manual.

If the coil is open (infinite resistance) or shorted (very low resistance, often 0 ohms), the relay won’t work.

To test the coil, set your multimeter to the resistance (Ω) setting. Place the multimeter probes on the two coil terminals (usually 85 and 86). You should get a reading that is typically between 50 and 150 ohms, though this can vary.

If you get an “OL” (Over Limit) or infinite resistance reading, the coil is open and the relay is bad. If you get a very low reading (close to 0 ohms), the coil is likely shorted, and the relay is also bad.

This test is done with the relay removed from the circuit and without any power applied to it. It provides a quick assessment of the coil’s integrity.

Testing the Relay Contacts (De-energized State)

Next, you’ll test the relay’s contacts in their resting state, meaning the coil is not energized. This test is performed with the relay still removed from its socket and no power applied. You’ll be checking for continuity between the common terminal (30) and the normally closed (NC) terminal (87a), if your relay has one.

Set your multimeter to the continuity setting (usually indicated by a speaker or diode symbol). Place one probe on terminal 30 and the other on terminal 87a. You should hear a beep or see a reading close to 0 ohms, indicating that the circuit is complete.

If there is no beep or a high resistance reading, the normally closed contact is faulty.

Warning: For relays without a terminal 87a (typically SPST relays), you will only test the normally open (NO) contact, which should show no continuity when de-energized.

For a standard SPST relay (usually four pins: 85, 86, 30, 87), you would check for continuity between terminal 30 and terminal 87. In the de-energized state, there should be NO continuity (no beep, OL reading). If there is continuity here, the normally open contact is stuck closed, and the relay is faulty.

Testing the Relay Contacts (Energized State)

The most important test is to check the contacts when the relay is energized, simulating its operation in the circuit. This is where you verify that the magnetic field from the coil successfully moves the contacts to complete the intended circuit. You can do this either by carefully applying power to the coil with jumper wires or by installing the relay back into its socket and briefly activating the circuit that powers the coil.

Method 1: Using Jumper Wires and a 12V Source

• Connect a jumper wire from terminal 85 to the positive (+) terminal of a 12V power source (like a car battery).
• Connect another jumper wire from terminal 86 to the negative (-) terminal of the 12V power source.
• As soon as power is applied to the coil, you should hear or feel the relay click.
• While the coil is energized, use your multimeter on continuity setting. Check for continuity between terminal 30 and terminal 87. You should hear a beep.
• If you have an 87a terminal, check for continuity between 30 and 87a. There should be NO continuity (no beep) in this energized state.

Method 2: In-Circuit Testing (More Advanced)

This method is more complex as it requires identifying which circuit powers the relay coil and then using your multimeter to check voltage and continuity at the relay socket. You’ll need to know which socket pins correspond to the coil and the contacts.

• With the relay installed and the vehicle’s ignition system on (or whichever system powers the coil), use your multimeter to check if voltage is being supplied to the coil terminals (85 and 86).
• Then, check for voltage at the contact terminals (e.g., output of the component the relay controls). If the relay is supposed to be activated, and voltage is applied to the coil but no voltage is present at the controlled device, the relay is likely the culprit.
• You can also try testing for continuity across the contacts (30 and 87) with the relay energized, similar to the jumper wire method.

If the relay clicks but the contacts don’t switch properly (i.e., no continuity on 87 when energized, or continuity still present on 87a), the relay is faulty.

Common Relay Failure Modes and Symptoms

Relays can fail in several ways, each leading to different symptoms in the system they control. Understanding these common failure modes helps in diagnosing electrical problems more accurately. The most frequent issues involve the coil becoming open or shorted, or the contacts sticking or failing to make proper contact.

Stuck Contacts: If the contacts become welded shut or stuck in the closed position, the device controlled by the relay will remain on constantly, even when it shouldn’t be. For example, headlights might stay on after the engine is turned off, or a fan might run continuously.

Important: A relay that clicks but doesn’t activate the device it controls is often a sign of faulty contacts, while a relay that doesn’t click at all might indicate a bad coil or no power to the coil.

Troubleshooting Specific Relay Types

Different relay types have slightly different testing procedures. The most common are four-pin (SPST) and five-pin (SPDT) relays. Understanding the difference is key to accurate testing.

Four-Pin Relays (SPST – Single Pole, Single Throw): These have two coil terminals (85, 86) and two contact terminals (30, 87). When the coil is energized, terminal 30 is connected to terminal 87. In the de-energized state, there is no connection between 30 and 87.

Five-Pin Relays (SPDT – Single Pole, Double Throw): These have two coil terminals (85, 86) and three contact terminals: common (30), normally open (87), and normally closed (87a). When the coil is energized, terminal 30 connects to 87. When de-energized, terminal 30 connects to 87a.

This provides a switching action.

• Coil Test: Same as for four-pin relays (85 & 86).
• De-energized Contact Test: Continuity between 30 and 87a. No continuity between 30 and 87.
• Energized Contact Test: Continuity between 30 and 87. No continuity between 30 and 87a.

The exact resistance values and specifications can be found in your vehicle’s service manual or the relay manufacturer’s datasheet. Some sources suggest that if you can’t find specific resistance values, a reading between 50 and 150 ohms is generally acceptable for most automotive relays.

Frequently Asked Questions

What is the typical resistance of a relay coil?

The typical resistance of an automotive relay coil is usually between 50 and 150 ohms. However, this can vary depending on the relay’s specific design and application. Always consult your vehicle’s service manual for precise specifications.

Can I test a relay while it’s still in the car?

Yes, but it’s often easier and more accurate to test a relay by removing it. In-circuit testing can sometimes be misleading due to other components in the circuit. However, you can test for voltage at the relay socket and listen for the relay to click when its coil is energized.

What does it mean if a relay clicks but doesn’t work?

If a relay clicks, it means the coil is receiving power and is activating. However, if the device it controls doesn’t function, it suggests the problem lies with the relay’s internal contacts. They may be worn, dirty, or fused, preventing the switched current from flowing properly.

How can I tell if my relay is bad?

A relay is generally considered bad if it shows open or shorted resistance in the coil, if its contacts fail to make or break connection as they should, or if it doesn’t click when it should. A visual inspection might reveal burnt contacts or a damaged casing.

Should I replace a relay if I suspect it’s faulty?

Yes, if your multimeter tests indicate that the relay coil has an abnormal resistance or the contacts are not switching correctly, replacing the relay is usually the most effective solution. It’s often more economical to replace a suspect relay than to spend extensive time troubleshooting other potential, more complex electrical issues.

Final Thoughts

Testing a relay with a multimeter is a fundamental skill for any DIY mechanic or electronics enthusiast. By systematically checking the coil’s resistance and the contacts’ continuity in both energized and de-energized states, you can accurately diagnose relay failures. This process not only confirms if a relay is the source of an electrical problem but also prevents unnecessary component replacements, saving you valuable time and resources.