Relays are essential components in countless electrical and electronic systems—from automobiles and industrial machinery to home appliances. They act as electrically operated switches, controlling high-current circuits with low-power signals. When a relay malfunctions, it can lead to problems such as non-responsive devices, intermittent failures, or complete system breakdowns.
If you’re wondering how to test a relay with a multimeter, you’re in the right place. This comprehensive guide will walk you through every step—from understanding how relays work to using a multimeter to diagnose whether your relay is functional or faulty.
Understanding the Basics: What Is a Relay?
A relay is an electromechanical switch that opens or closes circuits electronically or electromechanically. It consists of two main sections:
- Coil (electromagnet) – which activates the switching mechanism when current passes through.
- Contacts (switching terminals) – which open or close depending on the coil’s state.
When voltage is applied to the coil, it creates a magnetic field that either pulls or releases the internal contacts. This action allows or interrupts current flow through another part of the circuit.
In short, the relay allows a small electrical signal to control a much larger one—making it indispensable in automation and control systems.
Why Test a Relay?
Over time, relays can wear out due to electrical arcing, dirt buildup, corrosion, or coil failure. A faulty relay might cause:
- Lights or motors not turning on
- Electrical components working intermittently
- Clicking sounds but no operation
- Burnt smell or discolored casing
Testing a relay helps confirm whether it’s the cause of the malfunction or if another component is at fault. Fortunately, all you need is a digital multimeter and a few minutes of patience.
Tools You’ll Need
Before diving into the testing process, gather these simple tools:
- A digital multimeter (with continuity and resistance modes)
- The relay you want to test (removed from the circuit if possible)
- A small power source (optional, for coil testing under load)
- Protective gloves (recommended for safety)
Step 1: Identify the Relay Terminals
Every relay has multiple pins, usually between four and five. The exact pin configuration depends on the type of relay, but most follow these general connections:
- 85 and 86 → Coil terminals (used to energize the relay)
- 30 → Common terminal (main input/output)
- 87 → Normally Open (NO) terminal
- 87a → Normally Closed (NC) terminal (in 5-pin relays)
Most relays have a small schematic printed on the side showing the pin configuration. If not, you can look up the model number online to confirm which pin corresponds to which terminal.
Step 2: Check the Coil Resistance
Set your multimeter to the ohms (Ω) setting. Place the probes on the coil terminals (85 and 86).
Interpretation:
- Normal Reading: Most relay coils show a resistance between 50–1200 ohms, depending on the type and voltage rating.
- Zero or Very Low Resistance: Indicates a shorted coil, meaning the internal winding has fused together.
- Infinite or “OL” Reading: Means the coil is open—there’s a break in the winding, so it won’t energize.
If your coil shows either extreme reading, the relay is faulty and should be replaced.
Step 3: Test for Normally Closed (NC) and Normally Open (NO) Contacts
Next, switch your multimeter to the continuity mode (often symbolized by a sound wave icon).
- Place one probe on terminal 30 (common) and the other on terminal 87a (NC).
- You should hear a beep or see a near-zero reading—indicating that this path is normally closed.
- Now move the second probe to terminal 87 (NO).
- You should not hear a beep—this path is open when the relay is not energized.
This confirms that the internal switching contacts are behaving as expected in their default state.
Step 4: Energize the Relay Coil
To test whether the relay switches properly, you need to energize the coil.
- Connect terminal 85 to the negative (-) side of a DC power source (matching the relay’s rated coil voltage, usually 12V or 24V).
- Connect terminal 86 to the positive (+) side.
You should hear a distinct “click” sound—indicating that the relay’s internal switch has moved from the NC to the NO position.
Step 5: Test the Switching Function
While the coil is energized, use your multimeter again in continuity mode.
- Probe between terminals 30 and 87a → The continuity should disappear (no beep).
- Probe between terminals 30 and 87 → You should hear a beep (continuity established).
If you don’t observe this behavior, the relay’s internal contacts are likely damaged or stuck—time to replace it.
Step 6: Check for Mechanical and Burn Marks
Visual inspection is equally important. After testing, examine the relay’s body for:
- Burnt or melted spots
- Rust or corrosion on the terminals
- Sticky or sluggish clicking when energized
Even if electrical readings appear fine, physical wear can signal an impending failure.
Step 7: Confirm Results and Reinstall
If the relay passes all tests—correct resistance, proper switching, and clean mechanical action—it’s good to go. Reinstall it in the circuit, ensuring firm connections.
If it fails any of these tests, it’s best to replace the relay with one of the same voltage and current ratings to prevent damage to other components.
Common Mistakes to Avoid
- Testing in-circuit: Always remove the relay before testing to avoid false readings.
- Ignoring polarity: Some relays are polarity-sensitive; connecting the coil backward may damage it.
- Over-voltage testing: Never energize the coil with voltage higher than its rating.
- Skipping continuity check: Even if a relay clicks, it might not be making electrical contact properly.
The Role of Multimeters in Modern Diagnostics
A multimeter is more than just a measuring device—it’s your window into the unseen electrical world. With it, you can diagnose faults, verify connections, and ensure systems run smoothly. Learning how to test a relay with a multimeter empowers you to tackle electrical problems confidently, saving time and money.
Final Thoughts: Mastering Electrical Diagnostics
Understanding how to test a relay with a multimeter isn’t just a practical skill—it’s an essential foundation for anyone working with electronics or automotive systems. Relays are silent workhorses that keep our devices running reliably. Yet, when they fail, the symptoms can be frustratingly subtle.
By mastering this simple diagnostic method, you gain control over uncertainty. You can identify faults quickly, ensure safety, and maintain the longevity of your equipment.
In an increasingly electrified world—where automation, vehicles, and smart devices depend on precision—skills like relay testing bridge the gap between guesswork and expertise. So next time your circuit behaves unpredictably, grab your multimeter. A few minutes of testing might reveal more than hours of trial and error ever could.