Analysis of Relay Coil and Contact Faults: A Beginner's Guide

Key points

Have you ever turned your car key and heard nothing? Or discovered a home appliance suddenly stopped working? Often, a small, cheap part called a relay is the problem.

Think of a relay as a remote switch. A tiny electrical signal controls a much larger, more powerful circuit. This lets a small computer chip operate high-current devices like your car's starter or refrigerator compressor.

Two main parts tell this story: the coil and the contacts. The coil acts as the "brain" receiving small signals. The contacts serve as the "muscle" doing the heavy work of switching the main circuit.

This guide will teach you exactly why relays fail. You'll learn how to check if the relay is working properly. This knowledge will help you diagnose and fix problems yourself.

Why Relays Fail

Understanding analysis of relay coil and contact faults starts with knowing each part's job. The coil and contacts fail most often. Each has specific weaknesses. Let's see why they break down.

The Heart: The Relay Coil

The relay coil is basically a tiny electromagnet. Very thin copper wire wraps around an iron core hundreds or thousands of times.

Small voltage applied to this coil creates a magnetic field. This field pulls a small lever called an armature. The armature then operates the switch contacts.

Overheating and Burnout

Coil burnout from overheating happens most often. Higher voltage than the coil's rating makes it draw too much current. This creates excessive heat and melts the thin wire's protective coating.

Constant use also builds heat over time. This is especially true if the relay isn't made for continuous operation. You might smell burnt plastic or varnish. This odor clearly indicates a fried coil. The smell means the insulation has failed completely.

Open Circuit in Windings

An open circuit means the coil wire physically broke. This creates a gap in the electrical path. No current can flow.

Physical vibration over thousands of cycles can cause this break. Manufacturing weak points or internal rust also contribute. When the coil opens, it can't create a magnetic field. The relay does nothing.

Shorted Windings

Shorted windings happen when insulation between wire loops breaks down. Overheating or voltage spikes cause this.

When wires touch, they create a shortcut. This reduces the coil's total length and resistance. The coil then draws massive current. This can blow the control circuit fuse or damage sensitive electronics meant to activate the relay.

The Workhorse: The Relay Contacts

Relay contacts are physical switch points. They open and close to control electricity flow in the main, high-power circuit.

These contacts use special metal alloy. The material resists wear and conducts electricity well. They're the relay's working end and take lots of abuse.

Pitting from Electrical Arcing

This is the most serious and common contact failure. Every time contacts open or close under load, a tiny electrical spark jumps across the gap.

This arc resembles miniature lightning and gets incredibly hot. Over thousands of operations, these arcs blast away tiny bits of contact material. This creates pits and craters on the surface.

Relay contacts have specific cycle ratings, like 100,000 operations at rated load. Exceeding this or switching high-current DC loads drastically shortens life. More severe and longer arcing causes this.

Welding of Contacts

Contact welding happens when massive current surges flow through contacts. This occurs when starting large motors or powering circuits with big capacitors.

Current surge can be so intense it momentarily melts contact surfaces. If they're closed then, they can weld together. The relay sticks in the "ON" position. The controlled device won't turn off.

Carbon and Oxide Buildup

Arcing creates fine carbon deposits on contact surfaces over time. In other environments, humidity and air pollutants form oxide layers.

Carbon or oxide layers act as insulators. They prevent clean, low-resistance connections when contacts close. This causes intermittent connections or complete failure to conduct electricity. The relay still "clicks" and seems to work.

The Practical Test

Now comes the most important part: the practical, step-by-step guide. This shows how to perform complete analysis of relay coil and contact faults using basic tools. This process will help you confidently determine whether a relay is good or bad.

Safety and Tools First

Safety must be your top priority before any electrical testing. Always follow these simple rules.

Disconnect all power from the circuit you're working on.

Carefully remove the relay from its socket or board.

Wear safety glasses, especially when working with batteries.

You only need a few basic tools for this procedure.

A Digital Multimeter (DMM).

A suitable power source matching the relay's coil voltage (like a 9V battery, 12V car battery, or variable benchtop power supply).

Jumper wires, preferably with alligator clips, to make connections easier.

Step 1: Visual Inspection

The first test is always simple visual and sensory inspection. Experience shows many failed relays reveal themselves before you pick up any tool.

Look closely at the relay's plastic case. A swollen, cracked, or melted case often clearly indicates the relay severely overheated. This likely happened due to a shorted coil or overloaded contacts.

Check the pins or terminals. Are they clean and shiny, or corroded and blackened? Poor pin connections can cause heat buildup and failure.

Finally, smell the relay. That distinct burnt electronic odor strongly suggests the internal coil burned out.

Step 2: Understand the Pinout

To test a relay, you must know which pins do what. Most automotive and general-purpose relays use standard numbering called the ISO mini relay pinout.

Pins 85 and 86: These connect to the coil.

Pin 30: This is the "Common" contact, the switch input.

Pin 87: This is the "Normally Open" (NO) contact. It connects to pin 30 only when the coil is energized.

Pin 87a: This is the "Normally Closed" (NC) contact. It connects to pin 30 when the coil is off. (Note: Not all relays have an 87a pin).

Picture a simple diagram. Pins 85 and 86 are on one side, representing the control circuit. Pins 30, 87, and 87a are on the other, representing the switched circuit.

Step 3: Testing the Coil

This test checks the coil's internal wiring health. It tells you if the coil is open, shorted, or healthy.

First, set your multimeter to Resistance mode. This is shown by the Ohms (Ω) symbol.

Connect the multimeter probes to the two coil pins, 85 and 86. Polarity doesn't matter for this test.

Now observe your multimeter reading. Three outcomes are possible.

Good Coil: A healthy relay coil shows resistance reading, typically between 50 and 200 Ohms. The exact value depends on the relay but should be a stable, non-zero number.

Bad Coil (Open): If the multimeter reads "OL" (Over Limit), infinity (∞), or similar out-of-range indication, the coil wire is broken. The coil is open and the relay is bad.

Bad Coil (Shorted): If the multimeter reads very low resistance, like near-zero Ohms, the internal windings shorted together. The coil is shorted and the relay is bad.

Step 4: Testing the Contacts

This two-part test verifies the physical switch mechanism works correctly. It's often called the "click test" because you'll listen for the relay's operation.

Part A: Check Normally Closed

First, set your multimeter to Continuity mode. This is often shown by a sound wave or diode symbol. In this mode, the meter beeps when probes touch, indicating a closed circuit.

If your relay has a fifth pin (87a), connect multimeter probes to the Common pin (30) and Normally Closed pin (87a).

With no power applied to the coil, the multimeter should beep immediately. This confirms the normally closed connection is intact. If it doesn't beep, the internal switch is faulty.

Part B: The Click Test

This is the definitive test. It checks if the coil's magnetic field is strong enough to move the switch and if the normally open contacts can make good connection.

Keep your multimeter probes connected to Common pin (30) and Normally Open pin (87). The meter should be silent, as this connection is open by default.

Now carefully connect your power source (like a 9V or 12V battery) to the coil pins (85 and 86).

Listen carefully. As soon as you apply power, you should hear a crisp, distinct "click" from the relay. This is the internal switch moving.

Look at your multimeter. The moment the relay clicks, the meter should start beeping. This confirms the switch closed the circuit between pins 30 and 87.

To be thorough, disconnect power from the coil. You should hear a faint "clack" as the switch returns to rest, and the multimeter should stop beeping. If the relay clicks and continuity changes exactly as described, the relay is most likely functional.

A Deeper Fault Analysis

A simple pass/fail test is good. But true analysis of relay coil and contact faults requires connecting symptoms to root causes. Understanding this relationship moves you from parts-tester to genuine troubleshooter.

Coil vs. Contact Failures

The following chart organizes key differences between the two main failure categories. Use it as a quick reference during diagnosis.

This table clearly shows how a symptom like "relay clicks but nothing happens" points directly to contact fault, not coil fault. Conversely, a completely dead relay that won't click is almost certainly a coil issue or problem with the signal feeding the coil.

The "Hidden" Fault

There's a tricky failure mode that can fool basic continuity tests. This is high contact resistance.

Here's the scenario: The relay clicks perfectly. Your multimeter beeps, showing continuity across contacts. You reinstall the relay, but the high-power device it controls, like a fuel pump or large fan motor, still doesn't work correctly or seems weak.

This happens because contacts are severely pitted or carbonized. They can still pass tiny, millivolt current from your multimeter, enough to make it beep. However, they can't handle high amperage required by the actual device. The pitted surface creates high resistance that chokes current flow and generates lots of heat.

You can often diagnose this hidden fault by feeling the relay after it's been running for a few minutes. If the relay gets very hot to touch, it's a strong sign that contacts are failing under load, even if they passed simple continuity tests. This is an advanced diagnostic tip from real-world experience.

Proactive Care for Relays

Diagnosing failed relays is valuable. But preventing failure in the first place is even better. Proper application and circuit design can dramatically extend any relay's life. Following these best practices will help you avoid premature analysis of relay coil and contact faults.

Best Practices for Long Life

Use the Correct Voltage. Always match the relay's specified coil voltage to the control circuit's voltage. Under-voltage may prevent the relay from closing firmly, causing contact chatter and arcing. Over-voltage will quickly burn out the coil.

Respect the Current Rating. The contact current rating is a hard limit, not a suggestion. A 10-amp relay used in a 20-amp circuit will fail very quickly due to severe contact erosion and overheating.

Add a Flyback Diode. When switching inductive loads like motors or solenoids, the collapsing magnetic field can create large voltage spikes. A simple "flyback" diode placed across coil terminals safely dissipates this spike. This protects both the relay and sensitive electronics controlling it. This is a key expert tip for reliability.

Protect from the Elements. In harsh environments, such as under car hoods or in industrial settings, use sealed or shrouded relays. This prevents moisture, dust, and corrosive fumes from contaminating contacts and causing failure.

Ensure Secure Connections. Loose crimp terminals or poorly soldered joints commonly cause problems. They create resistance, which generates heat that can travel up the pin and damage the relay's internals. Always ensure connections are tight and secure.

Putting It All Together

Let's walk through a real-world example to see how all this knowledge comes together. This case study will synthesize the entire process, from symptom to solution.

Case Study: Radiator Fan

The Symptom

A car owner notices their engine temperature climbing while sitting in traffic. Quick check reveals the electric radiator fan isn't spinning, even though the engine is hot. The fan circuit fuse is inspected and found good. Suspicion immediately falls on the fan relay.

The Diagnosis

The technician locates and removes the fan relay from the under-hood fuse box. Visual inspection shows no obvious cracks or melting signs.

First is the coil test. The multimeter is set to Ohms and connected to pins 85 and 86. It reads steady 90Ω. Conclusion: the coil is good.

Next is the contact test. The multimeter is set to continuity. Probes are placed on common pin (30) and normally open pin (87). As expected, there's no beep.

Finally, the click test is performed. 12V power from a portable jump pack is carefully applied to coil pins 85 and 86. A clear, sharp "click" is heard from the relay. However, the multimeter connected to pins 30 and 87 still shows no beep.

The Conclusion

The diagnosis is clear. The relay coil works, as proven by correct resistance reading and audible click. However, internal power contacts have failed. They're so severely pitted, burned, or broken that they no longer make electrical connection when the switch activates.

The root cause is failed contacts, likely due to many years and thousands of cycles switching the high-current fan motor on and off. The solution is simple: replace the relay with a new one of the same rating.

You Are a Troubleshooter

Congratulations. You've just walked through the entire process of professional relay diagnostics. By understanding core principles, you're no longer just guessing.

Let's recap the three key skills you've learned.

First, you understand why the coil and contacts are primary weak points in a relay. You know the specific ways they fail.

Second, you have a complete step-by-step process for performing thorough analysis of relay coil and contact faults.

Third, and most importantly, you've mastered how to check if the relay is working properly using nothing more than a multimeter and power source.

You now possess the knowledge and confidence to tackle these common electrical problems yourself.