Can You Test A Relay With A Multimeter? | Check It

Yes, you can absolutely test a car relay with a multimeter, and it’s a fundamental skill for diagnosing common electrical issues.

Every car has a hidden network of electrical components working tirelessly. When something stops working, like a horn that won’t honk or headlights that stay dim, a small, often overlooked component called a relay might be the culprit.

Understanding how to check these little electrical workhorses can save you time and money. We’ll walk through the process together, just like we’re in the garage figuring things out.

Understanding Your Car’s Relays: The Electrical Traffic Cops

Think of a relay as an electrically operated switch. It uses a small electrical current to control a much larger current.

This is important because high-current components, like your starter motor or fuel pump, would quickly burn out a standard dashboard switch.

The relay acts as a buffer, taking a low-current signal from your dashboard switch and using it to activate a heavy-duty switch inside the relay itself.

Relays are found throughout your vehicle’s electrical system, protecting sensitive control modules and switches.

They ensure power gets where it needs to go efficiently and safely. A faulty relay can cause a component to fail completely, work intermittently, or even stay on when it shouldn’t.

Common Automotive Relay Applications:

• Fuel Pump
• Headlights and Fog Lights
• Horn
• Starter Motor
• Cooling Fans
• Power Windows and Door Locks
• Heated Seats

Can You Test A Relay With A Multimeter? Absolutely, Here’s How

Testing a relay with a multimeter is a straightforward process that requires a basic understanding of its internal workings. You’ll be checking both the control side (the coil) and the switched side (the contacts).

Before you begin, always disconnect your vehicle’s battery to prevent accidental shorts or damage. Safety is always the priority when working with electrical systems.

Most automotive relays are either “Normally Open” (NO) or “Normally Closed” (NC).

A Normally Open relay’s contacts are open (no current flows) until the coil is energized. A Normally Closed relay’s contacts are closed (current flows) until the coil is energized.

Getting Started: The Basic Setup

You’ll need a few simple tools for this diagnostic task.

A reliable multimeter is essential, along with some jumper wires and a 12-volt power source, like your car battery or a dedicated power supply.

Most automotive relays have four or five pins. These pins are usually numbered, typically 85, 86, 30, and 87 (and sometimes 87a).

• Pin 85 & 86: These are the coil terminals. Power across these energizes the relay.
• Pin 30: This is the common power input terminal for the switch.
• Pin 87: This is the normally open (NO) output terminal. Power flows here when the relay is energized.
• Pin 87a: This is the normally closed (NC) output terminal. Power flows here when the relay is de-energized.

The Resistance Test (Coil Check)

The first step is to check the relay’s coil. This coil creates an electromagnetic field that pulls the switch contacts together when energized.

Set your multimeter to the Ohms (Ω) setting, typically in the 200Ω range. Connect the multimeter leads to pins 85 and 86 of the relay.

You should get a resistance reading, usually between 50 and 100 Ohms. The exact value can vary by relay type, so a quick check of the relay’s specifications or a known good relay is helpful.

If your multimeter shows “OL” (Open Line) or “1” (indicating an open circuit), the coil is broken. If the reading is very low, like close to 0 Ohms, the coil might be shorted internally.

Either of these readings indicates a bad relay. A good coil will show a stable resistance value.

Table 1: Relay Coil Resistance Check

Multimeter Reading	Interpretation
50-100 Ohms (stable)	Coil is likely good
“OL” or “1”	Coil is open (bad)
Near 0 Ohms	Coil is shorted (bad)

The Continuity Test (Switch Check)

Next, we’ll check the relay’s internal switch contacts. Set your multimeter to the continuity setting (it usually beeps) or a low Ohms range (e.g., 200Ω).

With the relay de-energized (no power applied to pins 85 and 86), connect your multimeter leads to pin 30 and pin 87.

For a Normally Open relay, you should see an “OL” or “1” reading, indicating an open circuit. This is correct; the switch should be open when the relay is off.

If your relay has a pin 87a (Normally Closed), connect your multimeter to pin 30 and pin 87a. You should get a continuity beep or a very low Ohm reading, indicating a closed circuit.

The Activation Test (Functionality Check)

This is the definitive test to see if the relay works as intended. You’ll need your 12V power source and jumper wires.

Carefully connect a jumper wire from the 12V positive (+) terminal to pin 85 of the relay. Connect another jumper wire from the 12V negative (-) terminal (ground) to pin 86 of the relay.

When you connect the second wire, you should hear a distinct “click” sound from the relay. This click confirms the coil is energizing and pulling the contacts together.

While the relay is energized (and clicking), reconnect your multimeter to pin 30 and pin 87. For a Normally Open relay, you should now hear a continuity beep or get a very low Ohm reading.

This indicates the switch has closed, allowing current to flow. If you have a Normally Closed relay (with pin 87a), you should lose continuity between 30 and 87a when energized.

If the relay clicks but you don’t get the expected continuity change, the internal contacts are faulty. If it doesn’t click, the coil is likely bad, or there’s no power reaching it.

Table 2: Relay Activation Test Results (Normally Open Relay)

Action	Expected Result (Click)	Expected Continuity (30 & 87)
No Power to Coil	No	“OL” or “1” (Open)
Power to Coil	Yes	Beep or Low Ohms (Closed)

Common Relay Problems and What They Mean

A relay can fail in several ways, each with distinct symptoms. Understanding these helps in diagnosing the problem quickly.

If a relay doesn’t click when powered, the coil circuit is open or shorted. This means the magnetic field cannot form to pull the contacts together.

If it clicks but doesn’t change continuity, the internal switch contacts are likely worn, pitted, or stuck. This prevents the electrical path from completing or breaking.

Sometimes, relays fail intermittently. This can be caused by a weak coil that struggles to pull the contacts firmly, or by corroded or dirty contacts that make poor connections.

Symptoms of a bad relay often include a component not working at all, working only sometimes, or staying on continuously when it should turn off.

Beyond the Multimeter: Other Diagnostic Tips

While a multimeter is your best friend for relay testing, other methods can help confirm a diagnosis or rule out related issues.

If you suspect a relay and have an identical, non-critical relay in your vehicle (like for the horn if your problem is the fuel pump), you can try swapping them. If the problem moves with the relay, you’ve found your culprit.

Always inspect the relay visually. Look for any signs of burning, melting, or corrosion on the casing or pins. These are clear indicators of internal damage or overheating.

Remember that a relay is part of a circuit. Always check the fuse associated with the relay’s circuit. A blown fuse will prevent the relay from receiving power or sending power to the component.

Also, inspect the wiring harness leading to the relay socket. Frayed wires, loose connections, or corrosion in the socket itself can mimic a bad relay.

Keeping Your Electrical System Healthy: A Pro’s Advice

Preventative maintenance helps keep your vehicle’s electrical system running smoothly. Periodically check relay terminals for corrosion and clean them if necessary.

Avoid adding aftermarket accessories that draw excessive current without proper wiring and fusing. Overloading circuits can strain relays and other components.

Always replace a faulty relay with one of the correct amperage rating and type. Using an incorrect relay can lead to premature failure or even electrical fires.

Familiarize yourself with your vehicle’s electrical diagram. This diagram shows you which relays control which components and how they’re wired.

Understanding these diagrams is a powerful tool for diagnosing complex electrical issues. It provides a clear roadmap of your car’s electrical system.

Proper electrical maintenance ensures your car’s systems operate reliably. A well-maintained electrical system contributes to overall vehicle safety and performance.

Can You Test A Relay With A Multimeter? — FAQs

What are the common signs of a bad car relay?

Common signs include a component not working at all, like a horn that’s silent or headlights that don’t turn on. You might also experience intermittent operation, where a system works sometimes and then fails. A constant click from the relay without the component activating can also point to an issue.

Can a bad relay drain my car battery?

Yes, a bad relay can potentially drain your car battery. If a relay’s contacts get stuck in the closed position, it can continuously send power to a component, even when the car is off. This parasitic drain will slowly deplete your battery over time, leaving you with a dead battery.

Is it safe to swap relays for testing purposes?

Swapping relays for testing is generally safe, provided you swap it with an identical relay from a non-critical system. For example, you could swap a horn relay with a fuel pump relay if they are the same type. Always ensure the amperage rating and pin configuration match exactly to avoid damage.

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

If your relay clicks but the controlled component doesn’t activate, it usually means the relay’s internal switch contacts are faulty. The coil is energizing and pulling the contacts, but the contacts themselves are either corroded, pitted, or physically broken. This prevents the electrical current from flowing through to the component.

How long do automotive relays typically last?

Automotive relays are designed for durability and can often last for many years, sometimes for the entire lifespan of the vehicle. Their lifespan depends on factors like operating conditions, frequency of use, and the current load they handle. Relays in high-demand circuits or those exposed to heat can fail sooner.