How To Test a Relay: A Step-by-Step Guide for Automotive Enthusiasts

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Relays are small but extremely important components in a car’s electrical system.
They control high-current devices like fuel pumps, radiators fans, headlights, AC compressors, horn circuits, ignition systems, and more.

If a relay fails, the entire system it controls can stop working — leaving you confused about whether it’s a wiring issue, sensor issue, or component failure.

Good news: Testing a relay is simple, and any automotive enthusiast can do it at home with basic tools.

This guide explains how automotive relays work, how to identify the terminals, and step-by-step methods to test them safely.

1. What Is an Automotive Relay?

A relay is an electrically controlled switch.
It uses a small current to control a much larger current.

A standard 4-pin or 5-pin relay includes:

• Coil terminals → energize the relay

• Switch terminals → connect or disconnect power

• Armature → internal movable contact

• Housing → protective outer case

When the coil receives voltage (usually 12V), it creates a magnetic field, pulling the switch closed and allowing high current to flow.

2. Types of Automotive Relays

1️⃣ 4-Pin Relay (Most Common)

• 85 = Coil negative

• 86 = Coil positive

• 30 = Power input

• 87 = Power output

2️⃣ 5-Pin Relay

Includes an additional terminal 87a, used for normally-closed circuits.

3️⃣ Mini & Micro Relays

Used in modern compact fuse boxes.

3. Symptoms of a Bad Relay

⚠ Fuel pump not working
⚠ Radiator fan not turning on
⚠ AC compressor failing intermittently
⚠ Car cranks but won't start
⚠ Headlights or horn randomly not working
⚠ Clicking noises from fuse box

Most electrical “mystery issues” trace back to faulty relays or poor connectors.

4. Tools Needed to Test a Relay

✔ Multimeter (continuity + voltage mode)
✔ 12V battery or jump pack
✔ Jumper wires
✔ Needle probes (optional)

5. How To Test an Automotive Relay (Step-by-Step)

Method 1: Listen for the “Click” Test (Quick Check)

1. Remove the relay from the fuse box.

2. Connect 12V to pins 85 and 86.

3. You should hear a distinct click.

✔ Click = Coil is working

❌ No click = Coil is dead (relay is faulty)

This is the simplest initial test.

Method 2: Continuity Test for the Coil

1. Set multimeter to ohms (Ω).

2. Measure between pins 85 and 86.

3. Normal coil resistance = 40–200 ohms (varies by relay).

❌ Infinite resistance → coil broken

❌ 0 ohms → coil shorted

Either condition means the relay must be replaced.

Method 3: Continuity Test for Switching Circuit

1. With NO power applied:

   • Test continuity between 30 and 87

   • Should show NO continuity (open circuit)

2. Apply 12V across pins 85 and 86.

3. Now test continuity between 30 and 87 again.

✔ Should now show continuity

= Relay is switching properly.

If not → the internal contact is burnt or stuck.

Method 4: Testing a 5-Pin Relay (30–87a)

For a 5-pin relay:

• 30 ↔ 87a → Should show continuity when relay is NOT energized

• 30 ↔ 87 → Should show continuity ONLY when relay IS energized

If 87a remains open or stuck → relay is faulty.

6. Common Causes of Relay Failure

Relays often fail due to:

• Overheating caused by high current

• Corroded or loose pins

• Poor-quality connectors

• Moisture entering the fuse box

• Rapid cycling (AC relays)

• Damaged wiring

Many electrical faults result from poor connectors, not the relay itself.
This is why workshops often use heat-resistant connectors from WirestoneAuto.com.

7. Tips to Prevent Relay Failure

✔ Keep fuse box clean & dry

Moisture corrodes relay pins.

✔ Use proper relays with correct amperage

Underrated relays burn quickly.

✔ Inspect wiring and connectors regularly

Loose connections cause overheating.

✔ Replace old relays every 5–8 years

Cheap insurance against failure.

✔ Avoid mixing pin layouts

Always match your relay type.

8. Should You Replace or Repair a Relay?

✔ Replace if:

• Coil is dead

• No clicking sound

• Contacts burnt

• Housing melted

• Relay overheats

✔ Repair only if:

• Pins are dirty

• Slight corrosion (cleanable)

Relays are inexpensive — replacement is the safest solution.

Final Thoughts

Testing a relay is one of the easiest DIY electrical diagnostics in a car.
With a multimeter and a 12V power source, you can quickly determine whether a relay is good, weak, or completely dead.

Quick Summary:

✔ Identify relay pins (85, 86, 30, 87, 87a)
✔ Check for clicking sound
✔ Test coil resistance
✔ Test switching continuity
✔ Inspect pins, wiring, and connectors

A healthy relay ensures your car’s electrical components run reliably for years.