Testing Automotive Relays: Complete Guide with Multimeter Methods

By Alexander HamiltonLast Updated April 22, 2025

Understand automotive relays

Automotive relays are electromagnetic switches that control high current circuits use low current signals. They’re crucial components in your vehicle’s electrical system, control everything from headlights and fuel pumps to starter motors and cool fans. When these relays malfunction, they can cause frustrating electrical problems that might seem difficult to diagnose.

Fortuitously, with a basic multimeter and some straightforward testing methods, you can promptly determine whether a relay is function decent or need replacement. This guide will walk you through the process of will test automotive relays with a multimeter, will help you’ll solve electrical issues without unnecessary parts replacement.

Why test relays with a multimeter?

Before diving into testing procedures, it’s worth understand why a multimeter is the perfect tool for relay diagnosis:

Cost-effective diagnosis before replace parts
Precise measurements of resistance and continuity
Ability to test relays both in and out of the vehicle
Quick identification of intermittent problems
Verification of proper voltage reach the relay

A fail relay oftentimes look identical to a work one, make visual inspection unreliable. Multimeter testing provide objective data about the relay’s internal condition.

Types of automotive relays

Before testing, identify which type of relay you’re work with:

Standard 4 pin relays

These common relays have four pins or terminals:

Terminal 85: control circuit ground
Terminal 86: control circuit power
Terminal 30: main power input
Terminal 87: output to the control device

5 pin relays

These include an additional terminal (87a )that’s usually close when the relay isn’t enenergized

Terminal 85: control circuit ground
Terminal 86: control circuit power
Terminal 30: main power input
Terminal 87: usually open contact (close when relay is eenergize)
Terminal 87a: usually close contact (open when relay is eenergize)

ISO micro relays

Smaller but functionally similar to standard relays, these are become more common in modern vehicles.

Tools you will need

To decent test automotive relays, gather these items:

Digital multimeter with continuity and resistance functions
Small alligator clips or jumper wires (optional but helpful )
9v or 12v battery for bench testing
Service manual for your specific vehicle (recommend )
Small screwdriver or relay puller tool
Safety glasses

Safety precautions

Before test any electrical components:

Disconnect the vehicle’s battery when remove relays
Work in a wellspring ventilate area
Avoid test relays in potentially flammable environments
Wear safety glasses to protect against accidental spark
Ne’er probe a relay while it’s control a critical system (like airbags )

Locate the relay

Automotive relays are typically find in two locations:

Fuse box

Most relays are house in the main fuse box, which might be located:

Under the hood near the battery
Inside the cabin, oftentimes under the dashboard
In a secondary fuse box (some vehicles have multiple )

Near the component they control

Some relays are mount closing to the devices they operate, such as:

Near the starter motor
Adjacent to cool fans
Near to the fuel pump (oft under rear seats )

Your vehicle’s service manual will provide exact relay locations and identification. The fuse box cover typically includes a diagram show relay positions and functions.

Test a relay while it’s install

In some cases, you can test a relay without remove it from the vehicle:

Listen test

The simplest check is to listen for the relay’s activation click:

Turn the ignition key to the” on ” osition ( (n’t start the engine )
)
Activate the system control by the relay (turn on headlights, ac, etc. )
Listen cautiously for a distinct clicking sound from the relay

A click sound indicate the relay’s coil is energized, but doesn’t confirm the contacts are work decent.

Voltage drop test

To check if the relay is pass current:

Set your multimeter to DC voltage
Connect the black probe to a good ground point
Touch the red probe to the output terminal (normally 87 )
Activate the system control by the relay
You should read battery voltage (12 14v )if the relay is work

Remove the relay for testing

For thorough testing, remove the relay from its socket:

Locate the relay use your vehicle’s manual or fuse box diagram
Disconnect the vehicle’s battery for safety
Grip the relay hard and pull straightaway upwardly, or use a relay puller tool
Note the orientation of the relay for proper reinstallation

Many relays have the terminal numbers mold into their plastic housing or print on top, which help with identification during testing.

Multimeter testing methods

Coil resistance test

This test checks if the relay’s electromagnetic coil is intact:

Set your multimeter to the ohms (Ï‰ )function
Touch the probes to terminals 85 and 86 (the control circuit terminals )
For most automotive relays, you should read between 50 120 ohms
If you read infinite resistance (oof) the coil is open and the relay is defective
If you read rattling low resistance (near 0 ) the coil may be short

Continuity test for normally open contacts

This check the main switching function:

Set your multimeter to the continuity function (it may have a sound icon )
Touch the probes to terminals 30 and 87
With the relay de energized, you should have no continuity (no beep )
Apply 12v to terminal 86 and grind to terminal 85 to energize the relay
Today test terminals 30 and 87 again you should have continuity (beep )

Continuity test for normally closed contacts (5 pin relays )

For 5 pin relays with commonly close contacts:

Set your multimeter to continuity mode
Touch the probes to terminals 30 and 87a
With the relay de energized, you should have continuity (beep )
Apply 12v to terminal 86 and grind to terminal 85
Test terminal 30 and 87a again you should straightaway have no continuity

Bench testing with a battery

For a more dynamic test that simulate actual operation:

Connect a 9v or 12v battery positive terminal to relay terminal 86
Connect the battery negative terminal to relay terminal 85
Set your multimeter to continuity mode
Test between terminals 30 and 87 you should hear a click and have continuity
Remove one battery connection and test again continuity should be lost

This test confirm both the coil operation and the contact switching function.

Interpreting test results

Here’s how to understand what your multimeter readings indicate:

Signs of a good relay

Coil resistance within 50 120 ohms range
Distinct click when the relay is energized
No continuity between 30 and 87 when DE energized
Good continuity between 30 and 87 when energize
For 5 pin relays: continuity between 30 and 87a when DE energized

Signs of a faulty relay

Infinite resistance across the coil (open coil )
Rattling low coil resistance (short coil )
No clicking sound when energize
Continuity between 30 and 87 when DE energized (stuck contacts )
No continuity between 30 and 87 when energize (fail contacts )
Intermittent continuity readings (unreliable operation )

Test for intermittent relay problems

Some relay issues sole appear when components heat up. For intermittent problems:

Perform standard tests with the relay at room temperature
If the relay pass, try heat it slender ( (e a hair dryer at a distance )
)
Test again while warm to see if problems appear
Tap or vibrate the relay lightly while test to check for loose internal connections

Intermittent failures are oftentimes heat relate and may not show up in standard testing.

Common relay problems and their symptoms

Failed coil

Symptoms include:

Component ne’er turn on
No clicking sound from relay
Infinite resistance read across terminals 85 and 86

Burn or pit contacts

Symptoms include:

Intermittent operation
Higher than normal resistance across contacts
Relay clicks but component work periodically

Welded contacts

Symptoms include:

Component stay on eventide when it should be cancelled
Continuity between terminals 30 and 87 tied when relay is DE energized
No change in continuity when relay is energized

Moisture or corrosion damage

Symptoms include:

Visible corrosion on terminals
Inconsistent readings
Problems that worsen in humid conditions

Replace a faulty relay

If your tests confirm a bad relay:

Purchase an exact replacement match the original part number
Ensure the new relay have the same pin configuration and current rating
Disconnect the vehicle’s battery before installation
Insert the new relay in the correct orientation
Reconnect the battery and test the system

Keep the old relay for comparison if you need to purchase a replacement at an auto parts store.

Preventive maintenance for relays

To avoid relay failures:

Keep relay boxes clean and dry
Ensure proper voltage in the vehicle’s electrical system
Check for loose connections that could cause voltage spikes
Consider carry spare relays for critical systems during long trips
Test suspect relay during regular maintenance intervals

Troubleshoot beyond the relay

If replace the relay doesn’t solve the problem, investigate:

Blown fuses relate to the circuit
Damage wiring to or from the relay
Faulty switches that control the relay
Problems with the component the relay control
Improper ground connections

Sometimes what appear to be a relay problem is really an issue elsewhere in the circuit.

Advanced relay testing techniques

Load testing

For a more thorough test of high current relays:

Connect a test light or small bulb between terminals 30 and 87
Energize the relay by apply 12v to terminal 86 and grind to 85
The test light should illuminate at full brightness
Dim illumination suggest high resistance in the contacts

Voltage drop testing

To check for efficiency loss across the relay:

Install the relay in the vehicle
Set your multimeter to DC volts
Measure voltage at terminal 30, so at terminal 87 while the relay is energized
The difference should be minimal (less than 0.2v )
A larger voltage drop indicate resistance in the contacts

Conclusion

Test automotive relays with a multimeter is a straightforward diagnostic skill that can save you time and money. By understand how relays function and apply these testing methods, you can promptly determine whether a relay is at fault when electrical components malfunction.

Remember that relay testing is precisely one part of electrical troubleshooting. A systematic approach that will include will check fuses, wiring, and the components themselves will lead to more successful repairs. With practice, you’ll be able to will diagnose relay problems confidently and keep your vehicle’s electrical systems will function decently.