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By Olivia Parker•Last Updated April 18, 2025

Testing Automotive Relays: Complete Guide Using a Multimeter

Understand automotive relays

Automotive relays are small electrical devices that control high current circuits use low current signals. They function as electrically operate switches, allow a low current circuit to control a high current circuit safely. Relays are crucial components in modern vehicles, control everything from headlights and fuel pumps to cool fans and starter motors.

When a relay fails, it can cause various electrical problems in your vehicle. Symptoms of a faulty relay include components that don’t turn on, intermittent operation, or complete system failure. Test relays with a multimeter is a straightforward diagnostic technique that can save you time and money.

Types of automotive relays

Before testing, it’s important to understand the different types of relays you might encounter:

Standard 4 pin relays

These common relays have four terminals and are use for basic on / off functions. They include two pins for the control circuit (coil )and two for the load circuit ( (itch path ).)

Standard 5 pin relays

These relays have an additional terminal that provide usually close functionality, allow for more complex switching operations. The fifth pin create an alternative circuit path when the relay is not energize.

ISO micro relays

Smaller than standard relays, these are normally use in modern vehicles where space is limited. Despite their size, they function likewise to standard relays.

Solid state relays

These have no move parts and use semiconductor switch elements alternatively of mechanical contacts. They’re more durable but require different testing methods than conventional relays.

Tools require for testing

To efficaciously will test automotive relays, you will need:

Digital multimeter with continuity and resistance testing capabilities
Jumper wires (optional for some tests )
Basic hand tools to access and remove relays
Relay pin diagram or vehicle service manual
Safety glasses and gloves for protection

Identify relay terminals

Before testing, you must identify the terminals on your relay. Most automotive relays follow standard pin configurations:

For 4 pin relays:

Pins 85 and 86: control circuit (coil )
Pin 30: common terminal (power input )
Pin 87: usually open contact (connect to 30 when relay is eenergize)

For 5 pin relays:

Pins 85 and 86: control circuit (coil )
Pin 30: common terminal (power input )
Pin 87: usually open contact (connect to 30 when relay is eenergize)
Pin 87a: usually close contact (connect to 30 when relay is not energize )

Most relays have a diagram on their housing show the pin configuration. If not, consult your vehicle’s service manual or look up the relay part number online.

Remove the relay for testing

To right will test a relay, you’ll typically will need to will remove it from the vehicle:

Locate the relay in your vehicle’s fuse box or relay panel. Most vehicles have relaid diagrams on the fuse box cover.
Turn off the ignition and disconnect the battery’s negative terminal for safety.
Softly pull the relay directly upwardly from its socket. Some relays may have a locking tab that need to be press before removal.
Inspect the relay visually for signs of damage, such as melt plastic, discoloration, or corrosion.

Test relay coil resistance

The first test checks if the relay’s electromagnetic coil is function decent:

Set your multimeter to the ohms (Ï‰ )setting, typically in the 200 ohm range.
Connect the multimeter probe to pins 85 and 86 (the coil terminals )
Read the resistance value. A typical automotive relay coil should measure between 50 and 120 ohms, but check your specific relay’s specifications.
If you get an infinite reading (oofor overload ) the coil is open a, andhe relay is defective.
If the resistance is importantly lower than specifications, the coil may be short and the relay should be replaced.

Test relay continuity

Following, test the switching function of the relay:

For normally open contacts (pins 30 and 87 )

Set your multimeter to the continuity setting (ordinarily indicate by a sound wave symbol )or resistance setting.
Connect one probe to pin 30 and the other to pin 87.
With the relay not energize, there should be no continuity (infinite resistance )
Today, apply 12v to pins 85 and 86 to energize the relay (you can use a 9v battery in a pinch, though it may not full engage the relay )
With the relay energize, there should be continuity between pins 30 and 87 (near zero resistance )

For normally closed contacts (pins 30 and 87a on 5 pin relays )

Connect one probe to pin 30 and the other to pin 87a.
With the relay not energize, there should be continuity (near zero resistance )
Apply 12v to pins 85 and 86 to energize the relay.
With the relay energize, there should be no continuity between pins 30 and 87a (infinite resistance )

Test with the diode test function

Many digital multimeters have a diode test function that can be used to hear the relay click:

Set your multimeter to the diode test function (unremarkably indicate by a diode symbol )
Connect the positive (red )probe to pin 86 and the negative ( (ack ) )obe to pin 85.
The meter should provide enough current to energize the relay coil moment.
Listen for an audible click, which indicate the relay’s mechanical components are work.
If you don’t hear a click, the relay may be defective.

Voltage drop test for installed relays

You can besides test a relay without remove it from the vehicle by perform a voltage drop test:

Set your multimeter to DC voltage.
With the system power (key in the appropriate position ) measure the voltage between the relay’s input terminal ( ( ) )d ground.
You should read battery voltage (around 12 14v )
Following, measure the voltage between the output terminal (87 )and ground while the relay should bebe activated
If the relay is work decent, you should read near the same voltage as at terminal 30.
A significant voltage drop between terminals 30 and 87 when the relay is energized indicate high resistance in the contacts, suggest a fail relay.

Test relay control circuits

If the relay test good but the component inactive doesn’t work, the problem might be in the control circuit:

With the relay install and the key in the appropriate position to activate the system, set your multimeter toDCc voltage.
Measure the voltage between pin 85 and ground, or between pins 85 and 86 (depend on how the relay is wire )
When the system should be activated, you should read roughly 12v.
If you don’t get voltage at the control pins when the system should be activated, the problem is in the control circuit, not the relay.

Common relay failure modes

Understand how relays typically fail can help diagnose problems:

Open coil

The electromagnetic coil inside the relay can break, prevent the relay from activate. This will show as infinite resistance between pins 85 and 86.

Shorted coil

The coil’s insulation can break down, cause a short circuit. This will show as abnormally low resistance between pins 85 and 86.

Burnt contacts

The switch contacts can become pit or burn from arc, peculiarly if they’ve been switched high currents. This cause high resistance at the contacts, which cabe detectedct in the continuity test.

Mechanical failure

The spring mechanism can weaken, or the armature can become stuck, prevent proper operation. You might hear unusual sounds during the click test or notice inconsistent continuity readings.

Interpreting test results

After conduct these tests, you can diagnose your relay’s condition:

Good relay:

Proper coil resistance, clear switch action between appropriate pins, and an audible click when energized.
Bad coil:

Infinite or importantly incorrect resistance read across pins 85 and 86.
Bad contacts:

No continuity or high resistance between pins 30 and 87 when energize (for usually open contacts )or between pins 30 and 87a when not energize ( (r usually close contacts ).)
Intermittent problems:

If all tests pass but the component motionless functions unpredictably, try to tap the relay lightly while test to check for intermittent connections.

Replace a faulty relay

If you’ve determined that a relay is faulty, replacement is straightforward:

Purchase an exact replacement relay with the same part number or a compatible universal relay with the same pin configuration and specifications.
Ensure the vehicle’s ignition is remove, andd the battery is disconnect if necessary.
Orient the new relay right accord to the socket diagram and insert it hard until it seat altogether.
Reconnect the battery and test the system to confirm proper operation.

Advanced relay testing techniques

Load testing

For more thorough testing, you can perform a load test:

Connect a test light or small bulb between pin 87 and ground.
Apply 12v to pin 30.
Energize the relay by apply 12v to pins 85 and 86.
The test light should illuminate, indicate the relay can handle a load.

Temperature testing

Some relays fail sole when hot:

Use a hair dryer to warm the relay to operate temperature.
Perform the standard tests while the relay is warm.
If the relay passes cold tests but fail when warm, ihasve a temperature relate defect.

Troubleshoot specific relay systems

Fuel pump relay

The fuel pump relay control power to the fuel pump. When testing:

Listen for the fuel pump to run shortly when turn the key to the” on ” osition.
If the pump doesn’t run, test the relay use the methods supra.
Remember that many vehicles have safety shutoff switches that disable the fuel pump after an accident or if the engine isn’t crank.

Cool fan relay

This relay activates the engine cool fans:

Fan operation is temperature dependent, so the engine may need to reach operate temperature before testing.
Some vehicles have multiple fan speeds control by different relays.
Test by check for voltage at the fan motor when the relay should be activated.

Starter relay

This relay provides power to the starter solenoid:

If you hear a click, but the engine doesn’t crank, the problem might be the starter motor, not the relay.
If you hear nothing when turn the key, test the starter relay.
Remember that many vehicles have neutral safety switches that prevent start unless in park or neutral.

Safety precautions when testing relays

Invariably prioritize safety when work with automotive electrical systems:

Disconnect the battery before remove or install relays if possible.
Wear safety glasses to protect against potential sparks.
Ne’er short relay terminals unitedly as this could damage vehicle electronics.
Be careful when probe relay sockets in the vehicle to avoid damage terminals or create short circuits.
If test with the system power, ensure all tools are decent insulate.

Conclusion

Test automotive relays with a multimeter is a valuable diagnostic skill that can save time and money. By understand how relays work and follow these systematic testing procedures, you can accurately diagnose electrical problems in your vehicle and avoid replace components unnecessarily.

Remember that while relay testing is comparatively simple, it’s part of a larger diagnostic process. If replace a faulty relay doesn’t solve the problem, further investigation of the associate circuits and components may be necessary. With practice, you’ll become proficient at rapidly will identify relay issues and keep your vehicle’s electrical systems in top condition.