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Summary
  • To check if a relay is faulty, you need alligator clips, test leads, and a benchtop power supply or a 12V battery. Relays can be found under the hood inside the fuse box or under the dashboard, depending on the relay you want to find.
  • Checking a relay involves connecting pins 85 and 86 to a battery or power source. As the circuit is completed, the relay should make an audible click.
  • An easy way to check if a relay is bad is to replace a relay with a new unit, if you have one lying around. But be careful: just because a relay looks the same doesn’t mean it’s the right one. If the part works with the  proper replacement relay, then the previous relay is faulty.

Note: Some relays are integral parts of the junction box or relay module and cannot be tested or replaced separately. This article only covers replaceable relays. 

Relays are electronically operated switches that control power flow to a device. Relays allow the vehicle’s computer or the driver to control various vehicle electronics using small electric currents.

For example, the starter solenoid is, in effect, a heavy duty relay that is triggered by a small amount of current through a small switch and wire to close and carry a large amount of current from the battery directly into the starter motor. When the “start signal” is delivered to the solenoid windings, the core of the solenoid moves backward very suddenly, slamming a copper washer against the battery positive post and the starter motor feed post instantaneously.

diagram of a starter solenoid
Diagram showing how a starter solenoid, which is a heavy-duty relay, works. | Image Source: Richard McCuistian

Today’s starter solenoids are activated by a much smaller relay, so in that case, you have a small relay receiving commands from the PCM or ignition switch to energize a much larger relay.

Relays can also be used in pairs to reverse polarity of components like door lock actuators rather than having the actuators directly wired to the door switches.

Relays have an electrical winding with a non-moveable metal core so that when the winding is energized with power and ground, the metal core becomes a very strong magnet and pulls a lever inside the relay so that the contact point on the moveable lever quickly snaps against its non-moveable counterpart, and that set of points connect battery power to the load.

See also  What Is a Relay In a Car?

A faulty relay can prevent you or your vehicle’s computer from controlling devices like the vehicle’s air-conditioning, cooling fan, headlights, fuel pump, and starter motor, among many other components. The PCM and the fuel pump are also powered by relays, and so some relays can prevent the vehicle from even starting at all.

When a certain electronic part malfunctions, the relay associated with it is one of the first things to check. Sometimes, your vehicle might be showing symptoms of a faulty cooling fan or fuel pump, but the issue might just be a malfunctioning relay that corresponds to the faulty part. Hence, checking your vehicle’s relays is important when it comes to diagnosing issues. In this article, we’ll discuss how to test vehicle relays.

Preparing the Right Tools

You’ll need a multimeter, alligator clips, test leads, and a benchtop power supply or a 12V battery. The multimeter allows you to check the continuity between two separate points along an electrical circuit. Meanwhile, the 12V battery is meant to activate the electromagnet inside the vehicle relay, which can either establish or break the electric circuit that leads to the relay’s corresponding component.

Finding the Fuse Box

Relays can be found under the hood inside the fuse box or under the dashboard, depending on the relay you want to find.

You should refer to your vehicle’s owner’s manual because it will tell you where specific relays are located. That said, many vehicle relays like the ignition relay and fuel pump relay are located inside the fuse box, which is typically located under the hood.

Finding the Relay For a Device

Once you’ve found the relay for a specific device that you want to rest, you can pull it out with your fingers. However, gripping the relay can be difficult, as it’s typically wedged between other relays. You might want to use a tool like long-nose pliers to pull it out.

car relay image
Again, keep in mind that some relays are part of a module and cannot be replaced separately, as is the case with this multiple relay module from a 2005 Chrysler Crossfire (see photo). This relay module costs nearly $500, so you probably won’t have one lying around. | Image Source: Richard McCuistian
diagram of a honda relay
Most Hondas have a relay like the PGM-FI relay shown in the second Illustration. This is two relays built into one module and the vehicle won’t start if either of the internal relay circuits fails. | Image Source: Richard McCuistian

Switching To a New Relay

There’s an easy way to determine whether a relay is faulty. If a device like your vehicle’s headlights doesn’t function, simply replace the relay that corresponds to the headlights with one that you know is good and then check if they now work. If they do, then the relay is likely problematic.

Understanding the Relay Prongs

Vehicle relays typically have four or five metal prongs or pins. Relays have numbers that are printed or embossed to help you identify the function of the pins. Pins 85 and 86 are responsible for activating the relay’s electromagnet. When current flows between these two pins, they can either complete or close the circuit that powers the device.

See also  What Is a Relay In a Car?

Meanwhile, pins 87 and 30 are the switch pins. Pin 30 is typically connected to a power source like the vehicle’s battery and pin 87 is connected to the controlled device unless the relay is paired with another relay in a polarity reversal circuit.

In 5-pin relays, a relay could alternate power flow between two controlled devices. 87a would be a normally-closed pin and 87b would be normally open.

image showing various car relays
The photo shows some of the various different kinds of relays available. Usually there will be a schematic of the relay pins and their numbers on the side of the relay. In the case of the older Toyota mechanical relay, the large blades are the current carrying blades and the smaller blades are the relay coil. Terminals on smaller relays may be numbered 1,2,3,4, 5 rather than 30, 85, 86, 87, and 87a. | Image Source: Richard McCuistian
standard iso relay diagram
Diagram showing a standard ISO relay | Image Source: Richard McCuistian
, How to Check If a Relay Is Bad

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: In the polarity reversal setup using two relays, pin 30 on each of the relays will be connected to a separate side of the load, with 87a on both relays grounded and 87 on both relays connected to B+. Energizing just one relay switches one side of the load to power while the other side remains grounded, and vice versa.

Listening for a Clicking Sound

Checking a relay involves connecting pins 85 and 86 to a battery or power source. As the circuit is completed, the relay should make an audible click that indicates that the electromagnet is working and is completing the circuit. If the relay doesn’t make an audible click, it might mean that the relay has stopped working and isn’t completing the circuit.

photo of a sample car relay
Note, however, that just because the relay clicks, you can’t assume that the contacts in the relay are capable of carrying current, because sometimes they’re not even though they’re closed. This relay (see photo) would click, but the contacts were fouled with ants that had made their way into the relay housing through a small hole in the base of the relay. | Image Source: Richard McCuistian

Doing a Resistance Test

You can also test your relay’s functionality by switching your multimeter to test for resistance. This process involves switching your multimeter to read ohms and connecting the two leads to pins 85 and 86.

See also  What Is a Relay In a Car?

A functioning relay coil should have a reading of between 50 and 120 ohms, but usually it’s very near 75 ohms. If it reads a lot higher or is open (“OL” on the DVOM), the coil is faulty and you’ll need a new relay. But again, even if the copper winding is good, the contacts can fail to carry a load.

A functioning relay coil should have a reading of between 50 and 120 ohms, but usually it’s very near 75 ohms. If it reads a lot higher or is open (“OL” on the DVOM), the coil is faulty and you’ll need a new relay. But again, even if the copper winding is good, the contacts can fail to carry a load.

Richard McCuistian, ASE Certified Master Automobile Technician

Testing Switch Pin Continuity

Assuming the control pins are working, you can proceed to test the relay’s switch pins. The switch pins on a Normally Open (NO) circuit should have no continuity when not energized. Use a multimeter on the ohms setting to measure resistance between the switch pins. Any resistance indicates the pins are stuck closed, signaling a faulty relay. That being said, a DVOM doesn’t measure the relay contacts’ ability to carry current.

Once you’ve confirmed the relay’s integrity without power, it’s time to test its functionality when energized. Use a benchtop power supply, a 9-volt battery, or a 12-volt power source to energize the coil pins. This is typically 85 and 86 on a four-pin relay. Listen for an audible click as the electromagnetic coil activates and the internal switch closes. Switch your multimeter to continuity mode, listen for the audible beep, and test for continuity between the switch pins. If there’s no beep, it means the switch pins are stuck open and your relay is faulty.

Testing for Current and Voltage Delivery

illustration of a condenser fan circuit
To make sure the relay contacts will carry a load, you’ll need to test the relay by feeding power to Pin 30, connecting a power window motor or headlamp bulb between 87 (Normally Open) and ground. Then, energize the relay by grounding 85 and powering 86 or vice versa to see if the relay will run the motor or burn the headlamp bulb, which usually pulls about 5 ohms. The condenser fan circuit shown (see Illustration) will pull around 30 amps, as will the A/C blower motor, both of which are energized by relays. | Image Source: Richard McCuistian

It’s crucial to verify that the relay is healthy enough to deliver both voltage and current. But don’t trust a DVOM for this test. Always use a robust load like a motor or a headlamp bulb depending on the circuit the relay is carrying.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : Electrical System , DIY Tagged With :
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