We will discuss how to test a relay in this article.
Like other vehicle electrical components, relays can fail over time due to wear and tear.
And since these electromechanical switches control critical parts like headlights and starter motors, their failure needs immediate attention.
You can replace the device immediately or test it to determine the failure source.
We recommend relay testing because it gives an accurate diagnosis of the situation.
Plus, you can test this electromagnetic switch as a preventive measure to ensure everything performs optimally. And here’s how to do it.
Table of Contents
Types of Relays By Switch Pins
Relays come in different types, but the most typical ones are 4-pin and 5-pin relays. 4-pin relays have the following pins.
- 30 → From the fused battery positive terminal
- 85 → Relay coil negative terminal (to ground wire)
- 86 → Relay coil positive terminal
- 87 → To power the load
5-pin relays split the load connection (terminal 87) to NO (normally open) and NC (normally closed) contacts.
This wiring designates NO as terminal 87 and NC as terminal 87a.
NC is usually unused because it remains powered until you energize the armature switch.
And pin 30 becomes the common (COM) terminal.
A 5-pin relay switch
Causes of Relay Failure?
Automotive relay failure usually occurs on the contacts due to the high power supply voltage switching.
So faults, like cracked or welded contacts, can occur due to overheating and wear.
How To Test a Relay
You will need the following tools for the testing process.
- Digital Multimeter
- Jumper wires
- Replacement relay (if the existing one is faulty)
- A power source (9-volt or 12-volt battery)
- Vehicle manual
Let’s focus on the 5-pin relay, although the tests are similar to those of the 4-pin type.
A car mechanic testing relays and fuses in the fuse box
Step 1: Find the Relay
Depending on its function, the relay can be in the fuse box, under the dash, or in the engine bay.
The best and easiest way to locate it is in the vehicle manual or wiring diagram.
Step 2: Relay Testing
We don’t recommend swapping the relay immediately because there might be an underlying electrical issue.
So instead of running the risk of frying the new relay, test the old one first to determine if damaged or functional.
Only replace after testing and concluding the old one is faulty.
Conduct these four tests on the relay contacts.
Relay Coil Windings Test
The simplest way to test this component is by connecting the relay coil terminals to a 12V battery (you can use a 9V battery). It should click.
A clicking sound implies the coil is functioning normally.
It happens when the electromagnet attracts the armature, which taps terminal 87 to connect the current flow to the load.
A lack of this sound means a stuck armature, while faint clicks imply internal coil issues.
But this test is inconclusive because the problem might be somewhere else in the relay.
So use a digital multimeter to diagnose the issue further. And you can use this device to do resistance or continuity tests.
Coil windings in a relay
With the red probe in the VΩ port and black lead in the COM slot, rotate the center knob to set the device to resistance testing in the 200Ω setting.
Connect one probe to terminal 85 and the other to terminal 86, then check the readings (polarity does not matter).
The electromagnetic coil should have zero or low resistance. So any reading that indicates a high resistance means the coil is faulty.
Ideally, this control coil resistance should range between 50Ω and 200Ω, depending on the relay.
Values outside this range are causes of concern.
Alternatively, you can test these coil pins for continuity, a setting that checks for a continuous electrical path between the two points.
Adjust the center knob to continuity mode, then attach the probes to the terminals.
An audible beep implies continuity. This test is convenient because it lets you focus on the testing process, not looking at the display.
Normally-Open Terminal Testing
The normally-open terminal remains disconnected from the 12V power source when there’s no power on the coil pins.
So when testing this part of the relay, the resistance should be infinitely high.
Begin the process by setting the multimeter knob to resistance mode (200Ω), then connect the probes to the COM pin (30) and normally-open pin (87). Polarity does not matter.
A 5-pin electromagnetic relay switch with NO and NC terminals is shown on the printed wiring diagram
Check the readings on the multimeter display, which should indicate a high resistance or OL (infinite resistance).
This reading means the terminals have no leakages.
But if there is a low resistance, there is some continuity between the contacts, which can drain your battery if left for a long time.
For instance, if the relay controls the vehicle’s headlights, such continuity can make them glow a little throughout the night, draining the battery.
Normally-Closed Terminal Testing
Although rarely used, you can still test the normally-closed terminal to ensure the armature contact sits firmly on it when you haven’t energized the wire coil.
Since this NC connects to the COM terminal, the resistance between the two should be low or zero.
Use the same procedure for normally-open testing to measure the resistance between the two points. If the resistance is high, the relay is faulty.
A technician testing a relay using a multimeter
Energized Relay Testing
If the relay passes the tests in its off state, It is crucial to test it when energized to conclude its status.
Power the coil using switch pins 85 and 86, then use a multimeter in continuity mode to test the NO terminal.
Immediately after you connect the power source, you should hear a clicking sound to indicate successful switching.
If you don’t hear this sound, attach the multimeter probes to terminals 30 and 87, then do a continuity test (listen for a beep sound). Not beeping means the armature is stuck open.
And even if it beeps, don’t stop there. Test the voltage across these terminals to ensure the relay transmits the required energy.
Set the multimeter to DC voltage (20V), then measure the voltage across the COM (30) and NO (87) terminals.
A test probe connected to an electric relay terminal
The voltage reading on the multimeter should match the power source voltage. So if using a 12-volt power source, the voltage on terminal 87 should be 12V.
Step 3: Replace the Relay
If the tests indicate any signs of failure, fit in a new relay with the same power ratings as the old one.
But if the relay is okay and the end device is not working, check the electrical lines in and out of the switch.
Step 4: Test the Relay
The final step is to test the relay by activating the switch that energizes the control coil.
Car fuses and relays in a fuse box
The high-power circuit will resume normal operations if zero underlying electrical issues exist.
But if they are present, they will likely fry the new relay as they cooked the old unit.
So, troubleshoot the problem further if the new relay gets damaged after a short time in use.
Wrap Up
Covering all aspects of relay testing is relatively easy because the processes only require resistance or continuity measuring.
And we also recommend doing a voltage supply test to ensure the relay outputs what it receives with zero or minimal resistance.
That’s it for now. Comment below to let us know if your vehicle’s relays passed all tests.
