If you’re having electrical issues with your car, a faulty relay might be the culprit. Learning how do i test a car relay is a straightforward process that can save you time and money. The basic approach involves listening for an audible click and using a multimeter to verify continuity across its switch pins.
This guide will walk you through everything you need to know, from identifying the relay to interpreting your test results. You don’t need to be a master mechanic to perform these checks.
With a few simple tools and some patience, you can diagnose the problem confidently.
How Do I Test A Car Relay
A car relay is an electronically operated switch. It uses a small electrical current from a switch, like your ignition key or a control module, to activate an electromagnet. This magnet then closes a much larger electrical circuit that powers a high-current device, such as the fuel pump, headlights, or radiator fan.
By acting as a remote switch, relays protect delicate control switches from high amperage and allow shorter wiring runs for the main power circuits. When a relay fails, the component it controls will stop working, even if the component itself is fine.
Common Symptoms Of A Bad Car Relay
Before you start testing, it’s helpful to know the signs that point to a relay problem. These symptoms often mimic a failed component, so testing the relay first is a smart move.
- The component (like headlights or fuel pump) does not work at all.
- An intermittent operation, where the device works sometimes and not others.
- You hear a rapid clicking sound from the relay area when trying to activate the system.
- The component works but seems weak or underpowered, which could indicate burnt contacts inside the relay.
- A burning smell coming from the relay box or fuse panel.
Tools You Will Need For Testing
Gathering the right tools before you start makes the job easier and safer. You likely have most of these items already.
- A digital multimeter (DMM) with continuity and resistance (ohms) settings.
- A set of jumper wires or a small piece of wire with alligator clips.
- A 12-volt power source, such as your car’s battery (with the car off) or a separate 12V battery.
- The vehicle’s service manual or a wiring diagram to identify relay pins (helpful but not always essential).
- Needle-nose pliers for gently removing the relay from its socket.
Locating And Identifying The Faulty Relay
Your first task is to find the correct relay. Most modern cars have multiple relay boxes.
- Check your owner’s manual for a diagram of the fuse and relay boxes. Common locations include under the hood in the engine bay, under the dashboard near the driver’s knees, or in the trunk.
- Once you locate the box, the cover usually has a diagram showing which relay controls which circuit. If the cover is missing, you may need to consult a manual or look for labels on the box itself.
- Relays are often standardized cubes with 4 or 5 pins. They can be the same color or different colors based on their amperage rating.
Understanding Relay Pin Diagrams
Most standard automotive relays have a diagram on the side or top of the plastic housing. This is your key to testing. The pins are typically numbered.
- Pins 85 and 86: These are the coil pins. This is the low-current circuit that activates the electromagnet. These pins are not polarity-sensitive, meaning you can connect positive to either one for testing.
- Pins 30 and 87: These are the switch pins. Pin 30 is usually the common connection to the power source (battery). Pin 87 is the normally open (NO) contact that sends power to the device when the relay is activated.
- Pin 87a: Present on some 5-pin relays, this is a normally closed (NC) contact. It is connected to pin 30 when the relay is *off* and disconnected when the relay is *on*.
Preliminary Test The Audible Click Method
The quickest and easiest test requires no tools, just your ears. This checks if the relay’s internal electromagnet coil is functional.
- Locate the relay you want to test and gently remove it from its socket.
- Shake the relay near your ear. If you hear anything rattling inside, the relay is physically damaged and needs replacement.
- Hold the relay in your hand. You will need to apply 12 volts to the coil pins (85 and 86). You can use two jumper wires connected to your car battery (with the car off).
- Briefly touch the wires to pins 85 and 86. You should hear and feel a distinct “click” as the relay activates. Reverse the wires and touch them again; you should hear another click as it deactivates.
If you hear a click, the coil is good. If there is no click, the coil is open and the relay is bad. However, a click does not guarantee the switch contacts are good, which is why further testing with a multimeter is crucial.
Step By Step Multimeter Testing Procedures
For a definitive diagnosis, you need to test both the coil and the switch contacts with a multimeter. These tests are performed with the relay removed from the vehicle.
Testing The Relay Coil For Resistance
This test measures the health of the small wire coil inside the relay that creates the electromagnet.
- Set your multimeter to the resistance (ohms, Ω) setting.
- Touch the multimeter probes to pins 85 and 86. The order does not matter.
- Read the resistance value on the display. A typical automotive relay coil will have a resistance between 50 and 120 ohms. Consult your service manual for the exact specification if possible.
- Interpret the results:
- A reading within the expected range means the coil is electrically sound.
- A reading of “OL” (Over Limit) or infinity means the coil is broken (open circuit) and the relay is faulty.
- A reading of 0 ohms or very close to it means the coil is shorted and the relay is faulty.
Testing The Switch Contacts For Continuity
This test checks the main high-current circuit that powers your car’s accessory. You will test it in both the “off” and “on” states.
Testing Normally Open Contacts (Pins 30 and 87)
- Set your multimeter to the continuity setting (usually indicated by a diode or sound wave symbol).
- With the relay not activated (coil not powered), touch the probes to pins 30 and 87. The multimeter should show no continuity (no beep). This is correct because the contacts are open.
- Now, apply 12 volts to the coil pins (85 and 86) to activate the relay. You can use jumper wires from a battery or a specialized relay tester.
- While power is applied to the coil, touch the multimeter probes to pins 30 and 87 again. You should now have continuity (a beep). This indicates the switch has closed properly.
If you have continuity when the relay is off, the contacts are fused shut. If you have no continuity when the relay is on, the contacts are burned or damaged. In either case, the relay needs to be replaced.
Testing Normally Closed Contacts (Pins 30 and 87a)
Only perform this test if your relay has a fifth pin, 87a.
- Set your multimeter to continuity.
- With the relay deactivated, test between pins 30 and 87a. You should have continuity (beep), as these pins are connected when the relay is off.
- Apply 12V to the coil pins (85 and 86) to activate the relay.
- Test between pins 30 and 87a again. The continuity should now be gone (no beep), as the connection should open when the relay is energized.
Testing The Relay In Circuit
Sometimes you may want to test the relay while it is still plugged into its socket in the car. This tests the entire circuit, including the wiring and control signal.
- Locate the relay in its socket. Do not remove it.
- Set your multimeter to DC voltage (20V range is fine).
- Turn the car’s ignition to the “on” position, or activate the system the relay controls (e.g., turn on headlights).
- Back-probe the socket terminals for pins 85 and 86 with the multimeter probes. You should read 12 volts (or very close to it) when the control switch is activated. This confirms the vehicle’s computer or switch is sending the signal.
- Next, check for power at pin 30. It should have constant battery voltage (12V) at all times.
- Finally, with the system activated, check for power at pin 87. If pin 30 has power and the coil is receiving its signal, pin 87 should also show 12 volts. If it doesn’t, the relay is likely faulty.
Be very careful not to short terminals together with your probe during this test, as it could blow a fuse.
Interpreting Test Results And Next Steps
Once you’ve completed your tests, you need to decide what the results mean. Here is a simple guide.
- Relay clicks during the audible test and all multimeter continuity tests pass: The relay is functioning correctly. Your problem lies elsewhere, such as in the component itself, a fuse, or wiring.
- Relay does not click, and coil resistance is OL (open): The relay coil is broken. Replace the relay.
- Relay clicks, but switch contacts show no continuity when activated: The relay’s internal contacts are burned or damaged. Replace the relay.
- Relay shows continuity across switch pins when it should be open: The contacts are fused together. Replace the relay.
- All relay tests pass, but it still doesn’t work in the car: The issue is in the vehicle’s wiring, a bad ground connection, a blown fuse, or a faulty control switch. Further diagnosis of the circuit is needed.
How To Choose A Replacement Relay
When buying a new relay, it’s important to get the correct one. Using the wrong relay can cause electrical failures or even fires.
- Take the old relay with you to the parts store to match it physically.
- Match the pin configuration (4-pin vs. 5-pin).
- Match the amperage rating, which is usually printed on the relay (e.g., 30A, 40A).
- Match the part number if it is visible. This is the most reliable method.
- Consider purchasing an OEM (Original Equipment Manufacturer) relay from a dealership for critical systems like fuel or engine management, as the quality is often higher.
Frequently Asked Questions
Can You Test A Relay Without A Multimeter?
You can perform the basic audible click test without a multimeter, but it only checks the coil. To fully verify the relay’s switching function, a multimeter is necessary. A simple test light can partially substitute by checking for power at the relay socket pins, but it cannot check for proper continuity or resistance.
What Does It Mean If A Relay Is Clicking Rapidly?
A rapid clicking sound from a relay usually indicates a problem with the circuit it is controlling, not necessarily the relay itself. It often means the relay is trying to engage but the component (like a fuel pump or starter motor) is drawing too much current or has a bad connection, causing the relay to cycle on and off rapidly. The relay itself may be fine, but the root cause should be investigated.
How Can I Tell If A Relay Is Bad Or If Its Just The Fuse?
Always check the fuse first, as it’s easier. Locate the fuse for the circuit that’s not working and visually inspect it or test it for continuity. If the fuse is good, then proceed to test the relay. A blown fuse can sometimes be caused by a failing relay with internally shorted contacts, so if you replace a fuse and it blows again, the relay is a prime suspect.
Is It Safe To Bypass A Relay To Test A Component?
Bypassing a relay by jumping power directly from pin 30 to pin 87 in the socket can be a useful diagnostic step to see if a component (like a fan or pump) works. However, you must be certain of the wiring and use appropriate gauge wire with an in-line fuse for safety. Never bypass a relay for systems critical to safety or engine operation, like fuel pumps, for extended periods. It should only be a momentary test.
Why Do Car Relays Fail?
Relays are mechanical devices with moving parts and electrical contacts, so they eventually wear out. Common causes of failure include arcing and pitting of the contacts from repeated high-current switching, heat damage from being located near hot engine components, vibration causing internal connections to loosen, and moisture corrosion on the pins or inside the housing. Using a component that draws more current than the relay is rated for will also lead to premature failure.