Electrical gremlins in your car, from dead windows to silent radios, are frequently traced back to a single, small component called a relay. Learning how to test relay in car is a fundamental skill that can save you time, money, and a lot of frustration. This guide will walk you through the entire process, from identifying a suspect relay to confirming its failure with simple tools.
Relays are essentially remote-controlled switches. They use a small electrical signal from, say, your headlight switch, to activate an electromagnet. That magnet then pulls a contact closed, allowing a much larger current to flow to the headlights themselves. This protects delicate switches and controls high-power circuits safely. When a relay fails, the circuit it controls simply stops working.
Testing a relay is straightforward and requires minimal investment. With a basic understanding and a few common tools, you can diagnose problems that might otherwise lead to an expensive trip to the mechanic.
How To Test Relay In Car
Before you start testing, you need to locate the specific relay and gather the right tools. Most modern cars have several relay boxes. The primary locations are under the hood in the engine bay fuse box and inside the cabin, often under the dashboard or near the driver’s side kick panel. Your owner’s manual will have a diagram labeling each relay’s purpose.
Tools You Will Need For Testing
You do not need a professional garage setup. The following items are sufficient for most tests:
- A basic digital multimeter (DMM).
- A 9-volt or 12-volt battery (a car battery is perfect).
- A set of jumper wires with alligator clips.
- The vehicle’s owner’s manual for relay identification.
- A small pick or your fingers to remove the relay.
Identifying The Faulty Relay
Start by confirming which circuit is not working. Is it the fuel pump, the blower motor, or the horn? Consult your manual’s fuse and relay diagram to find the correct relay. Often, the diagram on the fuse box lid will show this. A quick auditory test is to listen for a click. Have a helper activate the switch (like turning on the headlights) while you listen and feel for a distinct click from the relay. No click can be a clue, but it’s not definitive proof of failure.
Sometimes, a visual inspection can reveal problems. Gently remove the relay from its socket. Look for signs of melting, cracking, corrosion on the pins, or a burnt smell. These are clear indicators the relay needs replacement, but testing will confirm it.
Understanding Relay Terminal Numbers
Most standard automotive relays have four or five pins, and the terminals are marked with standard numbers. Knowing these is crucial for testing:
- 85 and 86: These are the coil terminals. This is the low-current electromagnet circuit. Polarity usually doesn’t matter for a standard relay.
- 30: This is the common contact. It receives power from the battery (often via a fuse).
- 87: The normally open (NO) contact. When the coil is energized, a connection is made between terminal 30 and 87, completing the high-current circuit.
- 87a: (Only on 5-pin relays) The normally closed (NC) contact. It is connected to terminal 30 when the coil is *not* energized, and disconnects when the coil is activated.
Method 1: The Shake And Listen Test
This is a simple preliminary check. A badly worn relay may have internal contacts that are loose or arced. Pick up the relay and shake it gently near your ear. If you hear a faint rattling sound, it often means the internal armature or contacts are broken or detached. This relay has failed mechanically and should be replaced. No rattle does not guarantee it’s good, but a rattle guarantees it’s bad.
Method 2: The Bench Test Using A Power Source
This is the most reliable DIY method to check a relay’s core function. You will simulate the car’s electrical system on your workbench.
- Set Up Your Power Source: Use your car battery or a separate 12V battery. Ensure you have two jumper wires with alligator clips.
- Identify the Coil Pins: Locate terminals 85 and 86 on the relay.
- Energize the Coil: Attach one jumper wire from the battery’s positive (+) terminal to pin 85. Attach the other jumper from the battery’s negative (-) terminal to pin 86. You should hear and feel a solid CLICK as the relay activates. If you don’t hear a click, the coil is faulty and the relay is dead.
- Test the High-Current Switch: With the coil still energized, use your multimeter set to the resistance (Ohms) setting. Place one probe on terminal 30 and the other on terminal 87. You should get a very low resistance reading (close to 0 Ohms), indicating the contacts have closed properly. An infinite (OL) reading means the switch contacts are burned out.
- Test the Resting State: Disconnect power from pins 85/86. The meter between 30 and 87 should now show infinite resistance (OL), indicating the contacts have opened. For a 5-pin relay, test between 30 and 87a; it should show continuity when the coil is *off* and open when the coil is *on*.
Method 3: Testing The Relay In Its Socket
Sometimes you want to test the relay without removing it, or check if power is reaching the socket. This tests both the relay and the car’s wiring.
- Locate the Circuit Diagram: Use your manual to confirm which socket terminals correspond to 85, 86, 30, and 87.
- Test for Control Signal: Set your multimeter to DC Volts. With the car’s ignition in the appropriate position (like “ON”), probe terminals 85 and 86 in the socket while an helper activates the switch (e.g., turns on the A/C). You should see a 12-volt reading appear. This confirms the car’s computer or switch is sending the activation signal.
- Test for Constant Power: Check for constant 12V at terminal 30 in the socket (ignition may need to be in “ON” for some circuits). This is the main power feed.
- Listen and Feel: With the relay plugged in, activate the switch. You should feel the relay click. If you have signal power at the coil pins, constant power at pin 30, but no output at pin 87 when activated, the relay is likely faulty. A final check is to swap in a known-good identical relay from another non-critical circuit (like the horn) to see if the problem is fixed.
Method 4: Using A Multimeter For Coil Resistance
Every relay coil has a specific resistance. You can measure this with a multimeter to assess the coil’s health.
- Set your multimeter to the Ohms (Ω) setting.
- Place the probes on terminals 85 and 86.
- A typical automotive relay coil will show a resistance between 50 and 120 Ohms. Consult a relay datasheet if possible for the exact value.
- A reading of infinite resistance (OL) means the coil is open and broken. A reading of 0 Ohms means the coil is shorted. Both readings indicate a bad relay. A reading within the expected range suggests the coil is electrically sound.
Common Car Relay Symptoms And Locations
Knowing which relay to suspect can halve your diagnostic time. Here are common failures and where to find them.
Fuel Pump Relay Failure
A failed fuel pump relay is a common cause of a no-start condition. The engine may crank but never fire. You won’t hear the characteristic humming sound from the fuel tank when you turn the ignition to “ON” before cranking. This relay is often located in the under-hood fuse box or the interior fuse panel. It’s life is often shortened by a struggling fuel pump that draws to much current.
Blower Motor Relay Problems
If your heater or air conditioner fan does not work on any speed, the blower motor relay is a prime suspect. If it works on only some speeds (like only high), the problem is more likely the resistor pack. This relay is usually found in the under-dash fuse panel.
Headlight Or Horn Relay Issues
When your headlights or horn don’t work, but the fuses are good, the dedicated relay for that system is the next check. These are high-current circuits, so the contacts can burn out over time. The headlight relay is typically in the under-hood fuse box for easy access.
Starter Relay Symptoms
Sometimes called a starter solenoid, this relay carries massive current. Failure results in a single, loud click when you turn the key to “start,” but the engine does not crank. It’s often mounted directly on the starter motor or in the under-hood fuse box.
Safety Precautions When Testing Relays
Working with car electrics is safe if you follow basic rules. Always disconnect the negative battery terminal before removing or installing relays if you are unsure of the circuit. This prevents accidental short circuits. When probing live sockets, be careful not to let the multimeter probes touch two terminals at once and cause a short.
Use the correct type of relay as a replacement. They are rated for specific current loads (e.g., 20A, 30A, 40A). Using an underrated relay can cause it to overheat and fail quickly, or even become a fire hazard. If a new relay fails immediately, you likely have a deeper problem in the circuit, such as a shorted motor or component drawing excessive current.
FAQ: How To Test A Relay In A Car
What Are The Signs Of A Bad Relay In A Car?
The most common signs include a component that stops working entirely (like no headlights), intermittent operation (a horn that works sometimes), a relay that clicks repeatedly without turning on the device, or a component that stays on when it shouldn’t. A audible click doesn’t always mean the relay is good; the high-current contacts inside can still be burned out.
Can You Test A Car Relay Without A Multimeter?
Yes, you can perform a basic test without a multimeter. Use the bench test method with a 12V battery and jumper wires. The click test confirms the coil works. To test the switch contacts without a meter, you could wire the relay to turn on a simple 12V bulb. If the coil clicks and the bulb lights when the relay is energized, the relay is functional. This is a practical, real-world test.
How Do You Check If A Relay Is Stuck?
A stuck relay is one where the contacts have welded together. This results in the component being powered constantly, even when the switch is off. For example, a fuel pump that runs continuously with the key out is a classic sign of a stuck relay. To test, remove the relay and use a multimeter to check for continuity between terminals 30 and 87. If there is continuity when the coil is *not* powered, the relay is stuck closed and needs replacement.
Why Does My Relay Click But Nothing Happens?
A clicking relay means the low-current coil circuit is working. The problem lies in the high-current switch circuit. The most common cause is burned or pitted contacts inside the relay that no longer conduct electricity. The click happens, but no power flows to the device. This is exactly why testing the continuity between terminals 30 and 87 while the coil is energized is such a critical step.
Is It Safe To Bypass A Relay To Test A Circuit?
Bypassing a relay can be a useful diagnostic trick, but it must be done with extreme caution and only momentarily. Using a fused jumper wire, you can connect terminal 30 (power in) directly to terminal 87 (power out) in the relay socket. If the component (like a fuel pump) springs to life, you’ve confirmed the wiring and component are good, pointing to a bad relay. Never leave a circuit bypassed, as it removes all control and protection the relay provides.