Electrical issues like a non-starting engine or a failed accessory can often be traced to a faulty relay. Learning how to check relays on a car is a fundamental skill that can save you time and money. This guide will walk you through the entire process, from locating them to testing with basic tools.
Relays are simple switches operated by electricity. They allow a small current from a switch, like your ignition key, to control a much larger current needed for a component like the fuel pump or starter motor. When a relay fails, the system it controls will stop working, even if the main component is fine.
You do not need to be an expert mechanic to perform these checks. With a basic understanding and a few tools, you can diagnose many common car problems yourself. Let’s get started by understanding what you’re looking for.
How To Check Relays On A Car
Before you start testing, you need to know what a relay is and where to find it. A typical automotive relay is a small, cube-shaped plastic device with four or five metal pins or blades on the bottom. It will be plugged into a socket, which is connected to the car’s wiring harness.
Common locations for relay boxes include under the hood in the engine bay, often near the battery or fender walls, and inside the passenger compartment, under the dashboard or behind kick panels. Your vehicle’s owner manual is the best resource for finding the specific layout of your relay boxes.
Once you locate the general box, you’ll need to identify the specific relay. The cover of the relay box or a diagram on the inside of the lid should have a label, such as “Fuel Pump,” “A/C,” “Radiator Fan,” or “Starter.” If the diagram is missing or faded, you may need to consult a repair manual for your car’s make and model.
Tools You Will Need For Testing
You can perform effective relay checks with a minimal set of tools. Here is what you should gather before you begin:
- A basic digital multimeter (DMM). This is the most versatile tool for the job.
- A test light. A simple and inexpensive alternative for some checks.
- A set of jumper wires with alligator clips.
- A fully charged 12-volt battery (your car’s battery is perfect, but a spare can be useful for bench testing).
- Your car’s owner’s manual for relay identification.
With these tools in hand, you are ready to begin the diagnostic process. Always start with the simplest check first.
Preliminary Visual And Auditory Checks
Before removing any parts, perform these quick and easy inspections. They can often point you directly to the problem.
First, listen for the relay. Have a helper turn the key or activate the switch for the faulty component while you listen near the relay box. A working relay will usually produce a soft but audible “click” when it energizes and de-energizes. No click often indicates a problem with the control circuit or the relay itself.
Next, perform a visual inspection. Carefully remove the suspect relay from its socket. Look for signs of damage:
- Melted, cracked, or discolored plastic casing.
- Corroded or burnt metal pins.
- A burnt smell coming from the relay.
If you see any of these signs, the relay is likely faulty and should be replaced. If it looks fine, you need to proceed with electrical testing.
Understanding The Relay Pin Diagram
To test a relay properly, you must understand its terminal layout. Most standard automotive relays have four or five pins. The pins are usually labeled on the relay casing with small numbers or a diagram.
Here is the standard configuration for a common 5-pin relay:
- Terminals 85 and 86: This is the coil circuit. This is where the switch (like your ignition) sends a small control signal to activate the relay.
- Terminal 30: This is the common connection to the high-current power source (usually directly from the battery via a fuse).
- Terminal 87: This is the normally open (NO) contact. When the relay is activated, it connects terminal 30 to terminal 87, sending power to the component (like the fuel pump).
- Terminal 87a: This is the normally closed (NC) contact, only present on 5-pin “changeover” relays. When the relay is at rest, terminal 30 is connected to 87a. It breaks this connection when activated.
Four-pin relays simply omit the 87a terminal. Knowing this layout is crucial for the next steps.
Method One: The Swap Test
The fastest and easiest way to check a relay is by swapping it with an identical one. In your relay box, find another relay with the same part number. Often, relays for non-critical accessories like the horn or sunroof are the same as those for critical systems.
- Turn the car completely off.
- Pull out the relay you suspect is faulty.
- Pull out a known, identical relay from a different circuit.
- Insert the known good relay into the socket of the faulty circuit.
- Test the system. If it now works, you have confirmed the original relay was bad.
This test is highly reliable and requires no tools. Just be sure the relays are the same and you don’t disrupt a system you need for safety, like headlights, while driving.
Method Two: Testing The Relay Coil With A Multimeter
This test checks the electromagnet coil inside the relay. A damaged coil will prevent the relay from switching at all. Set your multimeter to the resistance (Ohms Ω) setting.
- Identify the coil terminals on the relay (usually 85 and 86).
- Place the multimeter probes on these two terminals.
- Read the resistance value. A typical automotive relay coil will show a resistance between 50 and 120 ohms. Consult your relay’s specifications if possible.
- If you read an extremely high resistance (OL or “open loop” on the meter) or zero resistance, the coil is faulty. A reading within the expected range means the coil is electrically sound.
Method Three: Bench Testing The Relay With A Battery
This is the definitive test to see if the relay’s internal switch works. You will need your 12-volt battery and two jumper wires.
- Connect one jumper wire from the battery’s positive (+) terminal to relay pin 86.
- Connect a second jumper wire from the battery’s negative (-) terminal to relay pin 85. You should hear and feel a distinct “click” as the relay activates.
- Now, use your multimeter set to continuity (the setting that beeps) or a low Ohms setting.
- With the relay energized (power still connected to 85/86), test between terminals 30 and 87. You should have continuity (a beep or very low resistance reading).
- If testing a 5-pin relay, also check that continuity between terminals 30 and 87a is now broken.
If the relay clicks but does not show continuity between 30 and 87, the internal switch contacts are worn out or burnt. The relay needs to be replaced. If it doesn’t click at all but the coil tested good, the internal mechanism may be jammed.
Method Four: Testing The Relay In The Vehicle
Sometimes you need to test the relay while it’s plugged into its socket to check the car’s wiring. For this, you’ll use your multimeter set to DC Volts.
Testing Control Circuit (Pins 85 & 86):
- Reinsert the relay into its socket.
- With the multimeter’s black probe on a good ground (bare metal), back-probe the wire at pin 85 or 86 (consult a diagram for which one is switched).
- Have your helper activate the switch (turn the key to “Run,” etc.). You should see battery voltage (around 12V) appear when the switch is on and drop to 0V when off. This confirms the control signal is reaching the relay.
Testing The Load Circuit (Pins 30 & 87):
- Check for constant power at pin 30. Back-probe it with the red probe, black probe to ground. You should see constant battery voltage at all times. If not, there’s a blown fuse or wiring problem upstream.
- To check if power is switching through, back-probe pin 87. Activate the switch. You should see battery voltage at pin 87 only when the switch is commanded on.
These in-circuit tests help determine if the problem is the relay itself or the car’s wiring supplying it.
How To Use A Test Light For Relay Diagnosis
A test light is a great tool if you don’t have a multimeter. It’s a simple probe with a grounded wire and a light inside. It lights up when voltage is present.
To check for power at pin 30, connect the test light’s ground clip to a good ground. Probe the pin 30 terminal in the socket (with the relay removed). The light should illuminate, showing constant power is present.
To check the control signal, probe the control circuit socket terminal (85 or 86) while an helper activates the switch. The test light should illuminate when the switch is on. This confirms the switch and wiring to the relay are working.
Remember, a test light shows voltage presence but not quality. A weak connection might light a test light dimly but not provide enough current to operate the relay properly, which is where a multimeter is superior.
Common Symptoms Of A Bad Car Relay
How do you know when to start checking relays? Look for these common signs:
- A component that doesn’t work at all (fuel pump, radiator fan, A/C compressor).
- Intermittent operation of an accessory.
- A clicking sound coming from the relay box but no action from the component.
- Related systems that fail together (e.g., turn signals and hazard lights often share a relay).
- A car that cranks but does not start (a classic symptom of a failed fuel pump or main relay).
If you experience any of these, the relay controlling that system is a prime suspect.
Safety Precautions And Best Practices
Working with car electrical systems requires caution. Always follow these safety guidelines:
- Disconnect the negative battery terminal before removing or installing relays if you are unsure of the circuit. This prevents accidental shorts.
- Use the correct replacement relay. The amperage rating (e.g., 20A, 30A) must meet or exceed the original specification.
- Do not bypass a relay by jumpering power directly to a component. This can overload wiring and create a fire hazard.
- Investigate why a relay failed. A relay that repeatedly burns out may indicate a problem with the component it controls, like a motor drawing too much current.
- Keep the relay box covers sealed to prevent water and dirt ingress, which are common causes of relay failure.
FAQ About Checking Car Relays
What Does A Relay Do In A Car?
A relay acts as a remote-controlled switch. It uses a low-current signal from a dashboard switch or computer to safely control a high-current circuit for components like headlights, the fuel pump, or the starter solenoid. This protects delicate switches from handling high amperage.
Can A Relay Work Intermittently?
Yes. This is a common failure mode. Internal contacts can become pitted or corroded, making an inconsistent connection. Heat from high resistance can also cause the relay to work only when cool and fail when the engine bay heats up. An intermittant fault can be very frustrating to diagnose.
How Much Does It Cost To Replace A Car Relay?
Most standard automotive relays are very inexpensive, typically ranging from $10 to $25 for the part itself. The main cost is usually the diagnosis if you take it to a shop. Replacing it yourself often only costs the price of the relay, which is why learning to check them is so valuable.
Is It Safe To Drive With A Bad Relay?
It depends entirely on which relay has failed. A bad horn or power window relay is an inconvenience. A failed fuel pump or engine control relay will cause your car to stall and not restart. A faulty cooling fan relay can lead to engine overheating. It is best to adress a suspected relay problem as soon as possible.
What Is The Difference Between A Fuse And A Relay?
A fuse is a safety device designed to protect a circuit by melting and breaking the connection if current exceeds a safe level. A relay is a control device designed to switch a circuit on and off. They often work together: the fuse protects the wiring to the relay, and the relay switches the power to the component.
Learning how to check relays on a car empowers you to fix many electrical gremlins quickly. By starting with a visual inspection and the simple swap test, then moving to multimeter checks, you can systematically isolate the problem. Always remember to prioritize safety and use the correct replacement parts. With this knowledge, you can confidently tackle one of the most common, yet often overlooked, causes of automotive electrical failure.