Every vehicle has electrical components that enable efficient engine performance and a seamless driving experience. Some of these electronic and electrical components are regulated or controlled by a relay fuse.
Whenever any electrical system ceases to function, there are chances that you have a blown relay fuse. Relay switches signal the components they’re assigned to work with so they will function properly.
For example, when you press the horn switch, the horn relay receives the input and signals the horn to blow. Common problems that could cause the relay to fail are exposure to water, a resistance problem, or computer failure.
Here, RX mechanic teams have provided a concise guide on how to tell if a relay fuse is blown. But first, what does a bad relay look like? You’ll find out in the subsequent sections.
Understanding Fuse Relays
Before we dive into the identification and troubleshooting process, it’s essential to understand how a fuse relay works. A fuse relay is an electrical device that combines the functions of a fuse and a relay.
It consists of a fuse element, a relay coil, and a set of contacts. The fuse element is designed to melt and break the circuit when excessive current flows, while the relay coil is responsible for switching the contacts on and off.
Types of Fuse Relays
There are two primary types of fuse relays: standard and smart. Standard fuse relays are simple devices that rely on a physical fuse element to break the circuit.
Smart fuse relays, on the other hand, use advanced technology, such as microcontrollers and sensors, to monitor the electrical system and detect potential faults.
How Do Relays Work In A Car?
A car relay works like a switch that’s controlled by electrical power. When you turn on something in your car that requires a lot of current, say your wipers, horn, or lights; the current is passed through the relay where two pints of contacts close to power the accessory.
A typical car relay has two circuits – an energizing circuit that has a coil and a contact circuit that has a lever. To better understand how these work, we will take the example of a vehicle’s horn.
A car’s horn comes with a four-pin relay. Two of the pins sit opposite each other and are connected to the coil.
The other two are connected to the horn with an open contact between them. Each pin will have its own number to make it easy to identify the power source.
When you press the horn button on your steering wheel, current flows through the two pins that are connected to the coil.
This coil then generates an electromagnetic field that attracts a lever found along the contact circuit. The lever closes the point of contact, allowing the current to reach and activate the horn.
How To Tell If A Relay Fuse Is Blown?
There are several ways of checking a blown relay fuse. We’ll examine all of them in the following paragraphs. But first, let’s look at how to tell if a relay fuse is blown without a multimeter.
#1. Visual check.
First off, locate the relay fuse box. This component is usually underneath the dashboard or under the hood. It is the black box next to the car battery.
The relays are housed in the same box as the fuses on some vehicles. In other cars, the manufacturers place them in a separate compartment from the fuses.
Once you locate the relay fuse box, have your assistant turn the ignition to the ON position, and you place your hand on the relay. You should hear and feel a clicking sound. The relay is long gone if it doesn’t make any clicking sound.
Return the ignition key to the OFF position for further examinations and pull off the relay. Connect a small wire on the two relay contact pins. Ensure you figure out the correct pins before connecting the wire.
The pins are usually called C (commons) and NO (normally open). On Bosch Automotive Relay, these pins have 30 and 87 inscriptions.
After connecting the wires, re-plug the relay. Next, turn the ignition to the ON position, and see if the relay will click. If you hear a clicking sound, it’s a confirmation that the relay is bad.
#2. Swapping the relay.
Another way to tell if you have a bad relay is to swap it with another good one. It works; the former relay in that sitting is bad. But if it doesn’t work, you have another issue to deal with. First, examine and clean the connectors. Then, try it again and see if it works.
#3. Using a multimeter.
A more simplified means of testing a relay is using a multimeter. First, get your multimeter and swipe it to ohm settings.
Next, measure the resistance by touching multimeter probes on the relay electromagnet coil pins. The measurement should be around 50 – 120. Any result out of this range shows a malfunctioning electromagnet coil.
#4. Using a car battery.
It’s also essential to learn how to test a relay with a 12v battery. This knowledge will be handy for a service technician and a DIY car owner. It requires a combination of a 12 volts battery and a multimeter.
You can use the 12 volts battery to energize the electromagnet coil pins. You should hear a click from the electromagnet coil as it closes a switch.
Place the positive cable on one of the relay pins. Connect a test light to the ground and the other switch terminal. The test light should illuminate. Disconnect the positive cable, and the light will turn off.
What’s next?
Test the voltage on the relay. Disconnect the test light and set your multimeter to DC. Place the probes on the relay connectors and see if it matches the battery voltage. Test the resistance on the connectors.
Remove the positive jumper wire and return the voltmeter to ohm settings. Test the connectors for resistance. The measurement should be close to zero. So 01, 02 is okay.
Signs and Symptoms of a Blown Fuse Relay
A blown fuse relay can manifest in various ways, depending on the affected circuit and the severity of the fault. Here are some common signs and symptoms to look out for:
- Loss of Power: If a fuse relay blows, it can cause a loss of power to one or more components, such as lights, wipers, or the fuel pump.
- Intermittent Operation: A faulty fuse relay can cause intermittent operation of the affected component, making it difficult to diagnose the issue.
- Clicking or Chattering Sounds: A blown fuse relay can produce clicking or chattering sounds, indicating that the relay is trying to switch on and off.
- Burning Smell: In severe cases, a blown fuse relay can cause a burning smell, indicating that the electrical system is overheating.
What Causes A Relay To Fail?
- Overloaded Circuit: Fuses are designed to handle a specific amount of current. If the circuit draws more current than the fuse rating, it can blow. This often happens when too many devices are connected to a single circuit.
- Short Circuit: A short circuit occurs when a hot wire comes into direct contact with a neutral or ground wire. This creates a low-resistance path, causing a surge in current that can quickly blow the fuse.
- Faulty Wiring: Damaged or faulty wiring can lead to increased resistance or unexpected current flows, potentially causing a fuse to blow. This might include issues like frayed wires, loose connections, or corrosion.
- Faulty Appliance or Device: A malfunctioning appliance or electronic device can draw excessive current and lead to a blown fuse. Check the devices connected to the circuit for any signs of damage or malfunction.
- Power Surges: Sudden spikes in electrical voltage, such as those caused by lightning or faulty equipment, can overwhelm a fuse and cause it to blow.
- Old or Worn-Out Fuses: Fuses have a lifespan and can degrade over time. If a fuse is old or worn out, it may blow more easily. Consider replacing old fuses with new ones.
- Incorrect Fuse Type: Using a fuse with a higher amp rating than the circuit requires can lead to unsafe conditions and potentially damage the connected devices. Always use the correct fuse type and rating.
- Ground Fault: A ground fault occurs when the hot wire comes into contact with the ground or a grounded object. This can cause a sudden surge in current, leading to a blown fuse.
- Environmental Factors: Harsh environmental conditions, such as extreme heat or moisture, can affect the integrity of electrical components and contribute to fuse failures. Ensure that electrical systems are installed and maintained in appropriate conditions.
What happens if you ignore the problems?
Snapping in an old relay that fits in or ignoring this problem can lead to severe problems in the engine compartment.
If you install a faulty relay or the vehicle relay is malfunctioning, it can burn electrical wires and possibly set up a fire in the engine compartment. Imagine this happens when cruising at 120km/h. You don’t want this to happen, even when idling the vehicle.
Diagnosing a Blown Fuse Relay
To diagnose a blown fuse relay, you’ll need a multimeter and a wiring diagram for your vehicle. Here’s a step-by-step guide:
Step 1: Consult the Wiring Diagram.
Consult the wiring diagram to identify the fuse relay and its location. The diagram will also show you the circuit layout and the components connected to the relay.
Step 2: Check the Fuse Relay.
Use a multimeter to check the fuse relay for continuity. Set the multimeter to the ohms function and measure the resistance across the relay terminals. A good fuse relay should show continuity, while a blown relay will show infinite resistance.
Step 3: Check the Circuit.
Use the multimeter to check the circuit for voltage and current flow. Measure the voltage at the relay terminals and compare it to the specified value in the wiring diagram. Check the current flow by measuring the amperage at the relay terminals.
Safety Precautions for Handling Fuse Relays
Before working with fuse relays, it’s crucial to follow specific safety precautions to prevent electrical hazards and ensure personal safety:
#1. Shut Off Power.
Before handling any fuse relay, always turn off the power supply to the circuit or device it controls. This can be done by unplugging the appliance or switching off the circuit breaker.
#2. Use Insulated Tools.
When handling fuse relays, use insulated tools such as needle-nose pliers or screwdrivers with insulated handles. These tools help protect against electrical shocks.
#3. Wear Protective Gear.
As an added safety measure, wear protective gear such as gloves and safety glasses when working with fuse relays. This gear provides an extra layer of protection in case of an electrical accident.
#4. Avoid Touching Metal Parts.
While holding a fuse relay, avoid touching any metal parts or exposed terminals. These can conduct electricity and pose a shock hazard.
#5. Inspect the Relay.
Before inserting a fuse relay into a circuit, inspect it for any visible damage or corrosion. Replace any damaged or corroded relay to ensure proper functionality and safety.
#6. Follow Specific Instructions for Each Relay Type.
Different types of fuse relays may have specific handling and replacement procedures. Refer to the manufacturer’s instructions or an automotive repair manual for the correct steps to ensure safe and effective operation.
How Should You Fix a Blown Fuse?
Once you’ve identified a blown fuse, follow these steps to replace it safely.
How to Replace a Blown Fuse?
You’ll follow the same basic steps no matter what type of fuse you need to replace.
#1. Prepare for Replacement.
- Wear rubber-soled shoes and insulated gloves.
- Turn off all appliances in the affected area.
- Pull the main fuse block to cut power.
- Get the correct replacement fuse—same type and amperage as the blown one.
#2. Swap the Fuse.
- Remove the blown fuse:
- Plug fuse: Twist it counterclockwise.
- Cartridge fuse: Use a fuse puller.
- Install the new fuse by screwing or pushing it into place.
- Slide the main fuse block back in and press firmly.
#3. Check Your Wiring.
- When in doubt, call an electrician. Using the wrong fuse can lead to serious safety risks.
- If a circuit has a 30-amp fuse, your wiring may be overloaded—this is a fire hazard.
- Fuse ratings matter:
- 15-amp fuse → Requires 14-gauge wiring
- 20-amp fuse → Requires 12-gauge wiring
How to Test a Blown Fuse with a Multimeter?
This is the foolproof way to test a fuse and determine if it’s blown or not. The process is simple:
- Switch off the current at the mains and disconnect the appliance.
- Remove the fuse from the appliance
- Use a multimeter and set it to measure continuity.
- Make sure that your probes are working by putting the negative and positive leads together and seeing if the meter beeps to indicate continuity.
- Place one probe on each end of the fuse. It doesn’t matter which end of the fuse touches the positive or negative terminal.
- Place the other probe on the other end of the fuse.
- If the meter beeps and shows continuity, then chances are that your fuse is intact. Otherwise, it’s blown.
You can also use the resistance setting on the multimeter to test a fuse. This will tell you how much resistance is present in the fuse. A blown fuse will have infinite resistance or ‘OL’ value.
FAQs.
Q: Is the relay a sensor?
No, a relay is not a sensing device. Instead, it is a switching device that consists of input connectors for multiple or single control signals and works as an operational terminal.
In other words, relays are switches that aim at opening and closing circuits electromechanically and electronically.
Q: What is the difference between a switch and a relay?
We described a relay as a switching device. However, it has several differences from a switch. Here’s a comparison between a relay and a switch.
| Switch | Relay |
| Switches are mechanically controlled | Relays are electronically controlled |
| It controls electric flow by opening or closing circuits | Relays control high electric flow on the power circuit by opening or closing the contacts |
| Switches are operated manually by pressing a button or by pulling a puller | It can send an optical or electromagnet signal to activate the load circuit |
| Switches are used to open or close circuits. | Relays are used to prevent systems from possible damage. |
| It works slower than relays because it needs a physical operation. | It works faster than switches. |
| Switches make a direct connection or contact. | It has a remote connection or contact. |
Q: What is the difference between a fuse and a relay?
Relays and fuses perform similar roles. They also have similarities and differences. However, here’s the typical difference between them. A fuse is a device designed to interrupt a circuit once. In contrast, a relay connects or interrupts the circuit.
Q: How much does it cost to test a relay?
The cost of replacing a relay depends on several factors. First, you need to check if you want to replay it yourself or contact a service technician to install a new one.
The part will cost around $2 to $20, depending on the relay and your vehicle. If you choose to have a mechanic replace it, budget to spend $32 to $55. However, the price can be lower or higher than that.
Final Words
At this juncture, you have learned how to tell if a relay fuse is blown. The methods on how to check a blown relay fuse above are effective. You can use any of them and be sure of an efficient result.
However, I recommend the visual check method for all DIYers. This method is the most simplified guide and requires no tools. Although if you have the tools mentioned in this article, you can explore any of the methods.