Opening the fuse box under the hood or in the cabin, you see colorful "plates" (fuses) and large black cubes (relays). Although they are next to each other and often confused, they serve completely different functions in the car's electrical system. In this article, we'll explain in detail the differences between them, how they work, how to identify them, and how to diagnose faults.
Fuse - The Guardian of the Wiring
A fuse is a protective element whose main role is to sacrifice itself for the good of the entire wiring. It is the weakest link in the electrical circuit - designed to blow (melt) before current damages more expensive components.
How does a fuse work?
A fuse consists of a thin wire made of a material with a low melting point (usually a tin and lead alloy). When the current flowing through the fuse exceeds its rated value (e.g., 10A, 15A, 20A), the wire heats up and melts, breaking the electrical circuit. This immediate action protects:
- Wires - from overheating and melting
- Devices - from damage by excessive current
- ECU - from burning
- Motors - from winding damage
- Entire wiring - from fire
Types of fuses
In cars, you'll mainly encounter "blade" type fuses, which are divided into three sizes:
- Mini - smallest (approx. 10.9 mm wide), dominate in modern cars
- Standard - medium (approx. 19.1 mm wide), found in older models
- Maxi - largest, handle 30A-100A currents
Fuses are color-coded according to DIN 72581 standard, allowing quick identification of their value even when the text wears off.
How to identify a fuse?
- Appearance: Colorful, flat "plate" with two metal legs
- Size: Small, compact element (Mini/Standard) or larger (Maxi)
- Markings: Amperage value (e.g., "10A", "15A") and often housing color
- Location: In fuse sockets, often in rows
Relay - Remotely Controlled Switch
A relay is an electromagnetic switch that allows a small current to control a large current. It works like a remotely controlled switch - a small current from the cabin switch activates an electromagnetic coil, which closes contacts conducting large current to the device.
How does a relay work?
A relay consists of several key elements:
- Electromagnetic coil - powered by small current (usually 12V, 0.1-0.5A)
- Contacts - metal plates that close under the influence of magnetic field from the coil
- Housing - plastic cube with terminals (usually 4-5 pins)
When small current flows through the coil, it generates a magnetic field that attracts the movable contact to fixed contacts, closing the high-current circuit. When current in the coil disappears, the contact returns to open position (thanks to a spring).
Why are relays used?
Relays are essential for several reasons:
- Protecting switches - a small light switch in the cabin wouldn't have to handle 10-20A needed to power headlights. Thanks to the relay, the switch handles only 0.1-0.5A
- Safety - large currents are routed through short wires under the hood, instead of through the entire cabin
- Functionality - one switch can control multiple devices through different relays
- Automation - relays can be controlled by the ECU (e.g., automatic light activation)
Typical relay applications in cars
- Headlights - headlight relay (usually 20-30A)
- Radiator fan - fan relay (usually 30-40A)
- Fuel pump - fuel pump relay (usually 15-20A)
- Air conditioning - compressor relay (usually 20-30A)
- Wipers - wiper motor relay (usually 20-30A)
- Heated rear window - heater relay (usually 30-40A)
How to identify a relay?
- Appearance: Large, black (or colored) plastic cube
- Size: Much larger than a fuse (approx. 25-30 mm wide)
- Terminals: 4-5 pins (legs) on the bottom
- Markings: Model number (e.g., "30/40", "87", "85/86") and sometimes connection diagram
- Sound: Often hear characteristic "click" when turning on/off
Main differences - comparison
| Feature | Fuse | Relay |
|---|---|---|
| Function | Protection against excessive current | Control large current with small current |
| Operation | One-time - blows and requires replacement | Multiple - turns on/off on demand |
| Size | Small (Mini/Standard) or large (Maxi) | Large (approx. 25-30 mm) |
| Appearance | Colorful plate with two legs | Black cube with 4-5 pins |
| Current | Passes current up to rated value | Controls current (coil: small, contacts: large) |
| Cost | Cheap ($0.25-$1) | More expensive ($2-$10) |
How to diagnose fuse damage?
A fuse blows when current exceeds its rated value. Signs of damage:
Visual method
- Turn off ignition - for safety
- Pull out the fuse - use a plastic fuse puller
- Check against light - look at the metal wire inside the housing
- Assess condition:
- ✅ Wire is continuous - fuse is good
- ❌ Wire is broken - fuse is blown
- ⚠️ Wire is scorched or dark - fuse is damaged
Multimeter method
- Set multimeter to continuity test (diode/speaker symbol)
- Touch probes to metal legs of the fuse
- Interpretation:
- ✅ Beep + value close to 0Ω - fuse is good
- ❌ Silence + "OL" - fuse is blown
How to diagnose relay damage?
A relay can fail in several ways. Below you'll find diagnostic methods:
Test 1: Audio test (fastest)
- Turn on device controlled by the relay (e.g., lights)
- Put ear to the relay
- Listen for sound:
- ✅ Hear "click" - coil works, but this doesn't mean relay is good
- ❌ No sound - coil is burnt or has no power
Test 2: Multimeter test - coil
Check if the relay coil is good:
- Turn off ignition and remove key
- Remove relay from socket
- Find coil pins - usually marked as 85 and 86 (check diagram on housing)
- Set multimeter to resistance measurement (Ω)
- Measure resistance between pins 85 and 86:
- ✅ Resistance 50-200Ω - coil is good
- ❌ Resistance "OL" or infinity - coil is burnt
- ❌ Resistance 0Ω - coil has short circuit
Test 3: Multimeter test - contacts
Check if relay contacts conduct current:
- Turn off ignition and remove key
- Remove relay from socket
- Find contact pins - usually marked as 30, 87, 87a (check diagram)
- Set multimeter to continuity test
- Without coil power - measure resistance between pins 30 and 87:
- ✅ Resistance "OL" (infinity) - contacts are open (normal for relay without power)
- ❌ Resistance < 10Ω - contacts are shorted (damaged)
- Power the coil - connect 12V to pins 85 and 86 (use power supply or battery):
- ✅ Resistance < 1Ω + beep - contacts are closed (relay works)
- ❌ Resistance "OL" - contacts don't close (damaged)
Test 4: Replacement test
Simplest method - if you have an identical relay (e.g., from another circuit):
- Turn off ignition and remove key
- Remove suspicious relay
- Insert replacement with identical model number
- Turn on device and check if it works
- If it works - old relay was damaged
Common relay problems
Problem 1: Relay doesn't click
Causes:
- Burnt coil (most common)
- No power on pins 85/86
- Damaged control switch
- Damaged wiring to relay
Solution: Check coil resistance with multimeter. If coil is good, check power and control switch.
Problem 2: Relay clicks, but device doesn't work
Causes:
- Burnt contacts (most common) - contacts are charred and don't conduct current
- Damaged contacts - mechanical damage
- No power on pin 30 (current input)
- Damaged wire to device
Solution: Check contact continuity with multimeter. If contacts don't close, replace relay.
Problem 3: Relay "chatters" (quickly turns on and off)
Causes:
- Low supply voltage (weak battery)
- Damaged coil (partial short)
- Ground problem (pin 86)
Solution: Check battery voltage. If voltage is correct, replace relay.
When to replace a relay?
A relay should be replaced when:
- Coil is burnt - coil resistance is "OL" or 0Ω
- Contacts are burnt - don't conduct current despite closing
- Relay "chatters" - quickly turns on and off
- Device doesn't work despite relay clicking
- Visible damage - cracked housing, melted elements
Fuse and relay - cooperation
In car wiring, fuse and relay often work together:
- Fuse protects relay - protects circuit from excessive current
- Relay protects switch - allows small switch to control large current
- Both protect device - fuse from excessive current, relay from switch overload
Example: In headlight circuit, 15A fuse protects entire circuit, relay allows light switch (0.1A) to control headlights (10-15A).
Summary
Fuse and relay are two different elements serving different functions:
- Fuse - protects wiring from excessive current, blows once
- Relay - controls large current with small current, works multiple times
Both elements are essential in modern car wiring and often work together, ensuring safety and functionality of electrical systems. Proper diagnosis and replacement of damaged elements requires understanding their operation and using appropriate diagnostic tools.