Introduction
Electrical issues are among the most frustrating problems a car owner can face. Often, the culprit is something small and inexpensive: a blown fuse or a bad relay. Before you spend hundreds of dollars on new parts or diagnostic fees, learning how to test these components can save you time and money. This guide will walk you through the basics of testing car relays and fuses using a simple multimeter.
Understanding the Basics
What is a Fuse?
A fuse is a sacrificial safety device. It contains a thin wire that melts (or “blows”) when too much current flows through it, breaking the circuit and protecting more expensive components like the radio, lights, or ECU from damage.
What is a Relay?
A relay is an electrically operated switch. It allows a low-current circuit (like your dashboard headlight switch) to control a high-current circuit (the actual headlights). This prevents high voltage from running through the dashboard switches.
How to Test a Car Fuse
Testing a fuse is the easiest place to start when diagnosing electrical faults.
- Visual Inspection: Pull the fuse out. Look at the U-shaped wire inside the translucent plastic. If the wire is broken or there is a dark burn mark, the fuse is blown.
- Multimeter Test (Continuity):
- Set your multimeter to the “Continuity” mode (often looks like a sound wave symbol).
- Touch the probes to the two metal blades of the fuse.
- If you hear a beep, the fuse is good. If there is silence, the fuse is bad.
- Multimeter Test (Voltage):
- Keep the fuse plugged in. Turn the ignition key to the “On” position.
- Set your multimeter to DC Volts (20V setting).
- Connect the black probe to a metal ground point on the car.
- Touch the red probe to the tiny metal test points on top of the fuse.
- You should see 12V on both sides. If you see 12V on one side but 0V on the other, the fuse is blown.
How to Test a Car Relay
Relays are slightly more complex but still easy to test. Most automotive relays have 4 or 5 pins.
Step 1: Identify the Pins
Look at the diagram printed on the side of the relay. You will typically see numbers:
- 85 & 86: The control circuit (coil).
- 30: Common power input.
- 87: Normally Open (NO) output (power flows when relay is on).
- 87a: Normally Closed (NC) output (power flows when relay is off).
Step 2: Test the Coil (Resistance)
Set your multimeter to Ohms (Ω). Touch the probes to pins 85 and 86. A healthy relay usually reads between 50 and 120 Ohms. If it reads “OL” (Open Loop) or infinite resistance, the coil is broken, and the relay needs replacing.
Step 3: The “Click” Test
Use jumper wires to connect pin 85 to the negative terminal of a 12V battery and pin 86 to the positive terminal. You should hear a distinct “click.” No click means the mechanical switch is stuck.
Step 4: Continuity Under Load
While the battery is connected to 85 and 86 (energizing the relay), use your multimeter in continuity mode to check pins 30 and 87. You should hear a beep (continuity). If the relay clicks but you don’t get continuity between 30 and 87, the internal contacts are burnt or corroded.
Conclusion
Testing fuses and relays is a fundamental skill for any DIY mechanic. It takes only a few minutes and requires minimal tools. By verifying these basics first, you avoid the trap of “firing the parts cannon”—replacing expensive parts like fuel pumps or starter motors when the real problem was just a $10 relay.
FAQ
Q: Can I replace a fuse with one of a higher amperage?
A: No! Never replace a fuse with a higher rating. If a 10A fuse blows and you replace it with a 20A fuse, you risk melting the wires and causing an electrical fire.
Q: How do I know which relay is which?
A: Consult your vehicle’s owner’s manual or look at the diagram usually printed on the underside of the fuse box cover.
Q: My relay clicks, but the component still doesn’t work. Is the relay good?
A: Not necessarily. A relay can click but still have burnt internal contacts that prevent electricity from flowing. Always test for continuity or voltage drop across the load pins (30 and 87) to be sure.
Leave a Reply