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What Happens When A Fuse Blows?

Just like your house, a vehicle uses a number of fuses that serve different functions. Located in a central fuse box, these small components protect your car from short circuits and overloads. When one trips or burns out, it must be replaced. Here’s what to do about a blown fuse.

Fuse Design

Fuses involve small parts that screw into threaded sockets, sort of how light bulbs fit into place. Inside the fuse, a metal ribbon–sized to match the circuit–sits as currents pass through. If too much current flows through the ribbon it will melt, blowing the fuse and killing the circuit. On the outside, the fuse has a tiny glass enclosure through which the ribbon can be viewed. Sometimes when a fuse blows, you can actually see the melted ribbon. Other times, the glass appears cloudy.

When a Fuse Blows

Two main circumstances lead to a blown fuse. Most commonly, too much power is drawn from the circuit, overloading fuse capacity and causing the metal ribbon to melt. When that happens, any systems powered by the circuit will die. The other reason fuses blow is a hot wire touches a neutral wire or a grounding pathway. Most people refer to this as a short circuit, and it means loose wire connections or damaged wires have caused a problem. Both a blown fuse and short circuit look the same, with a burned metal ribbon or cloudy glass surface.

Checking Auto Fuses

Before you can check a fuse, you must locate the fuse box (the owner’s manual will be your guide). You can use either a high-impedance digital multimeter (DMM) or fuse tester, and both can be found at your local auto parts store. Here are a few simple ways to check fuse capacity:

  • Check available voltage – After setting the DMM to measure voltage in the auto range, connect the negative test lead to the ground. Then, take the positive test lead and touch both sides of the fuse. Good fuses will measure between 12 and 13.5 volts and a blown one will show zero voltage on one side.
  • Check continuity – Remove the fuse and set the DMM to measure resistance. Then, probe both ends of the chosen fuse. A good fuse will measure 0 and a damaged one will measure OL.
  • Use fuse tester – For this option, all you have to do is touch the probe pins at the back of the fuse (on the contact points) and see if the indicator says “bad” or “good.”

For all your vehicle’s maintenance and repair needs, call Fairfax Auto Repair at 703-822-5911 and schedule an appointment today!

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