What Causes the Fusible Link to Blow on a Car?by Richard Rowe
You've got to love the old-school fusible link. A study in simplicity that even a child could understand, the fusible link dates back to a simpler time before cars used junction boxes or complex computer controls. While fusible links can be a bit of a pain if you don't remember where they all are, they do make diagnosis a lot simpler once you locate them.
Fusible Link Basics
A fusible link is essentially a fuse set directly in line with the wiring and works as much like a literal weak link in the chain as you can imagine. Electricity never passes through anything but a superconductor with a zero loss of energy; some of it's always going to turn into heat. How much depends on the wire's resistance, which correlates to its size. A fusible link is a "narrow spot" in your wire, a portion of higher resistance that will overheat, melt and break before the rest of your wiring does. Wiring has a certain amount of resistance -- rated in ohms -- per foot. The longer the wiring, the greater its resistance and the less power it can handle without overheating.
The easiest way to blow a fusible link is to pass more current through it than it can handle. There are a couple of ways to do this, but the simplest is to ground the wire directly to your vehicle's chassis. Doing this will open the floodgates for electricity, and the fusible link will blow out like a drinking straw on the end of a fire hose. While you'd never -- presumably -- connect a positive battery or alternator wire directly to the chassis, the same thing will happen if a sharp metal corner or hot part of the engine cuts or burns the wire insulation away.
A high-draw accessory like an electric motor, ignition coil or speaker amplifier acts something like a short circuit in the system. For instance, if a speaker amp presents 2 ohms of resistance, everything's fine as long as the fusible link and length of wire don't have any more than 2 ohms of resistance. Install a more powerful amplifier with 1 ohm of resistance, and all of the sudden it's asking for twice as much power as the fusible link can withstand. The same is true for any accessory or system incorporating a fusible link or a standard blade-type fuse.
Sizing the Fusible Link
Two factors come into play when sizing a fusible link. First, you need to know the accessory draw. As an example, we'll use an amplifier drawing 50 amps; measure the distance in feet from the accessory to the power source. You could look at an ohms-per-foot chart, but you'll probably find it easier to think in amps of current and look at a wire-gauge/amp-draw chart. Add at least 30 percent to the accessory's amp draw, then find that amperage on the chart. Go over on that line until you get to the measured distance in feet. A 50-amp amplifier mounted in the trunk 15 feet from the battery would need a 4-gauge wire, which is good for 70 amps at 16 feet. For the fusible link, add 10 to 15 percent to the accessory draw. In this case, you'd need a link rated at 55 amps, primarily because you'll have a hard time finding one rated at 57.5.
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