Fusible links—also spelled fuseable and fusable—serve a similar purpose to a fuse. (You can read more about fuses here.)
Just like a fuse, the link is designed to handle a lower current (amp) load than the rest of the system so, in the event of a short or overload, the link will be the first failure point. When it fails, it will break the continuity in the rest of the circuit, preventing damage to other components down the line.
A fusible link typically costs a couple of bucks and can be replaced in 30 minutes. A starter motor or ECU? Not so much.
But fusible links look much different than fuses and are used for different applications.
What’s the Difference Between a Fuse and a Fusible Link?
In an automotive context, fuses are typically used on circuits with a relatively low current draw, measured in amperes, ranging from one amp to 40 amps.
But certain automotive components require momentary higher peak current levels depending, so a fixed-rating fuse may not be ideal. Fusible links are also cheaper and easier to install than a dedicated fuse block, which is why you see them in OE applications.
How Does a Fusible Link Work?
Wire is measured in “gauge,” often abbreviated AWG for “American Wire Gauge.” The lower the number, the bigger the wire. Bigger wires handle more current (amp) draw. Check out this cable calculator chart to learn more.
Again, a fusible link’s function is very similar to a fuse. It’s designed to fail before your wiring harness melts to goo.
As a general rule, a fusible link is made of wire that is four gauges higher (smaller) than the rest of the circuit, making it the weakest link in your electrical chain. For example, a fusible link in a 10-gauge wire would be 14-gauge.
The smaller-gauge wire handles less current than the rest of the circuit, so it will overheat first and, by doing so, will break the physical connection between the wire to which it’s inserted.
What Does a Fusible Link Look Like?
Well, that’s the fun part. They look like wires, which can make troubleshooting a headache—we’ll get to that in a second.
When you’re combing over your wiring harness, you’ll want to look for a short section of wire (usually a few inches long) that has a smaller diameter than the wire into which it’s connected. There’s also a good chance that the link will be a different color than the wire.
Remember, a fusible link is going to be four gauges higher (smaller) than the rest of the circuit.
If it was a factory-installed link, it will probably have a nice jacket or boot covering the splices.
The link may be at the very end of the harness as well, ending at a ring terminal secured to a solenoid, battery, or motor.
Troubleshooting a Fusible Link
When a fusible link does its job, it will break the circuit continuity, which is akin to cutting the wire. That results in cutting off power to the component.
We see this a lot in starting systems. A fusible link will blow, and folks will incorrectly diagnose a failed starter motor—turn the ignition key…and nothing will happen.
If this (or something similar happens), you should check for blown fuses in your fuse box first. If everything looks okay, pop the hood and inspect the wiring harness for melted, scorched, or broken wires.
Don’t panic if you see one, as it may simply be the fusible link doing its job.
You can also check for electrical continuity using a multimeter‘s continuity setting. Clip a lead on one end of the circuit and the other to its opposite end. If continuity exists, then your problem is likely elsewhere.
Avoid using a simple 12-volt continuity test wand for something like this, as your circuit may not automatically have 12V present. (An upstream switch or relay may also prevent an accurate diagnosis.)
Replacing a Fusible Link
Replacing a link is as straightforward as cutting out the broken link and installing a new one.
Many fusible links now come with a crimp connector pre-installed, which simplifies the installation process.
Remember to weather-proof your connections—cover your joints with heat shrink tubing or tightly wrap your connections with electrical tape.
Need a good 101 tutorial on electrical wiring? Check this out.