What Are the Causes of Ignition Fuses Blowing?

by Richard Rowe

Fuses are current overload protection devices, specifically engineered to act as the weak link in an electrical circuit. A fuse keeps a circuit from passing excess current and destroying whatever's attached to it or melting the wires and starting a fire. Automotive ignition fuse malfunctions pose a few unique problems, primarily since they're usually not the only thing on that circuit.

Fuse Basics

Fuses come in all types and varieties, but they all work in a similar fashion. Current passes through the fuse via a small metal strip or a spring; that small metal strip creates a sort of bottleneck in the system, a point of high impedance where electricity will slow down and turn into heat. Once that strip gets hot enough, it melts, snaps and breaks the circuit connection. So, to find the source of your problem, you'll need to look for an electrical short circuit that pulls more energy through the system than it was designed to handle.

Multiple Circuits

The problem with automotive electrical troubleshooting is that multiple systems often run through the same circuit or fuse. For instance, your ignition system might share its power source with the starter, fuel pump, fuel injectors, ignition control computer or that flashing skull shifter knob you bought from Pep Boys. So, the malfunction might not even be in your ignition system; it could be a malfunction or short circuit in any of the connected systems.

Ignition System Faults

The good news is that there aren't too many things in the ignition system itself that can blow your fuse, particularly if the the ignition coil draws its current directly from the battery or alternator via a relay. If that's the case, then your fault is almost certainly in the ignition switch itself or the wires going to it. Within the distributor itself, a bad or bypassed ballast resistor can repeatedly blow fuses, but that's unlikely unless some hack mechanic did a bad wiring job on it. A bad coil might blow fuses, but it'll more than likely kill the engine before that happens.

Ancilliary Faults

If your ignition system shares a common circuit with something motorized -- a fuel pump, cooling fans, power window motor, starter, etc. -- then that's likely the source of your fault. Electric motors always draw a certain amount of wattage, or amperage multiplied by voltage. Higher voltage makes the motor spin faster, more amperage causes it to produce more torque. If the motor seizes up or something forces it to slow down, voltage draw will drop and amperage draw will increase to maintain the same wattage. This can easily blow a fuse, particularly if it's already heavily loaded by something as power-hungry as the ignition system.

Wiring and Computer Faults

Inspect your wires carefully; hot parts on the engine can easily burn through the wiring's insulation and short the wires, and metal edges will cut through the wires and short them out. If you've got fuel injection, then you might be experiencing an internal short in the computer itself. In that case, you're out of luck; it's off to the parts store or junkyard for a new computer.

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