Why Would Alternator Fuse Blown:7 Reason and Fixes With Tips

Reviewed by Dr. Deepakkumar Jani

The alternator fuse blows due to overcurrent from a short in the alternator/wiring, excessive charging voltage, or a fault in the electrical system drawing more current than the fuse rating. Key data points: short-circuit current, fuse rating (amps), alternator output (volts, amps).

Troubleshooting Blown Alternator Fuse-Quick Reference

Challenge/IssueTroubleshooting StepsSolution
Blown alternator fuseCheck for short circuits in alternator wiringRepair/replace wiring or alternator
Excessive alternator output voltageTest alternator voltage and regulationAdjust/replace voltage regulator
Electrical overloadInspect the wiring harness for wear and tearRemove excess load, upgrade fuse capacity
Recurring fuse blowoutsExamine alternator and battery conditionReplace faulty alternator or battery
Wiring harness damageInspect wiring harness for wear and tearRepair or replace damaged harness
why would alternator fuse blown

Top Causes of a Repeatedly Blown Alternator Fuse

Let’s try to understand what are the top causes of blown alternator fuse.

Electrical Short in the Alternator Wiring


Fraying or exposed wiring from the alternator to the fuse box can create an electrical short path. This gives the current an easier route to flow than along the normal conductors. With lower resistance, way too much current flows through the fuse and causes it to blow.

Inspect this wiring thoroughly for any rubbing, wear, or exposed copper strands. Also check connections at either end for loose, corroded, or burnt terminals. Faulty wiring is one of the most prevalent reasons alternator fuses fail.

Bad Diode in the Alternator


Inside your alternator, diodes rectify the AC voltage produced into the DC voltage needed for your battery and vehicle electronics. When these diodes fail, you lose that rectification.

Unrectified AC spikes flowing through the fuse lead it to blow. Have your alternator bench tested to see if bad diodes could be the culprit. This would call for a full alternator replacement or rebuild.

Too High of Belt Tension


While wanting to avoid loose belts that slip, it is also possible to over-tighten your alternator belt. This places excess strain on the alternator bearings. This tension can actually short the internal windings, causing voltage spikes.

Use a belt tension gauge to check if the tension is in the right specification range for your make/model. Adjust tension as needed – often a tricky balancing act.

Bearing Issues Allowing Internal Short


Sometimes a worn alternator bearing can also enable the rotor to rub internally, shorting out the windings. Again this leads to voltage spikes through the fuse. Catch and fix bearing issues early to prevent this and other charging problems.

Bad Regulator


The voltage regulator controls current flow leaving the alternator. If it fails to operate properly, you may get uncontrolled power surges through that fuse. This would call for the replacement of the regulator or even the whole alternator in many modern vehicles.

Battery Issues Allowing Backfeed


Issues like sulfation or a short inside the vehicle’s battery can also lead to the current flowing backward from the battery itself through the alternator fuse. A load test and diagnostic scan can uncover if the battery is at fault here. Address battery issues separately from alternator fuse diagnosis.

Identifying the Reason


With many culprits able to take down that fuse, systematic troubleshooting is key to narrowing down what needs fixing in your specific situation. Here is an expert-level troubleshooting roadmap to pinpoint why the fuse keeps blowing:

Replace the Fuse and Test the Voltage Output


Start by replacing the blown fuse with an identical new one while the engine is off. Then start the engine and use a multimeter to measure voltage output on the charge line at the back of the alternator. If voltage output drops suddenly and fuse blows again, continue diagnosis. If it stays steady, you likely have an intermittent issue only blowing the fuse sometimes.

Inspect Wiring


Next, turn off the engine and closely inspect the wiring from the alternator charge terminal all the way to the fuse box. Look for any rubbing wear, burnt insulation, corroded terminals, or loose connections that could enable a short. Evaluate routing too – wiring should not touch engine components. Repair wiring issues as needed before continuing.

Check Drive Belt Condition and Tension


Issues with drive belts that power the alternator can also cause problems. Inspect for cracking, severing, fraying, missing ribs, or extreme glazing. Next, use a belt tension gauge to measure if the tension is within specifications. Adjust as required – often around 100 lbs is ideal if lacking specs. This guards against shorts from over-tension. Replace the belt if worn or damaged.

Check Battery and Cables


Faulty batteries and cables can back feed voltage in the wrong direction. Use a multimeter to measure the DC voltage on the battery cables while the engine runs. You should see very little voltage present if the charging system is working properly. Load-test the battery using a dedicated battery tester tool for maintenance-free models. Check cable lugs for corrosion and terminal ends for loose fit or heat damage. Address battery and cable issues external to alternator diagnosis.

Bench Test Alternator


At this advanced stage, it is prudent to remove the alternator for bench testing either yourself or at an auto electric shop. They can diagnose if bad diodes, windings, regulator issues, or bearing wear internally could be allowing shorts and voltage spikes to blow the fuse. If defects are found internally, pursue replacement or rebuild.

Preventing Repeat Failures


Once you successfully uncover the root cause and address it, that fuse should stop blowing every time you drive. To help prevent repeat issues:

  • Inspect wiring over engine accessories for rubbing annually
  • Proactively replace alternator bearing every 100k miles
  • Check belt tension and condition at routine intervals
  • Load test battery every 1-2 years