alternator failure
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Alternator Failure Causes

by Larry Carley copyright

The alternator keeps the battery charged and provided current for the vehicle's lights and other electrical accessories. An alternator failure will allow the battery to run down, and may prevent the vehicle from starting.

If the charging voltage with the engine idling is not at least 13.5 volts or higher, there is a charging problem that most likely is due to a bad alternator. However, the problem might also be a fault in the wiring that connects the alternator to the vehicle's electrical system, or a problem in the Powertrain Control Module (PCM) that regulates the charging circuit.

An alternator can be removed from the vehicle and taken to an auto parts store for testing. Many auto parts stores have a bench tester that can measure alternator output voltage and current. If the alternator fails a bench test, it needs to be rebuilt or replaced. If it passes a bench test, the problem is not the alternator but a wiring fault or PCM problem.



One of the most common causes of low or no charging output from the alternator is one or more bad diodes in the back of the alternator. There are three diode pairs (6 total) in the "rectifier" assembly that convert the alternator's alternating current (AC) output to direct current (DC). This is necessary because the fuel pump, fuel injectors, ignition system, engine computer, lights and other onboard electronics and electrical accessories all require DC current. High temperatures and high current loads can damage diodes over time, causing them to fail. A bad diode may not pass any current at all, causing a loss of current output from the alternator, or a bad diode may leak AC current into the vehicle's electrical system, creating a ripple voltage can disrupt the normal functioning of some onboard modules. A leaking diode may also create a small electrical drain on the battery when the engine is off causing the battery to gradually run down over time if the vehicle is not driven.

alternator components

The diodes can run hot when high electrical loads are placed on the alternator during hot weather. There may not be enough airflow through the alternator to prevent the diodes from overheating. This tends to occur when outside air temperatures are high and the engine is idling for a prolonged period of time with the A/C on, headlights, radio, etc.


The shaft bearings that allow the rotor to spin inside the alternator can wear out over time. Worn shaft bearings may be noisy (rumbling, buzzing or squealing sounds), and may eventually bind up and seize. If the alternator locks up, it will usually snap or throw the drive belt.

Bearing failure can also be caused by dirt or water contamination. Driving a vehicle through water that is deep enough to splash or flood the alternator can damage the alternator shaft bearings and possibly the brushes and electronics inside the unit as well. Water can also cause corrosion that over time will damage the wiring and bearings inside the alternator.

Worn bearings can be replaced by disassembling the alternator and installing new bearings.


An alternator has two sets of windings inside. One set of wires is wrapped around the rotor that spins inside the alternator. These windings create a magnetic field that induces current in the other set of windings, which are mounted around the rotor inside the housing (the field coils or stator). If any of the individual wires in the rotor or stator short out or break, it may reduce the alternator's output current or kill it entirely.

There are also external wire connections to the alternator that energize the rotor and carry current from the field coils to the electrical system. If any of the connections are bad (loose, corroded or broken), it can disrupt the alternator's current output. A visual inspection may or may not reveal such problems because some breaks in external wiring connectors may be impossible to see because they are inside a rubber boot or covering. A Voltage Drop Test across the connections may be the only way to find such problems.

A broken wire may be soldered, but it may also be necessary to replace the external wiring harness or wiring connector depending on the damage.

BAD BRUSHES. Brushes inside the alternator make sliding contact with slip rings on the rotor shaft. This allows electricity to flow to the rotor so the alternator can make current. If the brushes and/or slip rings are worn, corroded or burned, it will inhibit the flow of electricity causing a drop in alternator output.

Worn brushes can be replaced with new ones by disassembling the alternator, and a worn slip rings can be replaced or polished to restore good contact.

Some alternators do not have brushes (such as brushless alternators used on diesel engines), and instead use a two-in-one alternator design. The exciter section of the rotor creates voltage that then flows to the rotating coils in the second part to induce current in the outer windings.


Rebuilding an alternator requires some skill and expertise, and some alternators may not be rebuildable because of damage to the shaft, rotor or housing. Because of this, and the difficulty of sourcing internal repair parts for alternators, it is usually faster and easier for the average motorist to simply replace a bad alternator with a new one, or one that has been professionally rebuilt or remanufactured.

Another repair option is to take your vehicle to a repair facility that specializes in electrical work. They may be able to rebuild your old alternator for less that what it would cost to buy a new or remanufactured alternator at a parts store or online.

If you are buying a replacement alternator online, make sure it has a warranty or has been tested to verify it is producing its rated output. We would NOT recommend buying an untested used alternator online or at a salvage yard unless the seller is willing to take it back if it fails to work.

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High Output Alternators (Why You May Need One)

Troubleshooting electrical problems

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