Alternator & Charging System Checks

The first sign of alternator or charging system trouble may be dim headlights or an engine that is slow to crank (or will not crank). The charging system keeps the battery charged, and supplies voltage for the entire electrical system. So if the alternator, voltage regulator or wiring that connects the charging system to the battery and electrical system goes bad, it can create serious problems.
Charging problems can be caused by electrical faults in the charging system itself, by poor wiring connections at the battery or elsewhere, or by a slipping or broken drive belt. If there is no charging output, the battery will quickly discharge. You may have 20 minutes to an hour of driving time before everything goes dead and the vehicle shuts down.
Once battery voltage drops below a certain threshold, the onboard electronics, ignition and fuel systems may stop working normally and cause the engine to stall. The battery will not have enough reserve power to restart the engine, so the vehicle will be stranded until the problem can be diagnosed and repaired.
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Recharging the battery or jump starting the battery with booster cables from another battery or vehicle may get the engine running again, but it will not be for long if the charging system is not producing normal voltage.
Warning: Never disconnect a battery cable while the engine is running to "test" the alternator. Doing so can cause high voltage spikes that can damage the alternator as well as other electronics.

ALTERNATOR CHARGING OUTPUT

The alternator is the heart of the charging system. It generates all the power needed to keep the battery fully charged and to operate everything electrical in the vehicle. The alternator is mounted on the engine and is belt-driven off the crankshaft pulley by a serpentine belt or v-belt. The alternator produces alternating current (AC), which is converted to direct current (DC) by a six diode rectifier, which is usually located inside the back of the unit. Diodes only pass current in one direction, which is how they convert AC current to DC. Three positive diodes control the positive side of the AC sine wave, while three negative diodes control the negative side.
The alternator's charging output increases in proportion to the electrical load on the charging system and engine speed. Output is low at idle and increases with RPM. Maximum output is typically achieved at speeds above 2,500 RPM.

ALTERNATOR VOLTAGE REGULATION

Charging output of the alternator is controlled by a voltage regulator which may be mounted inside or on the back of the alternator (internally regulated), or somewhere else under the hood (externally regulated). On most newer vehicles, the powertrain control module (PCM) regulates charging output.
On older vehicles, the voltage regulator was electro-mechanical and used magnetic contacts to control the charging output of the alternator. Since the 1980s, most voltage regulators are solid-state electronic and use transistors to control charging output.
The actual output voltage produced by the alternator will vary depending on temperature and load, but will typically be about 1-1/2 to 2 volts higher than battery voltage. At idle, most charging systems will produce 13.8 to 14.3 volts with no lights or accessories on (although some may charge at a slightly higher voltage depending on temperature, engine RPM, type of battery, and the battery's state of charge). This can be measured by connecting the positive (+) and negative (-) test leads of a voltmeter to the battery posts while the engine is running.
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The left photo shows normal alternator charging voltage with the engine idling. The photo at the right shows low charging voltage with the engine idling.
A low reading tells you the charging system is not generating enough voltage to keep the battery charged or to meet the vehicle's electrical needs.

ALTERNATOR CHARGING VOLTAGE

Most alternators that are charging properly should produce a voltage of about 13.8 to 14.2 volts at idle with the lights and accessories off. Always refer to the vehicle manufacturer's specifications. Many Asian vehicles, for example, have higher charging voltages of around 15 volts.
When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, then taper off, leveling out at the specified voltage.
The exact charging voltage will vary according to the battery's state of charge, the load on the vehicle's electrical system, and temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.9 to 15.1 volts at 77 degrees F. But at 20 degrees F. below zero, the charging voltage might jump as high as 14.9 to 15.8 volts for a short period of time. On a hot engine on a hot day, the normal charging voltage might drop to 13.5 to 14.3 volts.

ALTERNATOR AMPERAGE OUTPUT

In addition to checking the alternator's voltage output, you also need to check its current or amperage output. Amperage is how much current the alternator generates at a specified voltage and speed. Not long ago, an 80 amp alternator was considered a high output unit. Moat late model alternators produce 120 to 155 amps or more. Current output increases with engine speed, from around 20 to 50 amps at idle up to the unit's maximum output at 2,500 RPM or higher (refer to a service manual for the exact charging output specifications for your vehicle).
Charging output can be measured with an inductive amp probe clamped around the BAT (B+) wire that connects to the alternator. It can also be measured on an alternator bench tester in a auto parts store.
Alternator power ratings can also be given in Watts (which is volts times amps). Many alternators in foreign vehicles are rated in watts rather than amps. The important point here is to make sure a replacement alternator has the same power rating (in amps or watts) as the original so the charging system can maintain the same power output as before, should the alternator need to be replaced. In fact, on some applications upgrading to a higher output replacement alternator may be recommended if the vehicle has a history of alternator failures, or the vehicle has a megawatt aftermarket sound system, emergency or off-road lighting, or other power-hungry electrical accessories.