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How to assess the serviceability of an alternator

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A faulty voltage alternator can overcharge a vehicle battery. Dive into smart charging systems and all things alternators-related in our latest Masters of Motion video, and learn how to check that they're in serviceable condition.

Batteries How to Technicians 1 min read

What is the voltage output of a healthy alternator?

Around 14 volts (between 13.8 and 14.2 is good). Remember, modern alternators will have their output controlled by an ECU and may not always output this voltage.

How can I identify a fault?

Perform a voltage check. If the voltage of the alternator increases or decreases with engine speed, then it may indicate a fault. You could also switch on heavy electrical loads to make the alternator work hard and check to see if the voltage output of the alternator operates consistently at around 14 volts.

What happens if the voltage output is too high or low?

Too high means that the vehicle’s battery is being overcharged. Too low means the regulator is potentially faulty and is discharging the battery.

What’s the difference between a standard alternator and a smart charging system?

An alternator delivers full power, even if it’s not needed. This is inefficient as it wastes fuel and causes more pollution than necessary. A smart charging alternator’s output is variable and controlled by an engine management system. This means that it only delivers the power that the vehicle needs.

How can I tell if the engine is fitted with a smart charging system?

If the engine is Euro 5 emissions standard or later, it mostly likely has a smart charging system. The other indicator is the battery sensor found on the negative terminal of the battery – smart charging systems often have these.

How does a smart charging system affect the power output from the alternator?

If the vehicle is fitted with a smart charging system, the voltage output can vary depending on how much power is needed by the engine management system at the time. This means that it won’t always be around 14 volts.

For example, a lower voltage could be because the vehicle’s battery is already charged and the power demand is low, rather than a fault. Bear in mind that if the smart charging system output is constantly no higher than battery voltage, it could indicate a fault with the system.

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