Charging System Tests

Checking Alternator Function

If the battery is in good condition, check the alternator with the engine running.

Test the voltage across the battery terminals - it should be 13.5 to 14.5 volts.
Double check to see that the alternator belt is tensioned properly and in good condition.

Alternator Failures & Tests

When the charging system fails, usually a "battery" or "alternator" warning indicator or light will come on.

The indicator light is a direct comparison between the voltage output of charging system and the voltage output of the battery.

There are three main ways a alternator can fail.

The electrical part of the alternator shorts out and stops charging.
One of the two bearings on either end of the armature seize, causing a grumble or squealing noise.
The main charge connection is lose it can cause the main charge electrical connector to overheat and fail.

Simple Charging Tests

Engine Speed Test - If you increase the engine speed and the alternator light becomes brighter, then the battery needs to be fully recharged and tested.

If the light becomes dimmer then the problem is most likely in the charging system.

Headlight Test - with the engine running shine the headlights against a wall at night.

If you turn the engine off and the lights get dimmer; the charging system is producing a higher voltage.

If the light becomes brighter, then you probably have a charging system problem.

Voltmeter Test - Most vehicles with good charging systems will measure between 13.8 and 14.8 volts on a warm day, depending on the battery type that the charging system was designed for.

Other Tests

Turn off the car
Check the battery cable ends for whitish/bluish powdery corrosion. If present, clean with a wire brush.
Check the battery cable ends are tight. You shouldn't be able to turn them at all.
Check the negative battery cable end. Make sure it's tight and secure at both ends.

Check the alternator belt . A loose belt will cause the alternator to undercharge and trip the battery light.
Make sure all the connections at the back of the alternator are secure.

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