On Car Alternator Testing.
Be aware of the dangers of acid splashes and of explosive
gas build up when working near batteries especially when charging and testing
and also take care not to short out the battery.
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Before attempting to test any alternator it is essential to ensure that
the battery is at least a 75% charge, and also the battery is in good condition
and has no dead or shorted cells. Even when working perfectly an alternator
cannot charge a battery which is almost completely flat.
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You should also examine all starter cable connections and earth braid connections
to ensure they are clean and tight and in good condition. The connections
at the rear of the alternator should also be checked as these are a frequent
source of trouble particularly on cars which use spade connectors for the
power connections which come from the battery.
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For all these tests you will need either a 0-20 volt multimeter or a LED
type charging system tester. Suitable low cost digital multimeters and
testers can be bought from Maplin or Tandy.
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If the charge is low Charge the battery for a few hours - overnight if
possible. If no hydrometer is available, switch the charger off and allow
the battery to de-gas for at least half an hour then check the battery
voltage. A voltage of 12.8 volts offcharge is generally taken as indicating
that no more charging is required.
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With the battery in the car you can now perform a cranking voltage test.
This is a very simple test to check if the stater is not drawing excessive
current or if the battery has a dead or shorted cell. Disconnect the coil
to prevent the engine starting. If the battery has removable tops remove
them. Connect the volt meter across the battery ( use the 0-20 volt scale
) and measure the battery voltage across the terminals while an assistant
turns the engine over on the starter. The battery voltage while cranking
should not be less than about 10.5 volts. A low reading indicates either
a problem with the battery or the starter is drawing too much current.
A reading of about 6 volts indicates a dead cell in the battery usually
accompained by gas or boiling of the cell - beware of acid splashes and
explosive gas build up. A reading of 6 volts which then rises to 8 after
a few seconds is also a sure sign of a dead cell. A reading of 9 volts
could indicate a stater problem.
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With coil lead still disconnected you can also test the eath lead voltage
drop. Connect the voltmeter between the battery negative terminal and the
engine cylinder block. Use an low voltage scale to measure the voltage
drop while an asistant cranks the engine on the starter. The voltage drop
should not be more than 0.250 volts.
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Now reconnect the coil connect the voltmeter across the battery terminals.
Start the engine and allow it to idle. Increase the engine speed to about
2500 RPM. -- the voltage should now read between 13.5 to 14.5 volts. A
voltage below 13.4 indicates the alternator is not charging or performing
poorly, over 14.8 volts and the battery and other electrical and electronic
components will be damaged.
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Now repeat the above test with head light dip beams and heated rear window
switched on, the voltage may drop slightly but as long as the voltage doesn't
drop below 13.4 volts the alternator is coping with the load. With the
engine still at 2500 RPM switch off the lights, heated rear screen and
any other electrical loads, the battey voltage should not climb above 14.8
volts.
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