How Do You Test An Alternator? - The Truth About Cars - Navara

How Do You Test An Alternator?

An automobile has many electrical devices, such as the starting system, lights, radio, etc. These devices are operated by the electric current supplied from the battery and alternator. The amount of power generated depends on the alternator specification and engine speed. Power generation is automatically controlled according to the battery condition and the vehicle electrical consumption (load) at a specific time. Generally, when the engine runs at idle the battery discharges, and as the engine speed increases the battery is charged by the alternator. image
Car Alternator
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Major parts of an alternator image
Alternator Major Parts

Rotor (field coil)
The rotor generates magnetism as the current flows through it. When the rotor turns, electricity is generated in the stator.

Stator (armature coil)
The stator is attached to the body. As the rotor turns, the magnetic force passing through the stator changes, and this change causes an electric current to flow through the stator coil.

Diode assembly
The diodes are used to rectify the alternating current (AC) generated in the stator to obtain direct current (DC).

Brush and slip ring
Electricity is supplied to the rotating rotor through these parts.

The diode has comparatively low heat resistance and coil resistance decreases when cooled, so a fan is used to cool the diodes and coils in the alternator. The fan also cools the alternator with a built-in IC regulator.

The power from the engine is transmitted through the V-belt and the pulley to the rotor.

How can you test the alternator?
Check Alternator Belt and Terminal S Fuse
Check the alternator belt tension before starting the engine. Check the fuse of terminal S. When the ignition switch is ON, measure the voltage at terminal S with the terminal S connector disconnected. If it is 12V, the fuse and the wire from the fuse to terminal S are normal. image
Alternator Terminals

Test the Alternator Output Voltage
Measure the output voltage at alternator terminal B.
The battery voltage is usually about 12V. To charge the battery, higher voltage (about 13V to 15V) is required. If the alternator output voltage is 13V or less, the battery continues discharging, thus the battery runs down. image
Alternator Voltage Output

If the voltage is 13V or less, the alternator output is NG. As it is OK when the alternator output is 13V to 15V, measure the voltage at the positive terminal of the battery. If the battery voltage is 13V or less, there is a malfunction at terminal B or the wire harness. Check the connection status.
Vehicles with the “Variable voltage generation system” have different voltage values.

Inspection and disassembly of the alternator
Disassemble the alternator and inspect the internal part.
(1) Remove the front cover through bolts. image
Alternator Front Cover

(2) Before removing the front cover, warm the rear cover with a soldering iron as shown in the illustration. The bearing at the rear end of the rotor can be easily removed from the rear cover. Excessive heating may damage the IC regulator or the diode. Do not use a dryer. image

(3) By tapping the flange of the front cover with a plastic hammer, the front cover and the rotor can be removed as an assembly from the stator and the rear cover. image
Alternator Flange

(4) Loosen the screw behind the rear cover, the screw of the IC regulator and the nut of terminal B. Remove the stator assembly from the rear cover and check the following items. image
Alternator Rear Cover

4.1 Brush
Inspect the brush condition of the alternator shown by the arrow. Check if the brush moves smoothly. If it doesn't, clean the brush. Is the brush worn? If it is worn, replace it. Remove the brush from the holder on the IC regulator assembly with a soldering iron as shown in the illustration. image
Alternator Brush Inspection

In the illustration, the upper brush has been worn after being used for 130,000 km. The lower brush is a new one, and the arrow shows the wear limit line.

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Alternator Brush Condition

4.2 Test the alternator rotor
Test the alternator rotor using digital volt meter. Measure the resistance of the rotor at the slip rings. If it is not within the specified value, replace the rotor.

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Alternator Rotor Resistance Measurement

4.3 Insulation Test
Check the insulation resistance. Measure the resistance between the coil (slip ring) and the rotor body. Replace the rotor if continuity exists.

4.4 Slip Ring
Check the wear of the slip ring. If it is not within the specified value, replace the rotor.

4.5 Stator
Remove the diode assembly and check the following items.
Continuity test
No continuity: Replace the stator.
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Alternator Diode Continuity Test

Insulation test
Continuity exists: Replace the stator.
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Alternator Stator Insulation Test

Assemble alternator and diagnose alternator output
1. Install the stator coil assembly on the rear cover and push away the brush with a finger until it is not visible as shown in the photo.

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Alternator Stator Coil Assembly

2. While pushing away the brush with a finger, insert a pin of appropriate thickness into the pin hole near the bearing boss at the back of the rear cover, in order to hold the brushes.

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Alternator Brush Hole

3. The brushes are held by the pin so that the rotor can be easily installed. Do not forget to remove the pin after assembly. Be sure to check the bearing. If unusual noise is heard. After assembly, make sure that the pulley turns smoothly and quietly.

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Alternator Brush Hole

4. Diagnose diode. For the alternator, the diode can be diagnosed by measuring the output waveform as well. For measurement, the oscilloscope is used.
Measure the waveform with 0.1V/div and 2ms/div display mode. Since the alternator has 3-phase AC full wave rectification, 6 peaks are displayed for one cycle of the alternator output as shown in the photo.

If the diode(s) of the rectification circuit is (are) damaged, the waveform becomes unusual. If the output waveform is distorted, it becomes electric noise that affects the audio system and other electronic devices.

When diagnosing the alternator, measure the waveform. If an error is found, replace the diode. If the waveform is normal and the output voltage is excessively high, replace the IC regulator. If the output is low, check the brush, rotor and stator.

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