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How to Diagnose the Alternator

Model: (Various brands) Various
Part: Alternator

Video tutorial

To perform this diagnosis, ensure the battery and belt are in good condition. Connect a voltmeter to the battery and measure the resting voltage.

Turn on the ignition and check that the engine warning light and battery light illuminate; verify that they turn off when the engine starts. The voltage on the multimeter should be above 14 volts.

For subsequent measurements, switch on two high-power consumers, such as the interior fan and the rear window heater. At this point, the voltage indicated on the voltmeter should not be less than 13.5 volts; if you accelerate, it should rise above 14 volts.

If these values are not reached, check the connections to the battery. The voltage drop must not exceed 0.1 volts.

Measure the generator voltage between the terminal and its ground. Here, the voltage must not exceed 0.3 volts compared to the battery voltage; otherwise, locate the cable where the voltage drop occurs, then find the exact point of the drop.

Once the fault is found, repair the connection and repeat the measurement.


Updated on 19 June 2026 by ScegliAuto


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by ScegliAuto

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