This article is from the July 2008 AC Delco TECH CONNECT bulletins
Generator (Mis)Diagnosis
Generators (or alternators) are always working under a load. Recharging the battery, running the fuel pump and ignition system, and powering a vehicle’s lights and electrical accessories places a substantial load on the generator, creating a lot of internal heat. Heat can damage the diodes and wiring leads as well as the rotor and stator windings and the brush connections. Most generators fail electronically before they wear out mechanically. However, bushings and brushes don’t last forever.
Here are a few checks to make when diagnosing a charging system failure.
Voltage Check
The output voltage produced by a generator, depending on temperature and load, is usually about 1-1/2 to 2 volts higher than battery voltage. At idle, most charging systems produce 13.8 to 15.3 volts without any lights or accessories on. Check the charging voltage by touching the test leads of a DVOM to the positive and negative battery terminals. If the meter is not auto ranging, set the scale to 20 volts and select DC. Take the reading with the engine idling.
Bad Diodes
Diodes in the generator convert alternating current (AC) to direct current (DC). To detect bad diodes, switch the DVOM to AC and check voltage. If all of the diodes are functioning properly, there should not be an AC voltage reading at the battery. If there is a reading, one or more diodes are leaking. Bad diodes often cause a visible fluctuation in the output voltage of the charging system, which may cause the vehicle lights to brighten, dim or flicker. Bad diodes also can allow current to leak from the battery back through the generator to ground, causing the battery to run down overnight. Diodes usually fail due to overheating or overloading. They may also be damaged by a short in the charging light indicator circuit or a poor connection between the generator output terminal and the battery positive terminal.
Poor Connections
A generator may not be working properly because of poor electrical connections in the charging circuit. Loose or corroded connections on the generator as well as broken or frayed wires inside a connector or wiring harness can increase resistance and restrict the current flowing through the circuit. Connections may pass a visual inspection, but must be tested to know if they’re making good electrical contact. Test the generator wiring connectors by checking for voltage drops across the connections when the engine is running. The voltage drop across any connection should be zero, or less than 0.1 volts. In addition, check for voltage drops at the battery cable connections, the generator BAT+ power connection and the engine ground straps. Voltage drops on the positive side of the charging circuit can cause undercharging. Voltage drops on the negative side can cause overcharging.
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