![]() ![]() The most vulnerable parts in an alternator are not the mechanical components, but the diodes in the rectifier. The PCM may boost the charging current at low rpm when loads are high, or reduce charging output to smooth idle quality or temporarily reduce the charging voltage to maximize engine power when accelerating at full throttle. But the PCM may alter the curve depending on what kind of charging strategy has been programmed into it. Current typically increases in proportion to the electrical load on the charging system and engine speed. The alternator’s current output (the number of amps it produces) should also be tested. If a vehicle has a history of repeat alternator failures, one of these may be the cause. But they may also be damaged by a short in the charging light indicator circuit or by a poor connection between the alternator output terminal (B+) and the battery positive terminal. Always refer to the vehicle manufacturer’s key-off electrical drain specifications if they’re available.ĭiodes usually fail as a result of overheating or overloading. Once all the modules go to sleep, the current drain drops significantly. But as a rule, the key-off current drain should usually be less than 50 milliamps one hour after the engine has been shut off and left undisturbed. The normal key-off current drain on a battery may be as high as 300 to 400 milliamps right after the engine is shut off. Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to run down overnight. Leaking or shorted diodes can often cause a visible fluctuation in the output voltage of the charging system. If you get a voltage reading, it means one or more diodes are leaking and the alternator needs to be replaced. If all of the diodes are doing their job, there should be no AC voltage reading at the battery. Switch your DVOM to AC and check the voltage again. Here’s another check few technicians do, but it’s one that can easily detect bad diodes in an alternator. Then, take your reading with the engine idling. If the DVOM is not auto ranging, set the scale to 20 volts and select DC. You can check the charging voltage by touching the test leads of a DVOM to the positive and negative battery terminals. At idle, most charging systems produce 13.8 to 15.3 volts with no lights or accessories on. The actual output voltage produced by the alternator will vary depending on temperature and load, and will usually be about 1.5 to 2 volts higher than battery voltage. Using a voltmeter to check for voltage drops across connections can reveal bad connections that may reduce the charging voltage. ![]() In cases where the powertrain control module (PCM) controls voltage regulation, a problem in the voltage regulation circuit means the PCM will have to be replaced. Since most late-model import alternators are internally regulated, a failure of the regulator means the alternator must also be replaced. ![]() ![]() Charging problems can be caused by electrical faults in the alternator or voltage regulator, poor wiring connections at the battery or alternator, or a slipping or broken drive belt. A low battery also may not have enough reserve power to crank the engine, so the vehicle can be stranded until the charging problem can be diagnosed and repaired. Once battery voltage drops below a certain threshold, the onboard electronics, ignition and fuel systems may stop working normally or cause the engine to stall. If the charging system fails for any reason or does not produce enough power to meet all of the vehicle’s electrical demands, the battery quickly runs down. These parts can be ruined if the alternator is mounted low on the engine and is subjected to road splash that contains salt water or debris. Even so, the shaft bushings and brushes are wear items that don’t last forever. Most alternators fail electronically long before they wear out mechanically. Normal charging voltage can vary from 13.5 to 14.5 volts or even higher depending on the battery’s state of charge, temperature and electrical load on the alternator. ![]()
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