One reason why alternators have a high failure rate is because they’re always working under a load. Generating electricity to recharge the battery, run the fuel pump, injectors and ignition system, and power all of the vehicle’s lights and other electrical accessories places a substantial load on the alternator that generates a lot of internal heat. This can burn out the diodes in the back of the alternator that convert alternating current (AC) to direct current (DC) — especially in vehicles that spend a lot of time idling with high electrical loads. Heat can also damage the rotor and stator windings, brush connections and wiring leads.
Most alternators fail electronically long before they wear out mechanically. Even so, the shaft bushings and brushes are also wear items that don’t last forever. These parts can also be ruined if the alternator is mounted low on the engine and is subjected to road splash that contains salt water or debris.
On 1994 to 2004 Mitsubishi Montero SUVs with a 3.0L V6 engine, the alternator is mounted directly under the power steering pump. On these vehicles, the alternators have a higher-than-normal failure rate due to contamination by power steering fluid. If you’re replacing a bad alternator on one of these vehicles, be sure to inspect the power steering pump and hoses for leaks — and fix any leaks that are found before installing the new alternator.
FAILURE SYMPTOMS
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. 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. A low battery also may not have enough reserve power to crank the engine, so the vehicle will be stranded until the charging problem can be diagnosed and repaired. 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. Since most late-model import alternators are internally regulated, a failure of the regulator means the alternator must also be replaced. 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.
VOLTAGE & CURRENT CHECKS
The actual output voltage produced by the alternator will vary depending on temperature and load, and will usually be about 1-1/2 to 2 volts higher than battery voltage. At idle, most charging systems produce 13.8 to 15.3 volts with no lights or accessories on. You can check the charging voltage by touching the test leads of a DVOM to the positive and negative battery terminals. If the DVOM is not auto ranging, set the scale to 20 volts, and select DC. Then take your reading with the engine idling.
Here’s another check few technicians do, but it’s one that can easily detect bad diodes in an alternator. Switch your DVOM to AC and check the voltage again. If all of the diodes are doing their job, there should be no AC voltage reading at the battery. If you get a voltage reading, it means one or more diodes are leaking and the alternator needs to be replaced. Leaking or shorted diodes can often cause a visible fluctuation in the output voltage of the charging system. This may cause the headlights to brighten and dim, or the instrument lights to flicker.
Bad diodes can also allow current to leak from the battery back through the alternator to ground, causing the battery to rundown overnight. 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. 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. Once all the modules go to sleep, the current drain drops significantly. Always refer to the vehicle manufacturer’s key-off electrical drain specifications if they’re available.
Diodes usually fail as a result of overheating or overloading. But they may have been damaged by a short in the charging light indicator circuit, or a poor connection between the alternator output terminal (B+) and the battery positive terminal. If a vehicle has a history of repeat alternator failures, one of these may be the cause.
