The first sign of alternator trouble may be dim headlights or an engine that is slow to crank (or will not crank). The alternator keeps the battery charged, and supplies voltage for the entire electrical system. So if the alternator, voltage regulator or wiring that connects the charging system to the battery and electrical system goes bad, it can create serious problems. Alternator charging problems can be caused by electrical faults in the charging system itself, by poor wiring connections at the battery or elsewhere, or by a slipping or broken drive belt. If there is no charging output, the battery will quickly discharge. You may have 20 minutes to an hour of driving time before everything goes dead and the vehicle shuts down. Once battery voltage drops below a certain threshold, the onboard electronics, ignition and fuel systems may stop working normally and cause the engine to stall. The battery will not have enough reserve power to restart the engine, so the vehicle will be stranded until the problem can be diagnosed and repaired. Recharging the battery or jump starting the battery with booster cables from another battery or vehicle may get the engine running again, but it will not be for long if the charging system is not producing normal voltage. Warning: Never disconnect a battery cable while the engine is running to "test" your alternator. Doing so can produce a high voltage spike that may damage the alternator or other electronics. ALTERNATOR CHARGING OUTPUT The alternator is the heart of the charging system. It generates all the power needed to keep the battery fully charged and to operate everything electrical in the vehicle. The alternator is mounted on the engine and is belt-driven off the crankshaft pulley by a serpentine belt or v-belt. The alternator produces alternating current (AC), which is converted to direct current (DC) by a six diode rectifier, which is usually located inside the back of the unit. Diodes only pass current in one direction, which is how they convert AC current to DC. Three positive diodes control the positive side of the AC sine wave, while three negative diodes control the negative side. The alternator's charging output increases in proportion to the electrical load on the charging system and engine speed. Output is low at idle and increases with RPM. Maximum output is typically achieved at speeds above 2,500 RPM. ALTERNATOR VOLTAGE REGULATION Charging output of the alternator is controlled by a voltage regulator which may be mounted inside or on the back of the alternator (internally regulated), or somewhere else under the hood (externally regulated). On most newer vehicles, the powertrain control module (PCM) regulates charging output. On older vehicles, the voltage regulator was electro-mechanical and used magnetic contacts to control the charging output of the alternator. Since the 1980s, most voltage regulators are solid-state electronic and use transistors to control charging output. The actual output voltage produced by the alternator will vary depending on temperature and load, but will typically be about 1-1/2 to 2 volts higher than battery voltage. At idle, most charging systems will produce 13.8 to 14.3 volts with no lights or accessories on (although some may charge at a slightly higher voltage depending on temperature, engine RPM, type of battery, and the battery's state of charge). This can be measured by connecting the positive (+) and negative (-) test leads of a voltmeter to the battery posts while the engine is running. . . . . The left photo shows normal alternator charging voltage with the engine idling. The photo at the right shows low charging voltage with the engine idling. A low reading tells you the charging system is not generating enough voltage to keep the battery charged or to meet the vehicle's electrical needs. HOW TO TEST ALTERNATOR CHARGING VOLTAGE Most alternators that are charging properly should produce a voltage of about 13.8 to 14.2 volts at idle with the lights and accessories off. Always refer to the vehicle manufacturer's specifications. Many Asian vehicles, for example, have higher charging voltages of around 15 volts. When the engine is first started, the charging voltage should rise quickly to about two volts above base battery voltage, then taper off, leveling out at the specified voltage. The exact charging voltage will vary according to the battery's state of charge, the load on the vehicle's electrical system, and temperature. The lower the temperature the higher the charging voltage, and the higher the temperature the lower the charging voltage. The "normal" charging voltage on a typical application might be 13.9 to 15.1 volts at 77 degrees F. But at 20 degrees F. below zero, the charging voltage might jump as high as 14.9 to 15.8 volts for a short period of time. On a hot engine on a hot day, the normal charging voltage might drop to 13.5 to 14.3 volts. HOW TO TEST ALTERNATOR AMPERAGE OUTPUT In addition to checking the alternator's voltage output, you also need to check its current or amperage output. Amperage is how much current the alternator generates at a specified voltage and speed. Not long ago, an 80 amp alternator was considered a high output unit. Most late model alternators produce 120 to 155 amps or more. Current output increases with engine speed, from around 20 to 50 amps at idle up to the unit's maximum output at 2,500 RPM or higher (refer to a service manual for the exact charging output specifications for your vehicle). Charging output can be measured with an inductive amp probe clamped around the BAT (B+) wire that connects to the alternator. It can also be measured on an alternator bench tester in a auto parts store. Alternator power ratings can also be given in Watts (which is volts times amps). Many alternators in foreign vehicles are rated in watts rather than amps. The important point here is to make sure a replacement alternator has the same power rating (in amps or watts) as the original so the charging system can maintain the same power output as before, should the alternator need to be replaced. In fact, on some applications upgrading to a higher output replacement alternator may be recommended if the vehicle has a history of alternator failures, or the vehicle has a megawatt aftermarket sound system, emergency or off-road lighting, or other power-hungry electrical accessories. ALTERNOR DIAGNOSIS CHARTS ALTERNATOR OVERHEATING High underhood temperatures are hard on alternators, and high electrical loads create even more heat. The higher the charging load on the alternator, the hotter it runs. To control the heat, alternators have an internal and/or external fan that pulls air through the housing to help cool the "rotor" (the rotating part inside the alternator) and the "stator" (the stationary field coils or windings that surround the rotor). Some high output units have two fans to increase cooling. If the alternator is working hard under a heavy load at low RPM (especially during hot weather), there may not be enough cooling to prevent the unit from overheating. Excessive heat may damage the windings and/or wiring connections inside the unit, causing it to fail. This tends to be more of a problem on vehicles where the location of the alternator restricts airflow and cooling. BAD ALTERNATOR WIRING CONNECTIONS The alternator may be forced to work harder than normal if the battery cables, ground straps or other electrical connections in the charging circuit are dirty or loose. A poor connection increases resistance and causes a voltage drop across the connection. This, in turn, reduces the flow of current through the charging circuit. The electrical system is, after all, just a big series of loops that carry current from the charging system to the battery, and from the battery to all of the vehicle's electrical accessories and electronics. The return path is usually the vehicle body, which serves as the main ground circuit for almost everything. All the power supply and ground connections must therefore be in excellent condition to minimize resistance and the load on the charging system. In fact, poor ground connections are an often overlooked cause of low charging output and alternator failure. ALTERNATOR DIODE FAILURES One of the most common causes of charging problems is the failure of one or more diodes in the alternator. Alternators have six diodes (three negative and three positive) that convert the alternating current (AC) to direct current (DC). They are called a diode trio because each negative diode is paired with a positive diode. When the engine is running, charging current from the alternator flows through the diode trio via the BAT (B+) connection on the back of the alternator. A little current also flows through the charging light indicator circuit. On GM alternators, the indicator light circuit is terminal 1. On European alternators, the indicator light circuit is usually called 61 or D+. On Asian alternators, it is usually labeled L. This terminal leads to the ground side of the alternator warning light. When the alternator is charging, the diode trio supplies voltage to the ground side of the indicator light. This offsets the battery voltage applied to the positive side of the light, causing the light to go out once the engine starts. If the alternator stops charging, current flows though the light circuit from the positive side causing the charging system warning light to come on. If one of the diodes fails, it may cause the charging system indicator light to glow dimly. If two or more diodes fail, the light will get brighter. At the same time, the feedback current from the diode trio will reduce the alternators ability to produce current. So the more diodes that fail, the less power the alternator will generate. A bad connection or open circuit between the alternator output terminal and the positive battery terminal will force the charging current to follow a parallel route t…

Fonte: AA1Car.com