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.

 

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.

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.

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.

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 through the diode trio and out of the alternator. This heavier than normal current flow through the diodes will cause them to overheat and fail. Consequently, if you have replaced an alternator before because of bad diodes, and the replacement fails for the same reason, there is likely a bad connection or open circuit between the alternator BAT (B+) terminal and the positive side of the battery circuit. Do a voltage drop test to check the entire circuit.

Loose alternator mounting bolts and brackets can cause vibrations which may damage the alternator. A bad belt tensioner can also be another source of damaging vibrations (which is why the tensioner should always be checked when changing a serpentine belt).

A cycling buzzing noise may indicate an alternator bearing failure, or a bad diode that is allowing current to flow in the wrong direction. Either way, the alternator will have to be rebuilt or replaced.

Does your car battery contain enough power to start your engine during cold weather. The only way to know if your battery is fully charged or not is to measure the battery's state of charge with a voltmeter. You can use an analog or digital voltmeter, but a digital meter is easier to read and will give you a more accurate reading.
First, make sure the ignition key is OFF, and all the vehicle's lights are OFF.

To check your battery's state of charge, connect the RED or POSITIVE voltmeter test lead to your battery POSITIVE terminal. The positive terminal will be marked with a PLUS symbol (+), and the battery cable that is connected to the positive terminal is often color coded RED.
Connect the BLACK or NEGATIVE voltmeter test lead to your battery NEGATIVE terminal. The negative terminal will be marked with a minus symbol (-), and the battery cable that is connected to the negative terminal is often color coded BLACK.
Note the reading on your voltmeter and refer to the chart below:
Battery Voltage and State of Charge:

12.66v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%
(NOTE: these readings are at 80 degrees F. Battery voltage readings will drop with temperature roughly 0.01 volts for every 10 degrees F.)
(At 30 degrees F. a fully charged battery will measure about 12.588 volts, and at zero degrees F it will measure about 12.516 volts.)

The photo at the top of this page shows a battery voltage reading of 12.29 volts, which means the battery being tested is low with only about half a charge. The battery needs to be recharged.

Is Your Battery Low?

If your battery voltage is less than 12.45 volts (75 percent charged), it is low and should be recharged. This can be done by connecting a portable battery charger to your battery, or by driving your car for 15 to 20 minutes at 40 mph or faster.
Automotive lead-acid batteries should be maintained at a 75 percent charge level or higher for best performance and life. If the battery is allowed to run down and is not brought back up to 75 percent or higher charge within a few days, the battery may be permanently damaged. Sulfation can prevent the cell plates inside the battery from accepting a full charge. Over time, this will lead to diminished battery performance and life.

Why You Need a Good Battery for Reliable Cold Weather Starting

A good battery is essential for reliable starting, especially during cold weather because cold weather increases the cranking load on the battery. Oil gets thicker at low temperatures so it takes more amps to crank a cold engine when you try to start it. At 0 degrees F, the number of cranking amps it takes to start a cold engine may increase as much as 2X. At minus 15 degrees below zero F. , it can take 3X or more amps to crank the engine depending on the viscosity of the oil in the crankcase. The thicker the oil, the harder is it to crank the engine.

t the same time, cold temperatures also sap the battery's ability to supply amps. At 0 degrees F, most batteries can only deliver about 65% of their normal cranking amps. At -20 degrees, battery power is cut in half!
Warning: Do NOT attempt to recharge your battery if it has run down and the liquid inside is frozen. This may cause the battery to explode! Remove the battery and take it inside so it can thaw before recharging or testing it.

Is Your Battery Good or Bad?

A GOOD battery is one that will accept and hold a charge, and is capable of producing close to its rated amperage output. A BAD battery is one that will NOT accept or hold a charge, or cannot produce adequate cranking amps. A GOOD battery can be recharged and returned to service but a BAD battery needs to be replaced.

Most car batteries only last about 4 to 5 years, so if your battery is 4 or more years older and is not holding a charge (keeps running down), or it does not seem to crank your engine a normal speed, you probably need a new battery.

A low or dead battery does not mean your battery has failed, or that it needs to be replaced. A good battery can run down for any number of reasons: somebody left the lights on, you haven't been driving your vehicle enough to keep the battery fully charged, your vehicle has been sitting for a long period of time without being started, there is a problem with the charging system or alternator, or an electrical problem is draining power from the battery when your car is off.

Battery Testing

The only way to know if your battery is GOOD or BAD is to test it. Many auto parts stores will test your battery for free. If your vehicle is drivable or you can get it going with a jump start, drive to a nearby auto parts store that offers free testing and have them test your battery and charging system. If you can't get your car started, remove the battery and get a friend to give you a ride to the auto parts store so you can have the battery tested. Many repair shops will also test your battery and charging system, but they usually charge a fee for this service (some will test your battery for free or offer to apply their diagnostic fee towards the cost of repair).

CAUTION: Conventional wet cell car batteries are filled with a mixture of water and sulfuric acid. Wear gloves and handle the battery with care so no liquid spills on your skin or clothing. Battery acid can cause severe burns. If a spill does occur, wash with plenty of water and neutralize the acid by applying baking soda.

There are essentially two ways to test a battery. The "old fashioned way" is to use a Load Tester. For accurate results with a load tester, the battery must first be recharged before it is tested. The tester applies a calibrated load to the battery (typically half the battery cold cranking amp [CCA] capacity or three times its amp/hour rating). While the load is applied, the tester monitors battery voltage. If the battery voltage drops below 9.6 volts during the test, the battery is BAD and needs to be replaced. If the voltage remains about 9.6 volts, the battery is GOOD and can be returned to service.

The other (and must faster) method for testing your battery is to use an electronic "conductance" tester like the one shown here. A conductance tester sends a alternating frequency signal through the battery to determine the condition of the cell plates inside the battery. As a battery ages, its internal conductance declines. Shorts, opens and other cell defects also reduce conductance, so measuring conductance gives an accurate indication of battery condition. The best feature of this type of test is that the battery does NOT have to be recharged prior to testing. Most conductance testers will give an accurate reading even if the battery is almost dead.

Some electronic battery testers can also analyze the battery's CCA capacity, which can be used to estimate the battery's remaining service life. Some testers can also measure the amps drawn by the starter while cranking the engine, and analyze charging system output under load once the engine is running. Some testers even provide a built-in voltmeter for checking connections.