The first indication of alternator trouble may be dim headlights or even an engine that is slow to crank (or will not crank). The alternator ensures the battery is charged, and supplies voltage for the entire electrical system. So if the alternator, voltage regulator or wiring that links the charging system to the battery and electrical system goes faulty, it can create serious issues.
Alternator charging issues 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 car shuts down.
Once battery voltage goes below a certain threshold, the onboard electronics, ignition and fuel systems may stop functioning normally and cause the engine to stall. The battery will not have enough reserve energy to restart the engine, so the car will be stranded until the issue can be diagnosed and repaired.
Recharging the battery or jump starting the battery with booster cables from another battery or car may get the engine running again, but it will not be for long if the charging system is not giving out normal voltage.
Warning: Never disconnect a battery cable while the engine is running to “test” your alternator. Doing so can generate a high voltage spike that may damage the alternator or other electronics.
WHAT IS ALTERNATOR CHARGING OUTPUT
The alternator is the heart of the charging system. It produces all the power required to keep the battery fully charged and to operate everything electrical in the car. The alternator is installed on the engine and is belt-driven off the crankshaft pulley by a serpentine belt or v-belt. The alternator generates 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 proportionally to the electrical load on the charging system and engine speed. Output is low at idle and increases with RPM. Maximum output is typically gotten at speeds above 2,500 RPM.
THE 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 cars, 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 generated 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 generate 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.
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The left photo indicates normal alternator charging voltage with the engine idling. The photo at the right indicates low charging voltage with the engine idling.
A low reading informs you the charging system is not generating enough voltage to keep the battery charged or to meet the car’s electrical needs.
HOW TO TEST ALTERNATOR CHARGING VOLTAGE
Most alternators that are charging normally should produce a voltage of about 13.8 to 14.2 volts at idle with the lights and accessories off. Always refer to the car manufacturer’s specifications. Many Asian cars, 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 required voltage.
The exact charging voltage will differ 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 reduce to 13.5 to 14.3 volts.
HOW TO TEST ALTERNATOR AMPERAGE OUTPUT
In addition to inspecting the alternator’s voltage output, you also need to check its current or amperage output. Amperage is how much current the alternator produces 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 car).
Charging output can be measured with an inductive amp probe clamped around the BAT (B+) wire that connects to the alternator. It can equally be measured on an alternator bench tester in a auto parts store.
Alternator power ratings can also be offered in Watts (which is volts times amps). Many alternators in foreign cars 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 has to be replaced. In fact, on some applications upgrading to a higher output replacement alternator may be recommended if the car has a history of alternator failures, or the car has a megawatt aftermarket sound system, emergency or off-road lighting, or other power-hungry electrical accessories.
High underhood temperatures are hard on alternators, and high electrical loads produce 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 operating hard under a heavy load at low RPM (especially during hot weather), there may not be enough cooling to keep 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 cars where the location of the alternator restricts airflow and cooling.
BAD ALTERNATOR WIRING CONNECTIONS
The alternator may be made 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, lowers the flow of current through the charging circuit.
The electrical system is, after all, just a big series of loops that convey 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 often the car 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 because of low charging output and alternator failure.
ALTERNATOR DIODE FAILURES
One of the most common causes of charging issues 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 refered to as 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 equally 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 often 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 is started. If the alternator stops charging, current flows though the light circuit from the positive side making 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 lower 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 changed 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 inspect the entire circuit.
ALTERNATOR CIRCUIT VOLTAGE DROP TESTS
With the engine idling, touch one test lead of your voltmeter to the battery positive (+) post, and the other test lead to the BAT (B+) terminal on the alternator. Normally, the voltmeter should read less than 0.2 volts.
If you observe a voltage reading greater than 0.2 volts, it means there is excessive resistance somewhere in the circuit causing a voltage drop in the wiring circuit. Check all the wiring connections (use electronics cleaner to clean connections), and make sure the terminal connectors on the ends of the wires are clean and tight.
A negative side ground circuit test is made by touching one voltmeter test lead to the alternator housing, and the other test lead to the negative battery post (not the terminal clamp) with the engine running and charging system loaded. If good, the voltage drop should be 0.2 volts or less. If higher, inspect and clean all ground connections as needed. Equally, check for broken, loose or missing ground straps between the engine and body.
If the alternator output circuit and ground circuits test good (voltage drop less than 0.2 volts) and the car has a history of repeated alternator failures due to burned out diodes, inspect for a shorted indicator light terminal.
Checking for voltage drops in a circuit is a good way to find hidden problems that may be causing a charging problem. Voltage drop tests must be carried out while the engine is idling with a charging load on the system. In other words, there must be voltage flowing through the circuit for the voltage drop test to detect a problem. Voltage always follows the path of least resistance, so if the connection being tested has too much resistance some of the voltage will flow through the voltmeter and create a small voltage reading.
WHAT ARE ALTERNATOR VIBRATIONS
Loose alternator mounting bolts and brackets can create 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 inspected when changing a serpentine belt).
A cycling buzzing noise may show 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 changed.
ALTERNATOR CHANGE TIPS
Have your old alternator bench tested. Alternators have one of the highest warranty return rates of any parts on a vehicle. Many units are returned needlessly either because of faulty diagnosis (there was nothing wrong with the original unit or the replacement unit), or due to an overlooked problem caused a repeat failure. One way to lower this problem is to take your old alternator to an auto parts store with an alternator bench tester and have it tested BEFORE you buy a replacement. If the old alternator tests bad, you need to change it. But if it tests good, the problem is something else in the charging system.
Most auto parts stores have an alternator bench tester. Have your old unit tested to see if it is good or bad.
If your alternator tests good, the issue is not a bad alternator but something else.
Have the NEW alternator bench tested, too. For added insurance, you might also ask the component store to bench test the new or reman alternator they are selling you to make sure it is charging properly. Better to catch a defective unit at the store than after you have mounted it on your car.
Check the wiring harness and terminals. One way to lower the risk of premature failures and unnecessary warranty returns is to always check for resistance (voltage drops) in the charging circuit connections. This includes both the positive and negative battery cable connections, alternator power circuit and ground circuit as just described.
Voltage drops on the positive side can produce undercharging.
Voltage drops on the negative side can create overcharging (fools the voltage regulator into thinking the battery is low).
Use a battery charger to recharge the battery. Alternators are made to maintain battery charge, not to recharge a dead battery. If the battery is run-down or dead, therefore, it should be recharged with a battery charger before the car is driven, or before a replacement alternator is mounted. This will lower the stress on the charging system and lower the risk of overheating and failure.
Test the battery to make sure it is still good. The condition of the battery should always be tested if it fails to hold a charge or a charging issue is suspected. The problem may be an old battery that needs to be changed, not a bad alternator.
Get the correct alternator pulley.Make sure the pulley on the replacement alternator is the same as the one on the old unit. Many late model alternators are now equipped with an overrunning pulley decoupler that lets the alternator to momentarily disengage from the belt drive where there are sudden changes in belt speed. This lower noise and harshness, and prolongs the life of the serpentine belt. If a replacement alternator with an ordinary direct drive pulley is mounted, it could lead to premature belt failure.
Replace the serpentine belt. If the serpentine belt has more than 50,000 miles on it, throw it away and replace it with a new one.
Inspect the automatic belt tensioner. If the automatic belt tensioner is rusted, weak or stuck, it won’t keep the proper tension on the serpentine belt, letting it to slip.
MORE ALTERNATOR CHARGING CHECKS
* On some GM cars, a voltage drop of up to 0.5 volts on the positive side may be acceptable. Check service specifications.
* If a battery keeps running down and the charging system seems to be functioning normally, the issue may be a higher than normal parasitic electrical drain on the battery when the key is off. On most car, the normal drain on the battery should be 50 milliamps or less. But on some late model Fords, the normal drain may 300 to 400 milliamps with a few drawing as much as 850 milliamps for up to an hour after the engine is shut off (the modules are in stand-by mode during this period). After all the modules shut down, though, the current drain on the battery should drop to 50 milliamps or less.
- Peak loads and prolonged idle conditions can result in battery discharge, since the alternator cannot keep up with the power usage. Idling for long periods of time with the lights, defrosters, heater, radio on can pull more amps out of the battery than the charging system can put back into it. You may think you have a charging problem, but there is nothing wrong with the alternator.