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How to Testing Your Alternator and Avoiding Costly Repairs

Dim headlights or a slow or unresponsive engine may be the initial indication of alternator problems. The alternator plays a crucial role in keeping the battery charged and supplying the voltage for the entire electrical system. If the charging system’s alternator, voltage regulator, or the wiring connecting it to the battery and electrical system malfunctions, it can lead to severe consequences.

A lack of charging output can be caused by electrical issues in the charging system, inadequate wiring connections at the battery or other locations, or a broken or slipping drive belt. In such situations, the battery will discharge quickly, and the vehicle may function for only a short time, usually between 20 minutes and an hour, before shutting down entirely.

If the battery’s voltage drops below a specific threshold, the onboard electronics, fuel systems, and ignition may cease working correctly, resulting in engine stalling. Since the battery will not have enough reserve power to restart the engine, the vehicle will be immobilized until the issue is diagnosed and fixed.

Checking The Alternator

If you receive a low reading, it indicates that the charging system cannot produce sufficient voltage to maintain the battery’s charge or fulfill the vehicle’s electrical requirements.

While jump-starting the battery with booster cables or recharging the battery can restore engine functionality, the relief is temporary if the charging system fails to produce standard voltage.

It’s critical to note that disconnecting a battery cable while the engine is running to examine the alternator is never a good idea since it can generate a high voltage spike that may damage the alternator or other electronics.

Output Of Alternator Charging System

The alternator functions as the heart of the charging system, providing power to maintain the battery’s charge level and operate all electrical components in the vehicle. It is installed on the engine and driven by a serpentine or v-belt attached to the crankshaft pulley. The alternator generates alternating current (AC), which is converted to direct current (DC) through a six-diode rectifier at the back of the unit. Diodes enable the flow of current in one direction, enabling the conversion of AC to DC. Three positive and negative diodes control the positive and negative sides of the AC sine wave.

The alternator’s charging output varies with the charging system’s electrical load and engine speed. At idle, the output is low and increases with RPM, reaching maximum output at speeds above 2,500 RPM.

Alternator Voltage Regulation

Voltage regulation of the alternator charging output is accomplished through a voltage regulator, which may be internally mounted on the back of the alternator or externally mounted somewhere under the hood. The powertrain control module (PCM) regulates the charging output in most new vehicles. On older vehicles, the voltage regulator was electro-mechanical and used magnetic contacts to regulate the charging output of the alternator. Since the 1980s, most voltage regulators have been solid-state electronic and utilize transistors to control charging output.

The actual output voltage produced by the alternator may differ due to temperature and load but generally is about 1.5 to 2 volts higher than the battery voltage. Most charging systems produce 13.8 to 14.3 volts when idling with no lights or accessories on (although the voltage may slightly differ based on temperature, engine RPM, battery type, and the battery’s state of charge). To determine the output voltage, connect a voltmeter’s positive and negative test leads to the battery posts while the engine is running.

How To Testing Your Alternator

Testing The Charging Voltage Of An Alternator

To check if your alternator is charging properly, you should measure the voltage output using a voltmeter. While the engine is running and the lights and accessories are off, the voltage should be around 13.8 to 14.2 volts for most vehicles. However, it would be best if you always referred to the manufacturer’s specifications, as some Asian vehicles may have higher charging voltages of approximately 15 volts.

When you start the engine, the charging voltage should rise about two volts above the battery’s base voltage, then gradually decrease and stabilize at the specified voltage level. The actual charging voltage will depend on factors such as the battery’s state of charge, the electrical load on the vehicle, and the temperature. For instance, the colder the temperature, the higher the charging voltage, while a hotter temperature will lower the charging voltage. On an average day at 77 degrees F, the normal charging voltage might range from 13.9 to 15.1 volts. However, in extremely cold temperatures of 20 degrees F below zero, the charging voltage might briefly increase to 14.9 to 15.8 volts. Conversely, the normal charging voltage might drop to 13.5 to 14.3 volts on a hot day with an overheated engine.

How To Test Alternator Amperage Output

It’s necessary to measure its voltage output and amperage output, which refers to the amount of current the alternator generates at a specific voltage and speed to take the test alternator properly. While an 80 amp alternator was once considered high-output, most current models generate 120 to 155 amps or more. The amperage output ranges from 20 to 50 amps at idle and reaching and will increase with engine speed at maximum output at 2,500 RPM or above (refer to the vehicle’s service manual for exact charging output specifications).

To measure the charging output, you can use an inductive amp probe that clamps around the BAT (B+) wire connected to the alternator or use an alternator bench tester at an auto parts store.

Some alternator power ratings are given in watts (calculated by multiplying volts by amps). Some foreign vehicles may rate their alternators in watts instead of amps. It’s important to ensure that the replacement alternator has the same power rating (in amps or watts) as the original to maintain the same power output for the charging system. In some cases, upgrading to a higher output replacement alternator may be necessary, when some cases such as:

  • The vehicle has a history of alternator failures.
  • Has high-power aftermarket electrical accessories like sound systems
  • Emergency or off-road lighting
  • Or other power-hungry electrical components.

Charts For Diagnosing Alternator Problem.

Charts For Diagnosing Alternator Problem

  • Problem:The warning lamp does not light, with the ignition ON and engine off.
    Possible Cause:
    1. Blown fuse
    2. Lamp burned out
    3. Wiring connections lose
    4. Defective relay
    5. Defective regulator
    1. Check CHARGE, IGN, AND ENGINE fuses; replace as needed after correcting the cause
    2. Replace the lamp
    3. Check voltage drop in a circuit, tighten loose connections
    4. Check relays, if used, for continuity and proper operation
    5. Check the alternator output
  • Problem: Warning lamp does not go out with the engine running: battery overcharged or undercharged.
    Possible Cause:
    1. Loose or worn drive belt
    2. Defective battery or battery connections
    3. Blown fuse or fusible link
    4. Defective relay, regulator, or alternator
    5. Defective wiring
    1. Check the drive belt: adjust or replace as needed after correcting the cause
    2. Check the battery and its connections
    3. Check fuse and fusible link; replace as needed
    4. Check charging system outpu1and component operation, as needed
    5. Check voltage drop
  • Problem: Noise
    Possible Cause:
    1. Loose or worn drive belt
    2. Worn alternator bearings
    3. Defective diode
    1. Check drive belt: adjust or replace, as needed
    2. Replace the alternator
    3. Replace the alternator

Alternator Diagnosis Chart

Alternator Diagnosis Chart

Symptom: The alternator fails to charge (the indicator light may be on)
Possible Causes:

  • Slipping or broken alternator belt
  • Open diodes
  • Open circuit in stator windings
  • Open charging circuit
  • Open field circuit
  • Worn brushes or slip rings
  • Defective voltage regulator

Symptom: Low charging rate (battery run down)
Possible Causes:

  • Slipping alternator belt
  • Corroded/loose battery terminals
  • High resistance in charging circuit
  • Poor alternator/regulator ground
  • Open or grounded stator windings
  • Shorted or open alternator diodes
  • Defective regulator
  • Poor regulator ground (A-circuit)
  • No positive voltage to the regulator (B-circuit)

Symptom: Excessive charging rate (battery may have low water)
Possible Causes:

  • The regulator is not properly grounded (B-circuit alternators)
  • No positive voltage to the regulator (A-circuit alternators)
  • The regulator set too high or the contacts stuck (older mechanical regulators)
  • Defective regulator
  • Grounded alternator field circuit

Symptom: Battery fails to hold the charge (or battery dead)
Possible Causes:

  • Low charging rate (or battery dead)
  • Voltage drain on the battery when the ignition is off
  • Surface corrosion on the battery
  • Sulfated battery plates
  • Low or contaminated electrolyte
  • Damaged battery cells

Symptom: Alternator noise
Possible Causes:

  • Bad shaft bearing
  • Open or shorted diodes
  • Open or shorted stator windings
  • Mechanical interference or rubbing
  • Loose or bent pulley

Alternator Overheating

Overheating the alternator can occur when high underhood temperatures and electrical loads cause excessive heat generation. The charging load on the alternator is directly proportional to the heat it generates. Alternators have an internal or external fan that draws air through the housing to cool the rotor and stator. Some high-output units may even have two fans for additional cooling.

In situations where the alternator works strenuously under a heavy load at low RPM, particularly in hot weather, the cooling may not be sufficient to prevent overheating. This can cause damage to the windings and/or wiring connections inside the alternator, leading to eventual failure. Alternators located in areas with restricted airflow and cooling are more susceptible to this problem.

Bad Alternator Wiring Connections

If the battery cables, ground straps, or other electrical connections in the charging circuit are loose or dirty, the alternator may be compelled to work harder than normal. A dirty or loose connection increases resistance, resulting in a voltage drop across the connection. This voltage drop, in turn, reduces the current flow through the charging circuit.

The electrical system in a vehicle is essentially a large series of loops that transport current from the charging system to the battery, as well as from the battery to all of the vehicle’s electrical accessories and electronics. The vehicle body is the main ground circuit for almost everything, forming the return path. As a result, all power supply and ground connections must be in exceptional condition to minimize resistance and the load on the charging system. Poor ground connections are often overlooked as a cause of low charging output and alternator failure.

Alternator Diode Failures

The failure of one or more diodes in the alternator is one of the most common reasons for charging issues. Alternators have six diodes, three negative and three positive, that transform alternating current (AC) into direct current (DC). This group of diodes is known as the diode trio, with each negative diode matched with a positive diode.

During engine operation, the charging current from the alternator passes through the diode trio via the BAT (B+) connection on the back of the alternator. Some current also flows through the charging light indicator circuit. In GM alternators, the indicator light circuit is terminal 1, while in European alternators, it is usually referred to as 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, which offsets the battery voltage applied to the positive side of the light, causing the light to turn off after the engine starts. When the alternator stops charging, current flows through the light circuit from the positive side, illuminating the charging system warning light.

If one diode fails, it may cause the charging system indicator light to glow faintly. If two or more diodes fail, the light will become brighter. Simultaneously, the feedback current from the diode trio will decrease the alternator’s capacity to generate current. Thus, the more diodes that fail, the less power the alternator will produce.

A poor connection or an open circuit between the alternator output terminal and the positive battery terminal can force the charging current to follow an alternative path through the diode trio and out of the alternator. This elevated current flow through the diodes will cause them to overheat and eventually fail. As a result, if an alternator has been replaced previously due to diode issues and the new alternator fails for the same cause, there is probably a poor connection or an open circuit between the alternator’s BAT (B+) terminal and the positive side of the battery circuit. A voltage drop test on the entire circuit should be performed to verify this.

Testing Voltage Drop On Alternator Circuit

To test for excessive resistance in the charging circuit, perform a voltage drop test by connecting one test lead of your voltmeter to the battery positive (+) post and the other test lead to the BAT (B+) terminal on the alternator while the engine is idling. A reading of fewer than 0.2 volts is ideal. Suppose the voltmeter reading is greater than 0.2 volts. In that case, it indicates excessive resistance in the circuit. You should inspect all wiring connections and clean the terminal connectors with an electronics cleaner to ensure they are clean and tight.
Checking Voltage Drop
To test the negative side ground circuit, connect one voltmeter test lead to the alternator housing and the other to the negative battery post (not the terminal clamp) while the engine is running and the charging system is loaded. The ground circuit is good if the voltage drop is 0.2 volts or less. If the voltage drop is higher, inspect and clean all ground connections as needed, and check for broken, loose, or missing ground straps between the engine and body.

Check for a shorted indicator light terminal if the alternator output and ground circuits test well, and the vehicle has a history of repeated alternator failures due to burned-out diodes.

To detect hidden problems causing a charging problem, perform voltage drop tests while the engine is idling with a charging load on the system. Voltage follows the path of least resistance, so if the tested connection has too much resistance, some of the voltage will flow through the voltmeter and create a small voltage reading.

Alternator Vibrations

Vibrations caused by loose alternator mounting bolts and brackets can potentially harm the alternator. Additionally, a defective belt tensioner can also generate damaging vibrations, so it’s recommended to inspect the tensioner when replacing a serpentine belt.

If you hear a buzzing sound that comes and goes, this could indicate a problem with the alternator bearing or a faulty diode permitting the current to travel in the wrong direction. In either case, the alternator will need to be repaired or replaced.

Alternator Replacement Tips

Starter And Alternator Tester Bench

Consider having your old alternator tested on a bench before buying a replacement. Alternators have a high warranty return rate due to unnecessary returns from faulty diagnoses or overlooked problems causing repeat failures. To avoid this issue, take your old alternator to an auto parts store with a bench tester and test it before purchasing a replacement. If it tests poorly, then you need to replace it. However, if it tests well, the issue may be something else in the charging system.

To ensure added reliability, it is recommended to bench-test the new alternator before installation. You can take the alternator to an auto parts store with an alternator bench tester to verify if it is charging properly. This can help prevent potential issues that may arise after installation.

To reduce the likelihood of premature failures and warranty returns, inspecting the wiring harness and terminals for any resistance (voltage drops) in the charging circuit connections is important. This includes examining both the positive and negative battery cable connections and the alternator’s power and ground circuits.

Insufficient charging can result from voltage drops on the positive side.

Excessive voltage drops on the negative side can trick the voltage regulator into believing that the battery is depleted, leading to overcharging.

Before replacing the alternator or driving the vehicle, recharge the battery with a charger if it is dead or run down. This helps to avoid overheating and failure of the charging system.

Always test the battery condition if there is a charging problem or if it fails to hold a charge. It might be an old battery that needs replacement rather than a bad alternator.

Ensure that the replacement alternator has the correct pulley. Late-model alternators may have an overrunning pulley decoupler that momentarily disengages the alternator from the belt drive to reduce noise and harshness and extend the life of the serpentine belt. If a replacement alternator with a standard direct drive pulley is installed, it may cause premature belt failure.

More Alternator Charging Checks

Check the service specifications to determine whether a voltage drop of up to 0.5 volts on the positive side is acceptable for certain GM vehicles.

Suppose a battery repeatedly discharges, and the charging system functions normally. The cause may be a higher-than-normal parasitic electrical load on the battery when the key is turned off. Typically, the normal drain on a vehicle’s battery should be 50 milliamps or less. However, on some recent Ford models, the normal drain may be as high as 300 to 400 milliamps, with some modules drawing as much as 850 milliamps for up to an hour after the engine is shut off (while the modules are in standby mode). Once all the modules are shut down, the current drain on the battery should decrease to 50 milliamps or less.

Heavy power usage during peak loads and extended periods of idling can lead to battery discharge, as the alternator may be unable to keep up with the power requirements. When the lights, defrosters, heater, and radio are all on while idling for a long time, the battery can draw more amps than the charging system can replenish. This may create the impression of a charging problem, but the alternator itself may normally be functioning.

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