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Troubleshooting Alternator and Charging System problems

Updated: Apr 9, 2022

An alternator and charging problem can affect the vehicle in different ways :

  • Alternator or charge indicator light is on

  • Enging is too hard to start and cranks slowly, or doesn't crank

  • Battery damage ( acid vaporised)

  • Alternator make a noise

  • Headlight dim ( it doesn't get the correct voltage to run the engine)

  • Battery exploded ( over voltage)

  • Battery drain below 12,4 V

The alternator is a heart of the charging system of a vehicle.

Most of the alternator problems are not to difficult to fix. In the most cases there is a voltage leak of the alternator, the alternator is dirty and doesn't work efficiently or the belt is to lose.


The common symptoms of bad charging system

Alternator or charge indicator light is on

When the alternator or charging light is on while driving your charging system have a problem. The meaning of this indication light is that the alternator voltage output is below 12.6V, or above 12.6 V or the charging of the battery has stopped.

Enging is too hard to start and cranks slowly, or doesn't crank

When the battery is undercharged, the battery voltage has dropped below 12.6V, you may than have a difficulty with starting the vehicle. Also the engine will stall if the alternator can't produce enough power to run the ignition system.

Battery damage ( acid vaporised)

An overcharged battery causes the battery to overheat, you may add water frequently to the battery. If the problem doesn't solved the battery will be damaged or it could explode.

Alternator make a noise

If the vehicle make a noise, it maybe the alternator have a bad component or loose belt, faulty belt tensioner, loss alternator pulley or mounting bolts. This will effect the battery for not getting the good operating voltage.

Headlight dim ( it doesn't get the correct voltage to run the engine)

if you notice the headlights, instrument panel, interior lights dimming or flickering, power windows running slow, radio or gauges don't operate as they should be. The battery is getting than a low charge. The alternator diodes may have a fault or there can be a voltage leak.

Battery exploded (over voltage)

When the battery gets overcharged, the battery gets more than 14.4V the battery will vaporise. When the vaporise acid cannot escape the battery will explode.

Battery drain below 12,4 V

When starting the car after a day and the battery voltage has drope below 12.4 V mostly it would not start. Thereby the alternator could have a leak or it's not charging optimal.


Test Charging System

When you noticed a problem with your alternator you may check your alternator. There are some problems you can fix self.

Make a visual inspection of the system

When the engine is off, pop the hood open and visually inspect the different components of the charging system. Look for loose belt, alternator connection problems, corrosion or dirt in the alternator of the battery case. For the inspection on the belt, check for adjustment if needed.

Check for wiring Problems

Inspect the wires and connections at the back of the alternator, check the output voltage and the voltage on the battery. Look for corrosion at the connectors , damaged wires or wires that or cut that could make a electrical short.

Do this it indicator light is on, or goes on and off

The following steps :

  • Start the engine and apply the emergency brakes. Set the transmission to Neutral (manual) or Park (automatic).

  • Connect a voltmeter across the battery terminals.

  • While watching the voltmeter display, start wiggling wires at the back of the alternator, the battery, and the remote voltage regulator (if there is one).

  • If the voltage reading on the meter display goes up while wiggling one of the wires, you've found a bad electrical wire or connection.

Check for alternator noises

bad alternator bearings, rotor shaft, slip rings and brushes inside the alternator may make a noise

Check for Under-Charging and over-Charging

First, measure battery base voltage to make sure you have a fully charged battery.

  • Turn on the high beams for 10 seconds and then turn them off.

  • Wait for two minutes

  • Measure battery voltage across the battery posts with your DMM. You should get between 12.4 and 12.6V. This means your battery is fully charged. If you get a reading below 12.4V, charge the battery before continuing.

Measure the Charging System's No-Load Voltage

  • Ask an assistant to start the engine and hold engine speed at about 1500 RPM.

  • Measure voltage across the battery with your DMM. You should get 0.5 to 2 volts higher than base voltage. If you are getting more than 2 volts above base voltage, most likely your alternator is over-charging the battery or the battery is faulty. Other potential problems are a faulty voltage regulator or a problem in the charging system wiring. As part of your wiring checks, see the section "Troubleshooting for Unusual Charging System Voltage Drops" below.

Measure charging system load voltage with a high-current-condition system measurement.

  • Ask an assistant to start the engine and hold engine speed at about 2000 RPM. Turn on all electrical accessories like A/C, blower motor, headlights, defroster, wipers. But don't turn on the heated windshield if your vehicle has one; alternator voltage may increase to over 100V and this can be unsafe.

  • Take a voltage reading across the battery posts. Your reading should be at least 0.5 volts above base voltage for your system to keep up with electrical system demands. Otherwise, the charging system can't meet the demand and charge your battery. This fault could point to a faulty alternator or voltage regulator. Check the section "Troubleshooting for Unusual Charging System Voltage Drops" below to check for wiring problems that can lead to this condition.

Check for Alternator AC Voltage leak

Alternators use diodes to rectify alternating current produced by the alternator into direct current. When one or more diodes are bad the alternator will fail. The alternator could give several problems. AC voltage leak can cause your light to dim and drain power from your battery.

  • Start and let the engine idle.

  • Set the parking brake and your transmission to Park (automatic) or Neutral (manual).

  • Set your meter to a low AC voltage range.

  • Ask an assistant to increase engine RPM to 2000 rpm.

  • Touch the red meter lead to the alternator battery terminal B+.

  • Touch the black meter lead to the alternator case (ground).

  • If your DMM reads 0.4 AC volts or less, the diodes are good.

  • If your DMM reads 0.5 AC volts or more, the diodes or stator is faulty.


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