Alternator Basics

• An alternator is the device used to produce the electricity the car needs to run and keep the battery charged.

• The battery is the centre of your electricity system and it needs to be charged or it will lose its charge.  This is the job of the alternator.

• The alternator produces electricity and delivers it to the battery to maintain a full charge at all times.

• The alternator which is a type of generator transforms mechanical energy into electrical energy.

• Although your car’s battery supplies some electricity, most of the electrical mechanisms within the vehicle, require the alternator’s steady stream of power.

• If the alternator has to recharge an overly discharged battery, the alternator will become overworked, which will shorten its life.  This is largely due to the high amount of heat produced by the alternator during the charging process.  The greater the amperage flowing through it, the higher the heat an alternator creates. 

• So, anytime an alternator is replaced, the battery should be fully recharged with a battery charger or replaced.

How an Alternator Works

• When you switch ‘ON’ a red lamp on the dash lights, providing current through the lamp to the alternator rotor. This current in the rotor establishes a small magnetic field across the rotor.

• As the engine spins, the rotors magnetic field passes the stator windings such that the stator windings see a north pole followed by a south pole repetitively.

• This induces a voltage in the stator windings that similarly changes from a positive to negative.

(Note that the induction only occurs when the field changes).

• This stator voltage is sent through the rectifier that connects the negative voltage from the stator to the battery negative, and the positive voltage from the stator to the battery positive.

• The rectifier only performs this connection when the alternator voltage exceeds the battery voltage. Once connected the alternator starts to supply current to the battery.  The amount of this current is proportional to the current in the rotor.

• The regulator required an excess of about 0.7volts before it will pass significant current.  This voltage drop times the current going through the rectifier produces power that is lost through heat.  It gets hot and hotter the more work it does.

• The voltage regulator limits the battery voltage by reducing the rotor current.  The dash light can only supply a small current, not enough to realise the full potential of the alternator.

• So some extra diodes in the rectifier take some of the output power of the stator to supply rotor current via the regulator.  This extra diode supply also enables the red dash light to turn off.

Check if you need a new alternator

-Ignition ON and engine not running

Gauge readings should be between 12.0 and 12.6 volts with the ignition ON and the engine not running.

A reading below 12 volts could indicate insufficient charging, low battery, corroded, broken, loose or frayed wires/connectors.

-Ignition ON and engine running

Gauge readings should be between 13.0 and 14.2 volts with the ignition ON and the engine running.

A reading exceeding 14.2 volts could indicate a bad battery, failed regulator or poor wire connections.

A reading below 13.2 volts could indicate a failed alternator or corroded, broken, loose or frayed wires/connections.

Check the fuses in all the fuse box(s).

An open fuse indicates circuit problem(s) which may have an effect on the charging circuit.

There may be several fusible links controlling battery voltage to the vehicle’s electrical circuits.

If a fusible link is open, supply voltage will be completely lost to all electrical systems or to the electrical circuit(s) that the open fusible link controls.

Too loose

If the drive belt is too loose, it will slip around the pulley causing the alternator to charge irregularly or not at all.

Too tight

If the drive belt is too tight, internal bearing damage will cause premature alternator failure.

Worn or too narrow

If the alternator’s drive belt is worn or too narrow, it will slip around the pulley causing the alternator to charge irregularly or not at all.

New drive belt

It is important to check and adjust the belt’s tension to specification after the initial 5 minutes of operation.

Improper accessory Installation

Improper accessory installation procedures can cause charging problems. Some of these may include poor ground points, loose connections or improper wiring.

Electrical ground points

Check the ground circuits between the battery and engine and also the vehicle body to the frame for high resistance.

Many times when a vehicle has been repaired, the ground point(s) are disturbed and not re-secured properly.

Getting the right replacement alternator

Again, the most accurate method is to obtain the OEM number off the unit. If you have the reference number, drop us a line at parts@myautoparts.com.au and we can match your original starter or alternator to a replacement. You can also order using our vehicle make, model and year filter on our site. Check the specs of the alternator in the description and it will provide you enough information to match to your existing alternator.