Toyota Previa 1991-1997 Repair Information

General Information


The charging system is a negative (-) ground system which consists of an alternator, a regulator, a charge indicator lamp, a storage battery, circuit protection and wiring connecting the components.

The alternator is belt-driven from the engine. Energy is supplied from the alternator (with integral regulator) to the rotating field through brushes to slip-rings. The slip-rings are mounted on the rotor shaft and are connected to the field coil. This energy supplied to the rotating field from the battery is called excitation current and is used to initially energize the field to begin the generation of electricity. Once the alternator starts to generate electricity, the excitation current comes from its own output rather than the battery.

The alternator produces power in the form of alternating current. The alternating current is rectified by diodes into direct current. The direct current is used to charge the battery and power the rest of the electrical system. When the ignition key is turned on, current flows from the battery, through the charging system indicator light on the instrument panel, to the voltage regulator, and to the alternator. Since the alternator is not producing any current, the alternator warning light comes on. When the engine is started, the alternator begins to produce current and turns the alternator light off.

As the alternator turns and produces current, the current is divided in two ways: charging the battery and powering the electrical components of the vehicle. Part of the current is returned to the alternator to enable it to increase its output. In this situation, the alternator is receiving current from the battery and from itself. A voltage regulator is wired into the current supply to the alternator to prevent it from receiving too much current, which would cause it to overproduce current. Conversely, if the voltage regulator does not allow the alternator to receive enough current, the battery will not be fully charged and will eventually go dead.

The battery is connected to the alternator at all times, whether the ignition key is turned on or off. If the battery were shorted to ground, the alternator would also be shorted. This would damage the alternator. To prevent this, circuit protection (usually in the form of a fuse link) is installed in the wiring between the battery and the alternator. If the battery is shorted, the circuit protection will protect the alternator.