VW Passat & Audi A4 1990-2000

General Information


The charging system is a 12-volt Direct Current (DC) negative ( - ) ground system. The system consists of an alternator with an internal voltage regulator, an alternator belt, a charging system light, and a battery.

The alternator is mounted onto the engine and 2 sealed bearings within the alternator housing support the alternator rotor. One end of the alternator rotor has 2 electrical contacts called slip rings, and the other end of the rotor's shaft protrudes through the alternator housing and has a pulley attached to it. The rotor shaft's pulley is belt-driven by another pulley that is attached to the engine's crankshaft.

The slip rings are electrical contacts that allow voltage to be supplied to the rotor's electrical windings while it rotates. The voltage is supplied to the 2 slip rings via a pair of spring loaded brushes. The brushes are soft enough to not damage the slip rings, yet are made of a material that will conduct an electrical current. The rotor has an electrical winding that is surrounded by a series of metal fingers.

The initial voltage is supplied to the rotor from the battery and is called the excitation current. This electrical current is used to energize the field to begin the generation of electricity. When the electrical current is supplied to the rotor field winding via the slip rings, the rotor becomes an electromagnet. The rotor is surrounded by a series of small electrical coils called the stator assembly. When the rotor spins, the electrical coils of the stator assembly absorb the magnetic field from the rotor. This generates a series of positive and negative electrical pulses in the coils of the stator assembly creating an Alternating Current (AC) and AC voltage.

Because the vehicle's battery, electrical system and the electrical accessories are DC voltage, the AC voltage must be converted to DC voltage. The alternating current from the stator assembly is channeled through a series of diodes that are grouped together to form a component known as the rectifier. A diode is essentially a 1-way valve, and designed to allow current to pass in only one direction. The collection of diodes that form the rectifier assembly allows the current to flow in one direction, changing the electrical pulses from alternating current (AC voltage) to direct current (DC voltage). Once the alternator rotor begins to rotate and starts to generate electricity, the excitation current comes from its own output, rather than from the battery, although the battery remains as part of the electrical circuit.

Because the electrical needs of the vehicle change depending on operating conditions, the alternator's output needs to be regulated. To accomplish this, a voltage regulator is used to control the alternator's output. To do this, the regulator controls the voltage to the alternator rotor, which regulates the alternator's output by controlling the strength of the magnetic field. The more voltage the rotor receives, the stronger the magnetic field, and the more electrical current the alternator provides. Conversely, the less voltage the rotor receives, the weaker the magnetic field, and the less electrical current the alternator provides.

The alternator is used to maintain the charge of the battery and to power the components 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, and to the voltage regulator in the alternator. When the alternator rotor is not moving, the alternator is not producing an electrical current, and the alternator warning light remains on. When the engine is started, the alternator rotor begins to rotate. As the alternator rotor rotates, the alternator generates an electrical current and turns the alternator light off.

When the engine is running, the alternator produces an electrical current that is used to replenish the battery, which is drained slightly during start-up, and to power the electrical components of the vehicle.

Typical alternator component failures include:

Worn brushes
Failed diode(s)
A failed voltage regulator

Symptoms of worn brushes include:

Insufficient voltage output
Loss of electrical power while driving

Symptoms of a failed diode include:

Insufficient amperage output
Battery loses charge when not driving

Symptoms of a failed voltage regulator include:

Insufficient voltage output
Loss of electrical power while driving
Excessive voltage output
Battery fluid loss
Battery overheating
Warped battery casings