VW Passat & Audi A4 1990-2000

Electrical Components



Power is supplied to the vehicle by 2 devices: The battery and the alternator. The battery supplies electrical power during starting or during periods when the current demand of the vehicle's electrical system exceeds the output capacity of the alternator. The alternator supplies electrical current when the engine is running. Just not does the alternator supply the current needs of the vehicle, but it recharges the battery.

The Battery

In most modern vehicles, the battery is a lead/acid electrochemical device consisting of six 2-volt subsections (cells) connected in series, so that the unit is capable of producing approximately 12 volts of electrical pressure. Each subsection consists of a series of positive and negative plates held a short distance apart in a solution of sulfuric acid and water.

The 2 types of plates are of dissimilar metals. This sets up a chemical reaction, and it is this reaction, which produces current flow from the battery when its positive and negative terminals are connected to an electrical load. The alternator, restoring the battery to its original chemical state, replaces the power removed from the battery.

The Alternator

On some vehicles there isn't an alternator, but a generator. The difference is that an alternator produces alternating current (AC) which is then changed to direct current by a collection of diodes called the rectifier, which change the AC voltage to Direct Current (DC) voltage. A generator produces direct current and therefore does not require the use of a rectifier. Alternators tend to be much more efficient and that is why they are used.

Alternators and generators are devices that consist of coils of wires wound together making big electromagnets. One group of coils spins within another set and the interaction of the magnetic fields causes a current to flow. This current is then drawn off the coils and fed into the vehicles electrical system.


Two types of grounds are used in automotive electric circuits. Direct ground components are grounded to the frame through their mounting points. All other components use a ground wire that is attached to a related component, the frame or the chassis of the vehicle.

The electrical current runs through the chassis of the vehicle and returns to the battery through the negative or ground (-) cable; if you look, you'll see that the battery ground cable connects between the battery and the engine, transmission, the frame or chassis of the vehicle.

Whether or not the negative battery cable attaches to the chassis, engine or transmission, a heavy gauge ground cable must also be used to connect the engine and transmission to the chassis because of the high electrical demands of the starter motor and the electrical current output produced by the alternator. Because of these high electrical current demands, a sufficient sized ground cable capable of handling these loads must be attached between the chassis and the engine/transmission assembly.

It should be noted that a good percentage of electrical problems can be traced to bad grounds.


It is possible for large surges of current to pass through the electrical system of your vehicle. If this surge of current were to reach the load in the circuit, the surge could burn it out or severely damage it. It can also overload the wiring, causing the harness to get hot and melt the insulation. To prevent this, fuses, circuit breakers and/or fusible links are connected into the supply wires of the electrical system. These items are nothing more than a built-in weak spot in the system. When an abnormal amount of current flows through the system, these protective devices work as follows to protect the circuit:

Fuse-when an excessive electrical current passes through a fuse, the fuse "blows" (the conductor melts) and opens the circuit, preventing the passage of current.
Circuit Breaker-a circuit breaker is basically a self-repairing fuse. It will open the circuit in the same fashion as a fuse, but when the surge subsides, the circuit breaker can be reset and does not need replacement.

Always replace fuses and circuit breakers with identically rated components. Under no circumstances should a component of a higher or a lower amperage rating be substituted.

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Fig. The fuse panel lid shown being removed from the left side of the dash on A4 and 1998-00 Passat models-1.8L Passat model shown

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Fig. The cover is held in place with 3 press fit retainers-1.8L Passat model shown

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Fig. A fuse removal and installation tool is located on the fuse panel lid-1.8L Passat shown

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Fig. The A4 and 1998-00 Passat fuse panel is under a trim cover on the left side of the dash-1.8L Passat shown


Switches are used in electrical circuits to control the passage of current. The most common use is to open and close circuits between the battery and the various electric devices in the system. Switches are rated according to the amount of amperage they can handle. If a switch of sufficient amperage is not used in a circuit, the switch could overload and cause damage.

Some electrical components which require a large amount of current to operate use a special switch called a relay. Since these circuits carry a large amount of current, the thickness of the wire in the circuit is also greater. If this large wire were connected from the load to the control switch, the switch would have to carry the high amperage load and the fairing or dash would be twice as large to accommodate the increased size of the wiring harness. To prevent these problems, a relay is used.

Relays are composed of a coil and a set of contacts. When the coil has a current passed though it, a magnetic field is formed and this field causes the contacts to move together, completing the circuit. Most relays are normally open, preventing current from passing through the circuit, but they can take any electrical form depending on the job they are intended to do. Relays can be considered "remote control switches." They allow a smaller current to operate devices that require higher amperages. When a small current operates the coil, a larger current is allowed to pass by the contacts. Some common circuits that may use relays are the horn, headlights, starter, electric fuel pump and other high draw circuits.

When installing a switch or a relay always make sure their amperage rating is sufficient for the circuit in which they are to be used. One rule of thumb is to use a relay or switch that is rated at least 20% greater than the circuit in which it is to be used.

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Fig. Relays are composed of a coil and a switch. These 2 components are linked together so that when one operates, the other operates at the same time. The large wires in the circuit are connected from the battery to one side of the relay switch (B+) and from the opposite side of the relay switch to the load (component). Smaller wires are connected from the relay coil to the control switch for the circuit and from the opposite side of the relay coil to ground

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Fig. The underhood electrical box houses both fuses and relays


Every electrical circuit must include a "load" (something to use the electricity coming from the source). Without this load, the battery would attempt to deliver its entire power supply from one pole to another. This is called a "short circuit." All this electricity would take a short cut to ground and cause a great amount of damage to other components in the circuit by developing a tremendous amount of heat. This condition could develop sufficient heat to melt the insulation on all the surrounding wires and reduce a multiple wire cable to a lump of plastic and copper.


The average vehicle contains meters and meters of wiring, with hundreds of individual connections. To protect the many wires from damage and to keep them from becoming a confusing tangle, they are organized into bundles, enclosed in plastic or taped together and called wiring harnesses. Different harnesses serve different parts of the vehicle. Individual wires are color coded to help trace them through a harness where sections are hidden from view.

Automotive wiring or circuit conductors can be single strand wire, multi-strand wire or printed circuitry. Single strand wire has a solid metal core and is usually used inside such components as alternators, motors, relays and other devices. Multi-strand wire has a core made of many small strands of wire twisted together into a single conductor. Most of the wiring in an automotive electrical system is made up of multi-strand wire, either as a single conductor or grouped together in a harness.

All wiring is color coded on the insulator, either as a solid color or as a colored wire with an identification stripe.

A printed circuit is a thin film of copper or other conductor that is printed on an insulator backing. Occasionally, a printed circuit is sandwiched between 2 sheets of plastic for more protection and flexibility. A complete printed circuit, consisting of conductors, insulating material and connectors for lamps or other components is called a printed circuit board. Printed circuitry is used in place of individual wires or harnesses in places where space is limited, such as behind instrument panels.

Since automotive electrical systems are very sensitive to changes in resistance, the selection of properly sized wires is critical when systems are repaired. A loose or corroded connection or a replacement wire that is too small for the circuit will add extra resistance and an additional voltage drop to the circuit.

The wire gauge number is an expression of the cross-section area of the conductor. Vehicles from countries that use the metric system will typically describe the wire size as its cross-sectional area in square millimeters. In this method, the larger the wire, the greater the number. Another common system for expressing wire size is the American Wire Gauge (AWG) system. As gauge number increases, area decreases and the wire becomes smaller. An 18-gauge wire is smaller than a 4-gauge wire. A wire with a higher gauge number will carry less current than a wire with a lower gauge number. Gauge wire size refers to the size of the strands of the conductor, not the size of the complete wire with insulator. It is possible, therefore, to have 2 wires of the same gauge with different diameters because one may have thicker insulation than the other.

It is essential to understand how a circuit works before trying to figure out why it doesn't. An electrical schematic shows the electrical current paths when a circuit is operating properly. Schematics break the entire electrical system down into individual circuits. In a schematic, usually no attempt is made to represent wiring and components as they physically appear on the vehicle; switches and other components are shown as simply as possible. Face views of harness connectors show the cavity or terminal locations in all multi-pin connectors to help locate test points.

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Fig. A wire's size and length determines the electrical load it can handle. The chart assumes a maximum temperature of 150°F (65°C)


Three types of connectors are commonly used in automotive applications-weatherproof, molded and hard shell.

Weatherproof-these connectors are most commonly used where the connector is exposed to the elements. Terminals are protected against moisture and dirt by sealing rings, which provide a weather-tight seal. All repairs require the use of a special terminal and the tool required to service it. Unlike standard blade type terminals, these weatherproof terminals cannot be straightened once they are bent. Make certain that the connectors are properly seated and all of the sealing rings are in place when connecting leads.
Molded-these connectors require complete replacement of the connector if found to be defective. This means splicing a new connector assembly into the harness. All splices should be soldered to insure proper contact. Use care when probing the connections or replacing terminals in them, as it is possible to create a short circuit between opposite terminals. If this happens to the wrong terminal pair, it is possible to damage certain components. Always use jumper wires between connectors for circuit checking and NEVER probe through weatherproof seals.
Hard Shell-unlike molded connectors, the terminal contacts in hard-shell connectors can be replaced. Replacement usually involves the use of a special terminal removal tool that depresses the locking tangs (barbs) on the connector terminal and allows the connector to be removed from the rear of the shell. The connector shell should be replaced if it shows any evidence of burning, melting, cracks, or breaks. Replace individual terminals that are burnt, corroded, distorted or loose.

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Fig. Hard shell (left) and weatherproof (right) connectors have replaceable terminals

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Fig. Weatherproof connectors are most commonly used in the engine compartment or where the connector is exposed to the elements

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Fig. This shows a typical electrical connector. Knowing the terminal designations are essential when trouble shooting a problem