FAQ Understanding Wiring Diagrams Terminology and Symbols



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VW Tiguan 2.0 DBGC 2018, VW Touran 1.6 BSE 2008, Skoda Fabia 1.2 BME 2006
1.1 Capacitor
Capacitors are a passive electronic component that holds a charge in the form of an electrostatic field. They are often used in combination with transistors in DRAM, acting as storage cells to hold bits. Capacitors typically consist of conducting plates separated by thin layers of dielectric material, such as dry air or mica. The plates on opposite sides of the dielectric material are oppositely charged and the electrical energy of the charged system is stored in the polarized dielectric.

1.2 Connector
Connectors are the part of a cable that plugs into a port or interface to connect one device to another. Most connectors are either male (containing one or more exposed pins) or female (containing holes in which the male connector can be inserted).
Given a relatively fixed voltage in an automotive electrical system (12.6 volts), the current in the circuit is determined by the amount of resistance in the load and in the conductors and connections in the circuit.
Terminals, connectors and splices are used throughout the electrical wiring harness to aid in vehicle assembly and service. Any connection in a circuit creates a place where problems can occur.

Typical connector problems include:
- Disconnected connector
- Terminal backed out
- Terminal corrosion
- Terminal spread too wide
- Improperly made splice

Connector pin numbers are always shown from the mating side of the connector, not the harness side of the connector. If you are back-probing the connector for a voltage check, remember the pin numbering becomes the mirror of what is pictured in the diagram.

Push-on/multi-pin connectors use the “T” designation, and are identified in the legend. For example, T8a/5 designates an 8-pin connector, with the specified wire located in terminal 5 of the connector housing.
Most pin assignments are labeled on the plastic hardshell connector housing and/or the corresponding component. On larger connectors, pin assignments are labeled at either end of a row. For example, the Engine Control Module (ECM) plug often
has 2 or 3 rows of 12 or more terminals. Each row will be marked on each end to facilitate diagnosis.

1.3 Component Code Prefixes
- A - Battery
- B - Starter
- C - Alternator/generator
- D - Ignition/starter switch
- E - Switches — these are usually more complex switches: A/C control head, sunroof regulator
- F - Switches — door, hood, brake, clutch, trunk, multifunction, etc.
- G - Gauges and sensors
- H - Horn
- J - Control modules, relays, electronic relays
- K - Indicator/warning lights
- L - Lights
- M - Lights
- N - Solenoids/inductors/ignition — fuel injectors, purge, ignition coil, ignition module, etc.
- P - Spark plug connector
- Q - Spark plugs
- R - Radio, CD, telephone, navigation
- S - Fuse, circuit breaker, protection device
- T - Wire connector V Motor — window motors, vacuum pumps, etc.
- W - Lights — interior, trunk
- Z - Heating elements — O2 sensor heater, heated seats, heated mirrors, etc

1.4 DIN Standard 72 552
German institute for standards that establishes guidelines for manufacturing and nomenclature.

This standard applies to the terminal designations for circuits. The purpose of the terminal designation system is to enable accurate connection identification from conductors (wires) to various components when diagnosis and repair is necessary.
DIN standards for terminal designations are shown below:
- 1 - Ignition coil, ignition distributor — low voltage
- 1a - To contact breaker I (ignition distributor with two separate circuits)
- 1b - To contact breaker II (ignition distributor with two separate circuits)
- 4 - Ignition coil, ignition distributor — high voltage
- 4a - From ignition coil I (ignition distributor with two separate circuits)
- 4b - From ignition coil II (ignition distributor with two separate circuits)
- 15 - Switched (+) downstream of battery (output of ignition/driving switch)
- 15a - Output at ballast resistor to ignition coil and starter
- 17 - Glow plug and starter switch — start
- 19 - Glow plug and starter switch — preheat
- 30 - Input from battery (+) term., direct 12/24 V seriesparallel battery switch
- 30a - Input from (+) terminal of battery II
- 31 - Battery negative terminal, or ground, direct
- 31a - Return line to battery II, negative (12/24 V seriesparallel battery switch)
- 31b - Return line to battery negative terminal, or ground via switch or relay (switched negative)
- 31c - Return line to battery I, negative (12/24 V seriesparallel battery switch)
- 50 - Starter control (direct)

Electric Motors
- 32 - Return line
- 33 - Main terminal connection
- 33a - Self-parking switch-off
- 33b - Shunt fi eld
- 33f - For second lower-speed range
- 33g - For third lower-speed range
- 33h - For fourth lower-speed range
- 33L - Counter-clockwise rotation
- 33R - Clockwise rotation

- 45 - Separate starter relay, output; starter, input (main current) Two-starter parallel operation C — starting relay for engagement current
- 45a - Output, starter I, input, starters I and II
- 45b - Output, starter II
- 48 - Terminal ON starter and ON start-repeating relay for monitoring starting

Turn-Signal Flashers (Pulse Generators)
- 49 - Input
- 49a - Output
- 49b - Output, second circuit
- 49c - Output, third circuit

Starter Control
- 50 - Starter control (direct)
- 50a - Series-parallel battery switch — output for starter control
- 50b - With parallel operation of two starters with sequential control Starting relay for sequential control of the engagement current during parallel operation of two starters
- 50c - Input at starting relay for starter I
- 50d - Input at starting relay for starter II
- 50e - Start-locking relay input
- 50f - Start-locking relay output
- 50g - Start-repeating relay input
- 50h - Start-repeating relay output

Wiper Motors
- 53 - Wiper motor, input (+)
- 53a - Wiper (+), self-parking switch-off
- 53b - Wiper (shunt winding)
- 53c - Electric windshield-washer pump
- 53e - Wiper (brake winding)
- 53l - Wiper motor with permanent magnet and third brush (for higher speed)

- 55 - Fog lamp
- 56 - Headlamp
- 56a - High beam, high-beam indicator lamp
- 56b - Low beam
- 56d - Headlamp-fl asher contact
- 57a - Parking lamp
- 57L - Parking lamp, left
- 57R - Parking lamp, right
- 58 - Side-marker, tail, license plate, and instrument panel lamps
- 58b - Dimmer
- 58d - Dimmer
- 58L - License-plate lamp, left
- 58R - License-plate lamp, right

Alternators and Voltage Regulators
- 61 - Alternator charge-indicator lamp
- B+ - Battery positive
- B- - Battery negative
- D+ - Dynamo positive
- D- - Dynamo negative
- DF - Dynamo fi eld DF1 Dynamo fi eld 1
- DF2 - Dynamo fi eld 2
- U,V,W - Alternator terminals
- 75 - Radio, cigarette lighter
- 76 - Speakers

Break Contact (NC) and Changeover Switches

- 81 - Input
- 81a - Output 1, NC side
- 81b - Output 2, NC side

Multiple-Position Switches
- 83 - Input
- 83a - Output, position 1
- 83b - Output, position 2
- 83L - Output, left-hand position
- 83R - Output, right-hand position

Relays/Current Relays
- 84 - Input, actuator and relay contact
- 84a - Output, actuator
- 84b - Output, relay contact

Alternators and Voltage Regulators
- 85 - Output, actuator (end of winding to ground or negative)
- 86 - Input, actuator (start of winding)
- 86a - Start of winding or 1st winding
- 86b - Winding tap or 2nd winding

Relay Contact for Break (NC) and Changeover Contacts
- 87 - Input
- 87a - Output 1 (NC side)
- 87b - Output 2 87c Output 3
- 87z - Input 1
- 87y - Input 2
- 87x - Input 3

Relay Contact for Make (NO) Contact and Changeover Contacts
- 88 - Input
- 88a - Output 1
- 88b - Output 2
- 88c - Output 3
- 88z - Input 1
- 88y - Input 2
- 88x - Input 3

Directional Signals (Turn Signal Flashers)
- C - Indicator lamp 1
- C2 - Indicator lamp 2
- C0 - Main terminal connection for separate indicator circuits actuated by the turn signal switch
- C3 - Indicator lamp 3 (e.g., when towing two trailers)
- L - Turn-signal lamps, left
- R - Turn-signal lamps, right

1.5 Diode
Normal Diode


Zener Diode

Light Sensitive Diode
Diodes are an electric component that conducts electric current in only one direction, functioning as a one-way valve. Diodes typically are made from semiconductor materials such as silicon, germanium or selenium and are uses as voltage regulators, signal rectifiers, oscillators and signal modulators/demodulators.
The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.

1.6 Fuse
Normal Fuse


Thermal Fuse
A component installed in series with a circuit, designed to disrupt the circuit when carrying more than its specified amperage. Fuses are placed in circuits to protect other components in the circuit from damage.
Fuses are used to prevent excessive current from damaging other components in a circuit. There are various types including standard fuses, thermal fuses (circuit breakers) and strip fuses.
Fuses use the component designation S .

1.7 LED (Light-emitting Diode)
An LED is a light-emitting diode semiconductor device used to transmit light into a fiber in response to an electrical signal. It typically has a broad spectral width.
The diode is created such that light emitted at a p-n junction is proportional to the bias current; color output depends on the material used.

1.8 Module
A transistorized, rather than analog, switching circuit that relies on one or inputs to control the switching of a logic circuit.
Control Modules can make the task of reading and interpreting wiring diagrams more difficult because multiple signals enter and/or exit, but the internal schematic is not always shown.
Some control modules may be integrated with other components.
Take, for example, the window motor with control module. Both the motor and the control module are combined to make a single component.

The symbol below represents the combined components.

1.9 Motor
Single Speed Motor


Two Speed Motor
A motor is driven by electromotive force.
A device that converts electrical energy into rotational motion. On late model vehicles, the speed of most motors is controlled using PWM circuits. If a motor has low output, checking the motor amperage can determine if the problem is electrical or mechanical. Increasing the electrical resistance in a motor circuit will decrease the amperage in the circuit, while increasing the mechanical load on the motor shaft will increase the amperage in the circuit.
The direction of motor rotation can be changed by reversing the polarity of the signals to the motor.

1.10 Potentiometer or Rheostat
A resistor that varies its resistance based on a mechanical input. A three wire variable resistor that is used as a voltage divider. A reference voltage (usually 5 volts or battery voltage) and ground are placed across a resistance element. A wiper is moved across the element to produce an infinitely variable voltage signal from zero up to the reference voltage, which is measured on the third wire. In automotive applications, potentiometers are commonly used as position sensors for motors or measuring throttle plate position.
A rheostat, or potentiometer, varies its resistance based on mechanical input. An example of this is the Throttle Position Sensor (TPS) on a Motronic equipped car.

1.11 Relay
Component that uses a low current to switch a high current circuit. It contains a winding that is used to magnetically move a set of points (switch), similar to the operation of a solenoid. When an on/off signal is supplied to the winding, a magnetic field is generated which changes the position of the switch. Depending on the design of the relay, the switch may be normally open or normally closed in its rest state. The most common type of relay is a four wire relay, which uses two wires for the control circuit and two wires for the switched circuit. Relays that use more than four wires are usually variations of this design, usually containing multiple control and switched circuits. If a relay contains logic circuits, it is generally considered a control unit although it may still be called a relay.
Like solenoids, relays also produce an inductive kick. Volkswagen relays have a built in suppression circuit consisting of a resistor placed parallel to the winding.
Although there are numerous types of relays used in VW and Audi vehicles, two major distinctions might be made, conventional mechanical relays and solid state relays.
Solid state relays are actually transistors– miniaturized electronic components that have a purpose similar to mechanical relays.
Transistors, like relays, allow a circuit with low current to control a circuit with high current.
- Mechanical relays are always shown with the coil deenergized.
- Solid state relays may be part of a Relay Box/Junction Block.
If they are a transistor and they fail, the entire relay box/Junction Block must be replaced.

1.12 Resistor
Normal Resistor

A commonly used symbol is the resistor, used to create a voltage drop in a circuit.
A component that creates a voltage drop in a circuit. Resistors can be used for measurement of current flow, as well as to drop the applied voltage for certain consumers.
Resistors are an electronic component that resists, limits or regulates the flow of electrical current in an electronic circuit.
Since the device has resistance it is used in a circuit for protection and current control.
A standard resistor has a fixed resistance.

Temperature Dependent Resistor
A temperature dependent resistor or thermistor is a two wire sensor that utilizes a resistor whose resistance varies with temperature. The thermistors used in most automotive applications have a Negative Temperature Coefficient (NTC), where the resistance of the thermistor decreases as the temperature increases. In a Positive Temperature Coefficient (PTC) thermistor, the resistance of the thermistor increases as the temperature increases. NTC thermistors are commonly used as temperature sensors. The temperature value is not obtained by reading the sensor resistance directly, but instead by placing a reference voltage (usually 5 volts) and ground across the sensor and then reading the resulting voltage drop.
A temperature dependent resistor varies its resistance based on temperature. This type of resistor is used as the Engine Coolant Temperature (ECT) sensor on an engine management system, or the Interior Temperature sensor on models with climate control When a resistor is used as a sensor, it generally carries a component designator “G.”

1.13 Sensors
1.13.1 Analog Sensors


A two wire sensor that utilizes a resistor whose resistance varies with temperature. The thermistors used in most automotive applications have a Negative Temperature Coefficient (NTC), where the resistance of the thermistor decreases as the temperature increases. In a Positive Temperature Coefficient (PTC) thermistor, the resistance of the thermistor increases as the temperature increases. NTC thermistors are commonly used as temperature sensors. The temperature value is not obtained by reading the sensor resistance directly, but instead by placing a reference voltage (usually 5 volts) and ground across the sensor and then reading the resulting voltage drop.

A three wire variable resistor that is used as a voltage divider. A reference voltage (usually 5 volts or battery voltage) and ground are placed across a resistance element. A wiper is moved across the element to produce an infinitely variable voltage signal from zero up to the reference voltage, which is measured on the third wire. In automotive applications, potentiometers are commonly used as position sensors for motors or measuring throttle plate position.

Inductive Sensor: A two wire sensor that measures the rotation of a shaft. Unlike other sensors, this sensor does not have an external power supply. Instead, it contains a permanent magnet
that creates a magnetic field which collapses and expands when a sensor wheel is rotated through it, generating an AC sine wave signal. The frequency of the signal varies with changes in the RPM of the sensor wheel. Many crankshaft position (RPM) sensors and older ABS wheel speed sensors are inductive sensors.

Knock Sensor: A two wire sensor that is used to measure spark knock in an engine. This sensor uses a crystal material that generates an AC voltage when mechanical stress is applied to it (piezoelectric effect) when spark knock occurs. During installation, a knock sensor must be properly torqued to read spark knock correctly.

Analog Signal
An electrical signal whose amplitude can be measured at an infinite number of positions along the waveform.

1.13.2 Digital Sensors
Hall Sensors and Switches

A two or three wire electronic sensor that produces a variable frequency square wave signal. Power and ground are supplied to a Hall Effect Transistor which is located in a magnetic field generated by a permanent magnet. As the magnet field is altered by moving the magnet in relation to the transistor or by moving a shutter wheel through the magnetic field, the reference voltage is alternately pulled high or low resulting in a square wave signal.
Hall Sensors are often used to measure the position of rotating components such as camshaft position sensors.

Digital Signal
An electrical signal with an instantaneous change in amplitude (called a pulse) from low to high and high to low. Since the change in state is instantaneous, the amplitude can only be measured in two positions, high or low.
The pulse shown is a positive pulse, because the normal state of the waveform is low and the pulse goes high. However, with a negative pulse, the normal state of the waveform is high and the pulse goes low.

1.14 Solenoid
Used to actuate other components, a solenoid consists of a coil of wire wrapped around an iron core. When a current is passed through a wire, a magnetic field is induced. This magnetic "pull" is used to operate other components.
Solenoids are used to actuate many different components, including fuel injectors and relays.
A solenoid is a coil of wire wrapped around an iron core. When current is passed through the wire, a magnetic field is induced. This pulls a set of contacts in the relay closed, either opening or closing the circuit.

1.15 Switch
Normally Closed Switch


Normally Open Switch
The switch is another component used to control current flow. The basic symbols for an open and closed switch are shown in illustration SSP 8730/22. An example of this simple two-position switch would be a glove compartment light switch. A simple switch uses the component designation “F.” There are many types of designs, including mechanically actuated, pressure actuated, temperature actuated and momentary.

Manually Operated Switch

Pressure Operated Switch
Oil pressure switch (pressure actuated)


Thermally Operated Switch
Cooling fan thermo-switch (temperature actuated)


Mechanically Operated Switch
Brake Switch -F- (mechanically operated)


Complex Circuit with Multiple Switches
More complex circuits may require a switch with multiple sets of contacts. An example of this would be the cruise control switch.

As you can see in illustration above, there are multiple sets of contacts within the assembly. Depending on the position of the switch, various sets of contacts are open or closed. Careful study of the symbols allows the technician to follow the circuit through the switch under any condition.

A complex switch uses the component designation “E”

1.16 Solenoid
A two wire electromechanical device used to control the flow of liquids, gasses or the operation of mechanical components. To operate the solenoid, an on/off, PWM or variable frequency signal (commonly a switched ground) is supplied to a winding inside the solenoid, which in turn generates a magnetic field that moves a plunger. Depending on the design of the solenoid, the plunger may be normally open or normally closed in its rest state. A fuel injector is an example of a solenoid.
When the signal to the solenoid is switched off and the magnetic field around the winding collapses, the winding produces a phenomenon called “inductive kick”. Inductive kick is a high voltage pulse that is injected back into the control circuit and is similar in principal to the pulse produced by an ignition coil, although the voltage is much lower (generally around 30 to 60 volts).

1.17 Symbols
Airbag Spiral Spring


Antenna With Amplifier


Camshaft Sensor

Cigarette Lighter

Clock, Analog

Clock, Digital

Control Motor

Crankshaft Sensor



Glow Plug

Heated O2 Sensor

Heater Element


Ignition Coil

Instrument Gauge

Interior Light

Light Bulb, Single Filament

Light Bulb, Dual Filament

Magnetic Clutch

Malfunction Indicator


Rear Window Defogger

Slip Contact

Solenoid Valve


Speed Sensor


Spark Plug


Wiper Motor

1.17.1 Diodes


Zener Diode

Light Sensitive Diode

LED (Light Emitting Diode)

1.17.2 Switches
Normally Open Switch


Normally Closed Switch

Manually Operated Switch

Push Button Switch

Mechanically Operated Switch

Pressure Operated Switch

Thermally Operated Switch

Multiple Switch

1.17.3 Wiring
Wire Junction


Disconnected Wire Terminal

Multi Point Connection

Push On Connector

Connected Wire Terminal

1.18 Transistor
Transistor is a device composed of semiconductor material that amplifies a signal or opens or closes a circuit.

1.19 Wire
- A wire is a single conductor which usually has an outer layer of insulation

1.19.1 Wire Type
Single core wire

Single core wire: It consists of a solid wire with a plastic coating.
A - Strand
B - Insulating materials

Stranded wire
Stranded wire: It consists of many fine strands of wire covered with an outer plastic coating.
A - Strand
B - Insulating materials

Co-axial wire (cable)
- Co-axial wire (cable): It consists of insulated central wire which is used for signal, screen wires which are connected to 0 Volt to shield any electrical interference, and an outer sheath.
A - Signal wire
B - Insulation wire
C - Screen fine wires
D - Sheath

1.19.2 Wire color
Wire color is used to assist identify different circuit. It usually consists of one of two color: Base and Tracer. The base color is the dominant wire color whereas the tracer is thin color line.
The brown colored wire - whether is solid brown or traced brown - is typically used for ground or controlled ground.

Circuit color abbreviation summary

1.19.3 Wire Gauge
- Wire gauge: The term gague is essentially a standard measure of thickness. A rule of thumb when understanding wire gauge :
the lower the gauge number, the higher the bead.

Metric Gauge Size in mm2America Wire Gauge (AWG) Size

1.19.4 Wire Layout
In this example we will identify a basic wire layout.
1 - Wire connection
2 - Circuit color - base circuit color covers 60%
3 - Harness inline connector
4 - Wire gauge
5 - Wire color

1.19.5 Wire Connection
Wire connection: it is two or more spliced wires together of the same circuit function. There are two types of spliced wires:
negative connection or positive connection wires.

The vehicle chassis acts as a common Ground conductor throughout the vehicle, and is identified by the line at the bottom of the wiring diagram above the current track numbers. This Ground line will also show Grounds that are welded connections in the harness. The number in the circle will be repeated in the legend with a description of the Ground connection.
Ground connections are represented as a line at the bottom of the wiring diagram page, directly above the current track numbers. All ground connections, whether they occur as a splice in a harness, or the final ground source, are numbered and identified in the wiring diagram.

Some components, such as the starter or generator, may receive Ground where they are bolted to the engine or transmission. This is also shown as a thin black line.

Welded connections are used in wiring harnesses to join multiple, smaller gauge wires to a single larger wire which terminates at the fuse relay panel or chassis ground.

Sometimes a welded connection is shown with the thin line not terminating at another wire. This symbolizes that this welded connection is used in other diagrams for the car. The technician may need to reference other diagrams to locate components or Grounds related to this connection.

Internal conductors exist inside components, acting as bridges between the wiring harness and the final consumer. In some components these conductors are labeled in the component. An example would be the 30 circuit (Battery +) in the central/relay panel. In other components the conductor is not labeled. Internal conductors are shown as thin, black lines.

Threaded connectors are commonly used on the bottom of the fuse relay plate to distribute power and ground to components.
The common connectors include battery power, ground, and load reduction (X).
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