Introduction
The automotive industry has been massively transformed by globalisation.Design work can be
conducted in one country while manufacturing in another for vehicles to be shipped to nearly
all.This can have problems when not all manufactures use the same standards to present infor-
mation on the vehicles within the market. Something as simple as the results of measuring
temperature may need converting °C to °F. The answer is standards. Standards have been an
integral part of the automotive world since the earliest days of the automotive assembly line.
Standardisation of parts allowed automakers to transform their businesses from a one-at-a-
time proposition to a many-at-a-time operation.
In the US the Society of Automotive Engineers (SAE) is responsible for standards that apply
to automobile manufacturing. For example, when selecting a bolt for a US domestic vehicle
out of the bolt bin, chances are the standards and specifications concerning its thread pitch and
hardness were originally defined by SAE. Thanks to standardisation, that bolt should thread
into any nut made anywhere in the world, as long as it conforms to the same set of standards.
In Europe, the most widely recognised organisation responsible for establishing and pub-
lishing automotive standards is called Deutsches Institut für Normung e.V. Standards estab-
lished by this organisation are often referred to as DIN standards.
DIN standards (Europe)
Manufacturers may not fully conform to DIN standards; they may only conform to a number
of the standards and use what they feel work better for their service support data. For example,
Figures 3.126 and 3.127 show some relationships to DIN standards on terminal designation but
not all letter codes correspond to DIN standards, e.g. – C for connector DIN standard uses X.
It is important whenever using wiring system manuals, CDs or intranet support material to
ensure the charts and tables are checked against the current standard or codes.
Block diagrams (basic) and schematic (detailed) DIN 40 719/1
Description of an electrical system or circuit may begin with a circuit diagram.This is presented
in the form of symbols to provide a quick overview of circuit and device functions.The circuit dia-
gram illustrates the functional interrelationships and physical links that connect various devices.
A block diagram (Fig. 3.126) is another simplified representation of a circuit, showing only
Figure 3.126
the most significant elements. It is designed to furnish a broad overview of the function, struc-
ture, layout and operation of an electrical system.This format also serves as the initial reference
for understanding more detailed schematic diagrams. Squares, rectangles, circles and symbols
illustrate the components. Information about wire colours, terminal numbers, connectors etc.
are omitted to keep the diagram as simple as possible.
The schematic diagram in Figure 3.127 shows a circuit and its elements in greater detail. By
clearly depicting individual current paths, it also indicates how the electrical circuit operates.
Most DIN schematic diagrams are current flow diagrams.They are arranged from top to bottom,
Figure 3.127
so we can clearly see how the current flows through the circuit.Generally a terminal designation
number 15 (ignition switch) and or 30 (power from battery) runs along the top and 31 runs along
the bottom (earth). System codes, wire sizes and colours are also included.The symbols used to
define the components also conform to DIN specifications.A key explaining these symbols will
often be included with the schematic diagram. Even if you’re fairly familiar with a circuit on a
given car, a schematic diagram will help you find the correct location of a ground terminal,or help
you identify a specific pin number in a connector. The following will help you understand the
information within a schematic diagram.Where the components and wires are situated in relation
to one another in the diagram usually bears no resemblance to how they are actually arranged on
the vehicle but, generally,wiring diagram vehicle systems have loom layouts, component location
charts and connector charts to aid in finding all the information required.
Terminal designations DIN standard 72 552
DIN standard 72 552 establishes the terminal numbering system that is used for any wiring
schematic or component diagram that conforms to DIN specifications. The code tells the
reader about the wire as a connection and not as a wire itself. Chart 1 in the appendix offers a
full list of terminal numbers and definitions.All DIN standard wiring schematics have current
flowing from the top to the bottom. This starts with a fuse (depicted by a symbol and a letter
destination F meaning fuse).The fuse has the number 15 above it. 15 is the terminal destination
number which tells you the electrical state of the wire is only live when the ignition switch is
on start or run (chart 1).This DIN standard is used on any component connection or electrical
connection. For example, the wire leading to the heater blower motor M3 starts with the num-
ber 14 which is voltage supplied in start or run and is overload protected. This simply means
that voltage will be supplied when the ignition is switched to the start or run position and that
the connection is fused. See chart in the appendix.
Figure 3.128
Detached terminal diagrams DIN 40 719
These are used to show just one component and the connection details. This is common on
aftermarket wiring diagrams like Autodata.Autodata sells books which include lots of wiring
connections for a large range of manufacturers.This allows the information to be interpreted
using individual components and is particularly useful with ECU connections. Technicians
often test a range of input and output signals going to and from electronic control units.
Because they are central to the operation of a system they are often useful as a testing point
(if accessible). Figure 3.128 shows two detached device terminal diagrams. The left uses sym-
bols and the right uses a pictorial representation.
Example:
30 See chart 1 G2: See Device letter codes + See chart 1
50 See chart 1 S2: See Device letter codes 50a See chart 1
●—❙ Ground symbol (earth)
Table 3.5
Symbols DIN 40 900
Symbols are the smallest component of a circuit diagram and are the simplest way to illustrate
a whole or part of a device like a sensor or actuator. Standards are used so engineers can eas-
ily identify a component based on its symbol. For example, the M3 blower motor has the sym-
bol of a filter and a motor. These placed together show that the assembly of the component
includes a motor assembly and filter to reduce electron magnetic interference. Chart 3 in the
appendices shows a range of symbols used.
Device letter codes Din 40 719 Part 2
With reference to Figure 3.127 F16 means Fuse number 16. F is a device letter code and is a
standard letter used in all DIN schematics. This is the same for M for Motor, R for Resistor.
Not all device codes resemble the letter designated to them, e.g.B for transducer.Examples of
device codes can be found in the Appendices, chart 2.
Wire colour codes DIN 47 002
Colour codes for electrical wiring are defined in DIN 47 002.The main colour defines the pur-
pose of the wire and the tracer defines its use.
BSI Standards (UK)
BS AU 7a: 1983 are the British Standards for colour coding (Table 3.4).The main colour defines
the purpose of the wire and the tracer colour determines its use. Twelve colours are used for
the main colour coding system.
Tracers are used to define the function of the wire.Table 3.5 shows a small sample. See the
Appendices for more details.
Example of a manufacturer’s wiring system – Ford FSC system
Since the introduction of the Mondeo the Ford Motor Company uses a system for circuit num-
bering and wire identification.The system is called Function System Connection (FSC).
The Function is the same as DIN terminal designation DIN 72 552. The base colour is
directly related to the function of the wire (terminal designation).
Table 3.6
The next part are system codes (Table 3.7), denoted by two letters, the first is the system
group and the second the actual system within that group.
Finally the connection is a number from 1 to 99 which defines which component the wire
goes to.The information also includes a colour code and cable size.The base colour is related
to the terminal designation of the wire, i.e. 29 colour OG – orange.Colour codes are related to
the English name of the colour itself (DIN IEC 757).
Example schematic – heater control module
The heater control module is built into the A/C switch panel inside the vehicle (where the A/C
and heating system are turned on/off).Using the manufacturer’s information it should be pos-
sible to determine the function/terminal designation of most of the single wires, what systems
they are related to, the cross-sectional area of the wire and the colour.
Table 3.7
In Europe, there is often a large difference between the quality and quantity of information
supplied with aftermarket wiring manuals compared to those supplied by the manufacturer.
The Ford TIS system has the ability of locating almost every component, connector on the
wiring schematic as well as supplying a wealth of other information like technical updates,
Table 3.8
Figure 3.129
Table 3.9
workshop manuals etc. The information is supplied by a CD-ROM system (recently updated
via the intranet).European aftermarket technical information suppliers have not yet achieved
the coverage required to be used as a single source for information. Often they can provide
service information for a range of manufacturers and popular models.They may also provide
technical support via email or telephone lines. Sometimes they offer specialist information on
areas like engine management and air-conditioning via CD-ROM and textbooks.What they
are very successful at is providing information that is easily understood and often presented in
an easy to follow format.
Manual A/C systems
System components
To maintain a constant temperature within the vehicle using a simple manual system requires
continual changes to fan speed and temperature selection by the occupant.
This will be due to the following:
1. Changes in vehicle speed affecting natural flow rates.
2. UV light (ultraviolet radiation) causing additional heating through glassed areas.
3. Number of occupants.
4. Interior and exterior temperature.
5. Humidity levels.
To allow for a system to automatically account for all these plus other variables a control sys-
tem must be used. Because a manual control system uses the occupant to adjust the comfort
zone, only a simple control system is required. The module is generally an ASIC type con-
troller with no Electronically Programmable Read Only Memory (EPROM) containing data
tables (used for comparing data) which allow for closed loop control of a single or dual com-
fort zone. The controller will not be attached to a data bus system, or allow for OBD fault
codes or other serial type information.Often the A/C compressor will not be controlled by the
controller built into the A/C controls on a manual system. The A/C compressor will be con-
trolled by the powertrain control module (engine control module) based on information from
pressure switches. The temperature and air distribution are manually controlled by switches
and Bowden cables. Often the blower is a conventional DC blower motor with speed control
via a resistor pack although some systems may use a brushless blower motor due to greater
reliability and a reduction in noise.
The manual control system will soon be obsolete across the mass produced vehicle market.
A/C systems are evolving and graphical interfaces, even LCD displays, require more advanced
electronics which often include A/C selection.Electronic displays will replace conventional dis-
play systems and semi-automatic systems will be a part of the basic specification of a vehicle.
For more information see A/C modules and displays under section 3.2.
Semi-automatic systems
Semi-automatic systems always have an electronic control system using a module. Sensors
detect the temperature in the interior of the vehicle.The module compares the measured tem-
perature values (often integrated inside the module) with the temperature setting selected by
the user. If this comparison reveals a difference in temperature, the module activates heating
or air-conditioning according to needs and actuates the air temperature door control motor
and the blower motor.The system is semi-automatic due to the manual control of air distribution.
This means that the system does not include air distribution motors and air duct temperature
sensors. Depending on the circumstances, it may be necessary to activate the air-conditioning
Figure 3.130
automatically.This occurs in wet weather when the user selects ‘DEFROST’ mode.Warm air, pre-
viously dried by cooling, is best suited for drying internally misted windows.With semi-auto-
matic temperature control, the air distribution (except in ‘DEFROST’ mode) must be set by
the user. In addition, the user can switch off automatic operation and adjust the system manually.
For more information see A/C modules and displays under section 3.2.
Automatic climate control system
The main function of the automatic climate control system
As soon as a difference in the requested and actual temperature has been detected in the cli-
mate zone, the mixed air temperature and blower speed must be raised or lowered. The con-
trol module rotates the air mixing damper until the desired air mixture temperature is
achieved and then directs air distribution in order to ensure that comfortable conditions are
achieved as soon as possible and then maintained.
Electronic controlled temperature
The A/C control module supplies current to stepper motors (driver and passenger blend doors
if dual zone) which rotate air blend doors. The air blend door mixes cold air that has passed
through the evaporator with warm air that has passed through the evaporator and heat
exchanger (heater matrix).The interior temperature is calculated for the front seat passenger
and driver using a range of sensors. The calculated temperature is compared to the tempera-
ture set on the A/C module graphical display (if integrated).As soon as there is a difference,
the mixed-air temperature must be increased or decreased. The control module turns the air
blend door until the requested mixed-air temperature is achieved.When the lowest mixed-air
temperature is requested, the control module closes the coolant flow to the heat exchanger to
provide maximum cooling effect (only on some systems).
Electronic controlled air distribution
The control module supplies current to a stepper motor which rotates the air distribution
door. The air distribution control is dependent on the requested mixed-air temperature.
Demist/floor is selected at a high temperature value, floor/face vent or face vent is selected at
low temperature value. For cold engine starts, demist is selected initially. The calculated pos-
ition of the flap is dependent on how many pulses the control module has sent to the stepping
motors. For example, a rotary air distribution door may use 0% face vents, 20% face/floor,
50% floor, 75% floor/windscreen (demist) and 100% windscreen (demist).
Electronically controlled blower speed
The A/C control module communicates with the fan control which supplies current to the fan
motor. The fan current is at its lowest when the present mixed-air temperature matches the
requested temperature.The current increases as soon as cold or heated air is needed. For cold
starts, the value is determined by the coolant temperature and the outside temperature. The
fan current is limited if the engine is not running.
Electronically controlled air recirculation
The A/C control module supplies current often to a direct current motor which rotates the air
recirculation flap.
Recirculation is selected if the outside air temperature is high and the requested mixed-air
temperature is low (that is, the cabin has to be cooled significantly). It is also selected if an air
sensor is fitted and the ambient air contains pollution.
The automatic climate control system will often have a self-test facility which will be able to
display codes via the graphics interface.The system will have multiplex communication and be
able to provide data lists and fault codes via a serial connection.
For more information see A/C modules and displays under section 3.2.
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