Download Basic Vehicle Electric circuits

Introduction
As automotive technology continues to evolve and become even more complex,
mechanical systems are being enhanced or even replaced with electronic components
and systems. Electronic suspension and anti-lock brakes are two of the major changes
that have taken place under today’s vehicles. Electronic brake by wire systems have
already arrived and magnetic brake retarders may soon replace friction pads.
Basic Vehicle Electric circuits
Regardless of how complete a circuit or electronic system may be it will always
require a voltage supply and a path to ground in order to function correctly. Many
technicians are replacing parts following serial interrogation without further testing of
the circuit in question. Serial diagnostics alone cannot identify the problems and
often displays the symptoms leaving the technician vulnerable to an incorrect
diagnosis.
Actuators
Components that require current flow as a function of their operation e.g. ignition
coil, fuel injectors and an idle stepper motors require a stable voltage supply and a
secure ground reference. Without these basic requirements, the current flow through
the component will be compromised leading to incorrect operation or failure.
Sensors
Sensors operate from a stabilised 5v supply from the PCM and will be adversely
affected with discrete errors in voltage supply and earth reference points. Sensor
output errors often lead to misleading DTC’s.
Basic Diagnostic Rules
A very simple rule that will steer you clear of basic component error is to first check
the output of the component. Initially this can be done through serial communication
and confirmed later, if necessary, with a digital multimeter (DMM) or an
oscilloscope. The assumption being that if the output is correct and in range for the
condition it is monitoring then the power and ground supplies will also be correct. If
an error is detected in the output value then the power supply and ground reference
must be proven before replacing the component. If a trigger or a signal is required to
stimulate the operation the component, such as an immobiliser output to the PCM,
this must also be checked before condemning the suspected component. This rule
applies to simple circuits such as a coolant sensor as well as the more complex PCM.
Choice of Tools
Serial communication should always be considered as a first option due to its
simplicity and ease of connection. It is also non intrusive which means nothing is
disturbed during the procedure. Simple circuits such as a headlamp bulb or coolant
sensor can be evaluated using a DMM. However, more complex circuits such as an
immobiliser signal requires the use of an oscilloscope which will display very
accurate circuit activity.
Circuit Load
Circuits that require a substantial current to operate a component rely on a substantial
voltage supply from the battery and alternator. This must be stable and the circuit
must be able to carry enough current for the component to operate correctly. When
the supply voltage drops, so will the current flow. Supply voltage and ground checks
must be made when the circuit is in a loaded condition i.e. when the component in the
circuit is active. A component in the circuit should consume the entire load. For
example, a bulb when illuminated should have a supply voltage within 0.5v of the
battery potential and have an earth or ground reference of 0.25v. This ensures the
bulb is consuming all the current determined by the consumption rating measured in
watts.
Current = Watts
Voltage
Ground Errors
A total ground failure will result in no current flow, however, the supply voltage will
appear normal on the supply side of the component. No load is present and therefore
is not a valid circuit test. Some of the most challenging problems relate to discrete
errors in supply voltage or ground references and will adversely affect the
performance of a sensor without any tangible evidence through serial communication.
Circuit Overload
If you find a blown fuse or a component failure due to severe current overload i.e.
burnt out, you must not replace the component or the fuse until the circuit has been
tested for short circuits and/or damage to the wiring. Use a DMM to check the
resistance through the wiring and components from the power supply source through
to ground reference. It is also vital to check for short circuits between these reference
points. For example, where two cables have fused together due to a previous current
overload.
Most short circuits on a vehicle are caused by rubbing or chaffing to ground
especially where movement is excessive. Components which overheat due to current
overload often suffer excessive damage and possible damage may occur to the circuit
itself. Relays are used to switch a high current flow component controlled by a low
current flow in order to protect the control switch.
Simple terms you need to know
Voltage
Voltage is electrical pressure so the higher the voltage the greater the force pushing it
through the circuit e.g. a high tension spark. Voltage will flow if there is a potential
difference across a circuit. For example, a bulb with 12v on each side of the element
will have no potential difference therefore no current flow will take place and hence it
will not illuminate. The same bulb with 12v on one side and a ground reference on
the other will have a large potential difference and will shine brightly due to the
current flow. The same bulb with 12v on one side and 6v on the ground side caused
by a bad earth will consume only half the current and burn very dim, if at all. Voltage
can be measured by a DMM or an oscilloscope.
Voltage = Amps x Resistance
Current
Current is a unit of electrical flow and is used to drive a component such as a fuel
pump or a starter motor. Current should have no interruption, therefore a large
section cable is used in circuits where a high current demand is essential e.g. main
battery cables and starter motor cable. Today current is often controlled by
electronics, this allows a more accurate control and can lead to a reduction in the size
of the cables. For example, an injector or stepper motor can be driven by a pulse
width modulation signal (PWM) resulting in extremely accurate and rapid current
control. High current and high resistance will lead to excessive heat and a serious risk
of fire. Current can be measured by a DMM or an inductive current clamp.
Amps = Volts
Resistance
Resistance
Resistance is used to restrict voltage flow. A motor or solenoid with many windings
will have a higher resistance than a light bulb. Switches or connectors ideally should
have little or no resistance at all. Vehicles with CAN network or airbag systems
(SRS) are extremely sensitive to resistance change in their control circuits.
Resistance = Volts
Amps
Watts
The unit of electrical consumption or work done is the watt. The higher the
consumption rating, the more current will be required to meet demand. For example,
a 60watt bulb with a 12v supply will consume 5amps of current, whereas a 100watt
bulb with a 12v supply will consume 8.3 amps.
Watts = Volts x Amps
Vehicle electronics, despite being complex, still follow basic electrical theory: voltage
supply and an earth reference with continuity must exist for the circuit to be complete.
Complex circuits however can only be qualified after serial interrogation with the
application of an oscilloscope. This allows the technician to see the normally
invisible voltage or current patterns within the circuit. Electronic control operates in
time frames often down to 1/100000 second (micro). A DMM cannot function at this
speed leaving only high performance oscilloscope as the technician’s tool of choice.
Our Future
Electronics have changed our future forever. Do not be afraid of the change they
represent but accept it as an opportunity and challenge it with investment and
continuous training. The future beyond will bring prosperity and respect that our
industry deserves. Failure to meet this demand will guarantee our demise. At ADS,
we are proud to be part of the changing industry providing skills training and
incentive to all those who wish to join. We welcome you all and good luck.