Toyota training body electrical diagnosis ch01

Technician Handbook
652 Body Electrical Diagnosis

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Technician Handbook
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Use this space to write any questions you may have for your instructor.
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Automotive
Electrical Circuits

Every vehicle electrical circuit contains the following:
•  Load
•  Voltage source
•  Ground
•  Over-current protection
•  Control device

•  Conductors
•  Connectors
•  Continuous path for current from power source
to ground

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Load

Working devices – or loads – consume electricity. They change
electrical energy into another form of energy to do work.
The purpose of a circuit is to provide electrical current to the load so
it can perform a function in the vehicle.
Examples of loads include:
•  Light bulb
•  Motor
•  Solenoid
•  Heating resistor

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Voltage Sources

The voltage source is the “electron pump” that provides the
pressure differential in the circuit. This pressure differential gets the
electrons moving within the conductors.
On the vehicle, voltage is provided by the battery and the generator.
In some cases, a capacitor is used to temporarily maintain voltage
inside a component (such as an ECU or ECM) after the key is
turned off or power removed.

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Other Voltage Sources

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Sensors used in ECU controlled systems such as TCCS and ABS
produce an AC voltage that the ECU uses to determine RPM, or other
operating conditions. Noise filters are capacitors which act as voltage
“accumulators ” to dampen sudden voltage changes. The ignition coil
uses magnetic induction to step up the 12V battery voltage into the KV

(thousands of volts) necessary to fire the spark plug.

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Technician Handbook
652 Body Electrical Diagnosis

Over-current
Protection Devices

Fuses, circuit breakers, and fusible links are built into every
automotive circuit to prevent damage from excessive current.
Excessive current can be caused by a short to ground before a load.
When the load is bypassed there is nothing to limit current in the
circuit. Without a fuse in the circuit, wires and connectors could be
damaged by high current flow.
The circuits protected by a single fuse are designed so that normal
current does not exceed 70% of the fuse’s rating.

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Control Devices

Most circuits on the vehicle are switched off and on to control when
they operate. This switching is done through the use of relay contacts,
transistors, and of course, mechanical switches.
To limit or control the amount of current in a circuit, a series resistor
or pulse width control can be used. Examples of this are found in A/
C blower motor circuits and in interior light circuits.
Several circuits are designed to be ON at all times. These include
Theft Deterrent Systems, and components with electronic memory
such as radios, clocks and ECUs (Electronic Control Units). Circuits
that are ON all the time create parasitic loads which draw current from
the battery even when the engine is off and the vehicle is parked.

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Conductors

A conductor provides a low resistance pathway for current. All
conductors have a certain amount of resistance. The resistance is
determined by the conductor’s:
•  Diameter - Larger area has less resistance.
•  Length - Shorter wire has less resistance.
•  Material - Some materials conduct better than others.
•  Temperature - Heat increases the resistance in the conductor.
•  Physical Condition - Corrosion or nicks in the wire

increase resistance.
Current flow occurs through the conductor any time there is a
continuous connection between the positive and negative terminals
of the voltage source.

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Connectors

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

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Circuit Operation

When the switch in the circuit is closed, there is a continuous path
for current flow from the voltage source to ground.
Current flows through the conductors and connectors to the loads
where voltage is used up illuminating the bulbs.
Though most of the voltage is used up, current continues to flow
through to the ground and back to the voltage source.

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Principles of
Electricity

Voltage

The first step in effectively diagnosing electrical problems is to have a

good understanding of basic electrical principles. These concepts
were covered in depth in the 623 course. In this course, we will review
each concept, emphasizing how they are applied on the car when
diagnosing an electrical problem.
Simply put, think of voltage as electrical pressure or pressure
differential. The difference in pressure that makes a hydraulic pump
work applies similarly to electricity. The pressure differential created
between the positive and negative terminals of the battery causes
the electrons in a conductor to move when the two terminals are
connected. This movement or flow of electrons performs useful work.
Whenever work is done, pressure is used up. We can measure where
work is done in an electrical circuit by measuring where voltage is
used up.

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Voltage Measurements

Two types of voltage measurement are:
•  Available voltage is the amount of electrical pressure (voltage)
present at a test point in the circuit.
•  Voltage drop is the decrease in electrical pressure (voltage)
between two points in a circuit. This is the amount of voltage used
up between the two test points.

In both of these tests, the circuit must be operating and current flowing.
Both available voltage and voltage drop testing have their place in the
diagnostic process. Information from each of these tests will be used to
diagnose the most likely place to look for a problem in a malfunctioning
electrical circuit.

Available Voltage

When current is flowing and the negative probe is on a ground point,
the voltmeter measures available voltage.
In the example above, 12.5 volts is available to the lamp because a
small amount of battery voltage was used up in the wiring before the
test point.
Only 0.05 volt is left in the circuit after the lamp, because most of the
voltage was used up in illuminating the lamp.

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Open Circuit Voltage

Voltage measured when current is not flowing in the circuit is
called open circuit voltage. The reason current would not be flowing
is because of an open in the circuit disrupting the path for current
flow back to the voltage source. An open can be caused by a switch

or other control device in the open position, or a break or
disconnection in the wiring or connectors.
With an open in the circuit:
•  Voltage measured anywhere in the circuit before the open
(power side) will be equal to source voltage.
•  Voltage measured anywhere in the circuit after the open (ground
side) will be zero volts.

HINT

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If you are measuring voltage in a circuit that is not operating and
find source voltage at one test point and zero volts at another test
point, the open in the circuit is between the two test points.

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Technician Handbook
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Voltage Drop

Whenever current passes through any component (i.e., current is
flowing), voltage is used up. The voltage used up is called voltage
drop (ΔV).
Δ is the Greek letter delta. The symbol delta means “change in.”
When we use the abbreviation ΔV it indicates “change in voltage”
or voltage drop.

The voltage drop of a component is directly proportional to the
resistance of the component. Low resistance components like fuses,
switches, wires and connectors should have very low voltage drop. As a
general rule the maximum voltage drop allowed for these components is
less than 0.1V per component or connector in a body electrical circuit.
Higher resistance components are usually referred to as loads. Loads
use their resistance to convert current into work (light, heat, motion). This
conversion causes voltage drop as the electrical pressure is used up.
Typical loads include lamps, motors, relay coils and most sensors. When
loads are in series, each component’s voltage drop is proportional to its
resistance. The higher the resistance, the higher the ΔV.
In any electrical circuit, all the voltage is always used up. Adding up
all the voltage drops in a circuit always equals source voltage.

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Measuring Voltage Drop

Voltage drop is measured with current flowing. The measurement is
taken between the power side and the ground side of the
component (or series of components) being tested.

Understanding Voltage
Drops

In a normally operating circuit, the loads consume most of the
available voltage. If the circuit has an unwanted high resistance,
however, the high resistance will consume part of the available
voltage, and the loads will use up the remainder. This will result in a
lower-than-normal voltage drop across the loads, and a higher-thannormal voltage drop elsewhere in the circuit.

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Technician Handbook
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Both lamps in this circuit are dim. This could be due to an unwanted high
resistance in the circuit. Would you expect the problem to be in the series or
parallel portion of the circuit?
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The normal voltage drop across wiring and connectors is less than 0.1 volt. Is there
a problem in this connector?

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The voltage drop across this connector is much higher than the typical 0.1v or less.
This indicates a high resistance in this connection, which is using up a significant
amount of the circuit’s voltage.
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The bulbs are burning dimly because they are receiving less than their normal 12v.
Note that the total of the voltage drops across the bad connector and the bulb
equals 12.5v.
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Current

Current is the term used to describe the flow of electrons through
the circuit. This flow of electrons does the “work” in the circuit.
The unit for measuring the amount of current flow is the ampere or
amp (A). One amp equals 628 billion billion electrons per second
flowing through a circuit.
Current will only flow if there is a complete circuit between a source
of higher voltage (power) and a lower voltage (ground). Voltage is
the pressure that pushes the electrons through the circuit and
amperes are the measure of current flow.

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Measuring Amperage

An ammeter is used to measure current flow, also called amperage
because it is measured in amperes (amps, for short). To measure amps,

current must flow through the ammeter. The circuit must be opened so
the ammeter can be placed “in-line” with the circuit.
Small values of current flow are expressed in milliamps (mA). One amp
equals 1000 milliamps. A value of 650 milliamps equals 0.650 amp.

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Power

Though you normally won’t need to calculate power to diagnose a
circuit, it may be useful to know that a component’s power
consumption is determined by multiplying the component’s voltage
drop by the current flowing through it.
Power is measured in watts (W). One horsepower equals 746 watts.

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Resistance

Some materials allow current to flow through them easily. Other
materials do not. Electrical resistance describes how much a
material opposes current flow.
This opposition to current is measured in Ohms (Ω) or in kilohms
(kΩ). One kΩ equals 1,000 Ohms. In every circuit resistance
Resistance determines the amount of current flow as long as the
voltage stays constant.
Materials with very low resistance are called conductors. Copper,
aluminum, gold, and silver are good conductors.
Materials with extremely high resistance are called insulators.
Rubber, glass, paper, ceramics, plastics and air are good insulators.
Materials that are neither good conductors nor good insulators are
called semiconductors. Silicon is an example of a semiconductor.
In a “perfect” circuit, the only resistance would be the load you are
operating with the circuit. But even the best conductors have a
certain amount of electrical resistance.
Resistance is measured using an ohmmeter.

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Measuring Resistance

Resistance tests are primarily used for determining:

•  Circuit continuity to locate opens, shorts, or grounds.
•  Component resistance to determine if it meets specifications.
When using an ohmmeter, disconnect the component from the
circuit to isolate the measurement from other current flow paths and
voltage sources.
Voltage drop tests are quicker and more reliable for diagnosis
because high resistance can be missed by resistance testing.
Sometimes high resistance can only be found when attempting to
operate the load.

CAUTION

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Applying voltage to the test leads when the meter is set to measure
resistance can damage some meters. Always disconnect power
before measuring resistance.

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