Toyota training body electrical diagnosis ch01

652 Body Electrical Diagnosis The voltage source is the “electron pump” that provides the pressure differential in the circuit.. 652 Body Electrical Diagnosis Given a relatively fixed vo

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652 Body Electrical Diagnosis

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Use this space to write any questions you may have for your instructor

NOTES:

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Every vehicle electrical circuit contains the following:

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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:

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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

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

Other Voltage Sources

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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

Over-current Protection Devices

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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

Control Devices

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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

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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

Connectors

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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

Circuit Operation

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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

Principles of

Electricity

Voltage

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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

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

Voltage Measurements

Available Voltage

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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

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

Open Circuit Voltage

HINT

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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”

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

Voltage Drop

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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

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-than-normal voltage drop elsewhere in the circuit

Measuring Voltage Drop

Understanding Voltage

Drops

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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?

NOTES:

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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 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

Current

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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

Measuring Amperage

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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

Power

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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

Resistance

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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

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

Measuring Resistance

CAUTION

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The three types of electrical circuits are:

•  Series

•  Parallel

•  Series-Parallel Series circuit resistance is probably the easiest concept to understand because it is simply adding up of all the resistances in a circuit to get the total resistance

Total resistance determines the number of amps that can flow through the circuit Ohm’s Law says that amps can be determined

by dividing ohms into voltage A circuit with 12 volts and 4 Ohms total resistance would have 3 amps of current flowing

Amps = Volts / Ohms Amps = 12v / 4Ω

Basic Circuit

Principles

Series Resistance

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In parallel circuits, the total circuit resistance is always less than the smallest branch resistance

Moreover, adding a new parallel branch lowers the total circuit resistance and increases total circuit current flow Using the example above, assume a third branch is added with a 2Ω load

What would the total current flow and resistance be then?

This explains why adding a parallel component in a circuit can sometimes blow a fuse

Parallel Resistance

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A true series-parallel circuit has a load in series connected to multiple loads in parallel In this type of circuit, the load in series causes a voltage drop so the parallel loads receive less than full source voltage

To find the total resistance of a series-parallel circuit, add the resistance of the series part of the circuit to the resistance of the parallel part In a sense, when the total resistance of the parallel part is determined, the entire parallel segment is treated as if it were

a single series component

Calculating voltage, current, and resistance in a circuit is not essential for repairing automobiles

What is important is understanding the relationships between voltage, current, and resistance You need to be able to predict what

should happen and compare it to what is happening in a problem Series-Parallel Resistance

Why Does It Matter?

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Ohm’s Law explains the relationship between voltage, current, and resistance

Ohm’s Law states: The current in a circuit is directly proportional

to the applied voltage and inversely proportional to the amount

of resistance

Mathematically, we can always predict what electricity is going to

do in a circuit, as long as we know what any two of the three values are

According to Ohm’s Law:

•  If applied voltage increases and resistance stays the same, current flow increases

•  If applied voltage decreases and resistance stays the same, current flow decreases

•  If voltage is constant and resistance increases, current flow decreases

•  If voltage is constant and resistance decreases, current flow increases

Ohm’s Law

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