chapter 08 bennet

8/10/2019 Chapter 08 Bennet

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Chapter 8

Charging Systems


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Objectives (1 of 2)

Identify charging circuit components.

Navigate a charging circuit schematic.

Voltage drop-test charging circuit wiring and

components.

Describe the construction of an alternator.

Explain full-wave rectification.


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Objectives (2 of 2)

Full-field an alternator.

Measure AC leakage in the charging circuit.

Verify the performance of an alternator.

Use Intelli-check to assess charging circuit

performance.

Disassemble and reassemble a Delcotron

40SI alternator.


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Charging Systems


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Alternator Construction (1 of 2)


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Alternator Construction (2 of 2)

To generate electricity, the alternator uses this basic

law of physics:

When magnetic lines of force move across a

conductor (such as a wire or bundle of wires), anelectrical current is produced in the conductor.

Actual current flow induced depends on several

factors:

The strength of the magnetic field The speed of the wire passing through the field

The size and number of wires


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Rotor

The rotor is the only moving component within thealternator.

It is responsible for producing the rotating magneticfield.

The rotor consists of a coil, two pole pieces, and ashaft.

The magnetic field is produced when current flowsthrough the coil; this coil is simply a series ofwindings wrapped around an iron core.

Increasing or decreasing the current flow through thecoil varies the strength of the magnetic field, which inturn defines alternator output.


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Slip Rings and Brushes

The wiring of the rotor coil is connected toslip rings.

The slip rings and brushes conduct current to

the rotor. Most alternators have two slip rings mounted

directly on the rotor shaft; they are insulatedfrom the shaft and from each other.

A spring-loaded carbon brush is located oneach slip ring to carry the current to and fromthe rotor windings.


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Stator

The stator is made up of many conductors, or wires,into which the spinning rotor induces voltage.

The wires are wound into slots in the alternatorframe, with each wire forming several coils spaced

evenly around the frame. The wires are grouped into three separate bundles,or windings.

The coils of the three windings are staggered in thealternator frame so that the electrical pulses created

in each coil will also be staggered. This produces an even flow of current out of the

alternator.


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Alternator Operation


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Voltage Regulators

Microprocessors and electronic sensors and

switches are easily damaged by voltage spikes and

high voltage levels.

The voltage regulator receives battery voltage as aninput.

This is called the sensing voltage; it allows the

regulator to sense and monitor the battery voltage

level. When the battery voltage rises to a particular level

(approximately 13.5 volts), the regulator will turn the

field current off.


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Charging System Failures and Testing

A malfunction in the charging system results

in either:

An overcharged battery

An undercharged battery


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Overcharging

An overcharged battery will produce water

loss, eventually resulting in hardened plates

and the inability to accept a charge.

Overcharging can be caused by one or acombination of the following:

Defective battery

Defective or improperly adjusted regulator Poor sensing lead contact to the regulator or

rectifier assembly


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Undercharging (1 of 2)

An undercharged battery will result in slow

cranking speeds and a low specific gravity of

the electrolyte.

A loose drive belt

Loose, broken, corroded, or dirty terminals on

either the battery or alternator

Undersized wiring between the alternator andthe battery

Defective battery that will not accept a charge


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Undercharging (2 of 2)

Undercharging can also be caused by one or a combination of thefollowing defects in the alternator field circuit: Poor contacts between the regulator and brushes

Defective diode trio

No residual magnetism in the rotor/shorted, open, or grounded

rotor coil Defective or improperly adjusted regulator

Damaged or worn brushes/damaged or worn slip rings

Poor connection between the slip rings and field coils

Undercharging can also be the result of a malfunction in thegenerating circuits: One or more of the stator windings (phases) can be shorted,

open, or grounded.

The rectifier assembly might be grounded.

One or more of the diodes might be shorted or open.


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Charging System Testing

The battery must be at least 75 percent chargedbefore the alternator will perform to specifications.

The output of the alternator is first tested.

If the output is below specifications, the voltageregulator is bypassed and battery current is wireddirectly to the field circuit of the rotor.

This is called full-fielding the rotor.

If this corrects the problem, the fault is in the

regulator. If the output remains low with the regulator bypassed

by full-fielding, the alternator might be defective.


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Full-field Testing

the Alternator (1 of 2)

By applying full battery voltage directly to the

field windings in the rotor, it can be

determined whether or not the regulator is

the cause of the undercharging condition. There are two variations of this procedure

that apply to alternators with external

regulators.


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Caution

When testing the output of a full-fielded alternator,

carefully observe the rise in system voltage.

Because the current output is not regulated, battery

voltage can quickly rise to an excessive level, highenough to overheat the batteries, causing electrolyte

to spew from the vent holes, and possibly damage

sensitive electronic components.

Do not allow system voltage to rise above 15V.


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Shop Talk

Some alternators with remote-mounted electronic

regulators are connected to the regulators by a

wiring harness and a multi-pin connector.

Full-fielding the field circuit is accomplished byremoving the connector from the regulator and

connecting a jumper wire between two pins

(terminals) in the harness connector (consult OEM

service literature to correctly identify the pinassignments). Doing so bypasses the regulator,

sending battery current directly to the field circuit.


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Caution

Never full-field an alternator without applying

an electrical load to the batteries.

Whenever the voltage regulator is bypassed,

there is nothing to control peak alternatoroutput; this can cause voltage spikes that can

damage both electrical and electronic

components.


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Full-field Testing

the Alternator (2 of 2) If the field circuit is grounded through the

regulator (an A circuit), the regulator isdisconnected from the field terminal on the

alternator and a jumper is connectedbetween the terminal and a ground.

If the alternator receives battery voltagethrough the regulator (a B circuit), the

regulator is disconnected from the field (F)terminal and a jumper is connected to theterminal and to the insulated battery terminal.


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Full-fielding Internally

Regulated Alternators

Full-field testing is not possible on some

alternators with internal voltage regulators.

To isolate and test the regulator on these

types of alternators, the alternator must beremoved from the truck and disassembled.

Other internally regulated alternators have a

hole through which the field circuit can betested.


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AC Leakage Test

Over time, the diode bridge can start to leak

AC current to ground.

Typically AC leakage should not exceed 0.3

volts AC.

Use a DMM set to the AC voltage scale.

Place one test lead on the alternator

insulated terminal and connect the other leadto chassis ground.


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Intelli-Check

Intelli-check should first be plugged into the two-way connector attached to the alternator output andground terminal.

Next, the engine should be started and run with no

chassis electrical loads turned on. Rev up the engine once to high idle.

Then turn on the lights and the heater blower to high,and once again rev up the engine again to high idle.

This completes the test, and the charging circuit

condition will be displayed by illuminating one of fiveLEDs.


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Scope Testing (1 of 2)

Oscilloscopes can be used to diagnose

alternator conditions.

The scope used may be either a full-sized

oscilloscope or a good-quality, hand-held labscope.

Use the scope manufacturers instructions to

make the connections.


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Scope Testing (2 of 2)


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Caution

Truck alternators are not provided with

reverse-polarity protection.

Reverse-polarity connections such as those

caused by jump-starting can destroy alternatordiodes and numerous other chassis solid-

state devices.


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Shop Talk

Alternators that energize the field circuit with current

produced by the stator windings rely on residual

magnetism in the rotor to initially energize the stator

when the engine is being started.

During handling or repair, this residual magnetism

can be lost.

It must be restored before testing the system. This is

done by connecting a jumper wire between the diodetrio terminal and the alternator output terminal as

shown in the following figure.


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Summary (1 of 4)

A truck charging system consists of batteries,alternator, voltage regulator, associated wiring, andthe electrical loads of the chassis.

The purpose of the charging system is to recharge

the batteries whenever necessary and to provide thecurrent required to power the electrical componentson the truck chassis.

A malfunction in the charging system results in either

undercharged or overcharged batteries. The subcomponents of an alternator consist of

stator, rotor, slip rings, brushes, and rectifier.


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Summary (2 of 4)

A magnetic field is established in the rotor windings,and this is used to induce current flow in a stationarystator.

Slip rings are used to conduct current to the rotor to

establish a magnetic field. Because the brushes in slip rings conduct very low

current to the rotor windings, they significantly outlastthe brushes used in now-obsolete generators.

The alternator rectifier is located in the end frameassembly.


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Summary (3 of 4)

Most current truck alternators use a deltaconfiguration of diodes in the rectifier to achieve fullwave rectification.

Current truck alternators use solid-state, electronic

voltage regulators. TR-type alternators use electronic switching in

12/24V systems, eliminating the need forelectromechanical series-parallel switches.

When the voltage regulator is shorted out of thecircuit, the alternator is full-fielded and will producemaximum output.


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Summary (4 of 4)

The sensing voltage of the charging system shouldbe battery voltage at any given moment of operation.

AC leakage testing can verify the performance of thediodes in the rectifier and help predict imminent

alternator failures. Scope testing can graphically identify rectifier solid-

state component failures.

Delco-Remys Intelli-check tool enables

technicians to make a rapid test of charging systemperformance.

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