Chapter 09 Bennet

8/10/2019 Chapter 09 Bennet

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Chapter 9

Cranking Systems


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Objectives (1 of 2)

Identify the components in a truck cranking

system.

Explain the operating principles of magnetic

switches, solenoids, and starter motors.

Describe the operating principles of

lightweight, planetary gear reduction starter

motors. Test and troubleshoot a cranking circuit using

voltage drop testing.


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Objectives (2 of 2)

Disassemble a heavy-duty truck starter

motor.

Test an armature for shorts using a growler.

Test an armature for grounds and opens.

Use a test light to check out field coils.

Outline the procedure required to rebuild a

Delco-Remy 42MT starter motor.


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Cranking Systems (1 of 3)


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The cranking system in any vehicle is

designed to turn the engine over until it can

operate under its own power.

A cranking system can be divided into twosub-circuits:

Control circuit

Starter circuit


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In a typical cranking system, the components

are:

Battery

Key switch (or starter button)

Battery cables

Magnetic switch

Starter motor


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Battery Terminals

Cleaning terminals

If any evidence of corrosion is observed on

visual inspection, a battery terminal should be

removed from the post and cleaned.

Testing terminals

The only way to determine whether a battery

terminal is doing its job is to test it bymeasuring voltage drop.


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Cables

Cables conduct the heavy current load from the

battery to the starter and from the starter back to

the battery.

All cables must be in good condition. Cables must be heavy enough to carry the required

current load.

Cables can be corroded by battery acid.

Corrosion will cause a voltage drop and decrease

circuit amperage, reducing power available to the

starter.


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Cranking Circuit Components

See Table 9-1 on page 230 of the textbook.


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Starter Motor Operation (1 of 2)


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Starter Motor Operation (2 of 2)


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Starter Solenoids


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Override Clutches


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Reduction-gearing

Starter Motors (1 of 4)

Reduction gearing starters are lightweight

starters capable of cranking engines up to 16

liters.

Lightweight starters are able to crank largebore, high-compression engines because

they use a planetary gearset.

This arrangement provides for a reductionratio of about 3.5:1.


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Reduction-gearing



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Reduction-gearing



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Reduction-gearing



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Cranking Circuit Testing

The cranking circuit requires testing when:

The engine will not crank

The engine cranks slowly

The starter motor will not turn


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Preliminary Checks

Cranking output obtained from the starter

motor is affected:

By the condition and charge of the battery

The wiring circuit

The engine cranking requirements


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Troubleshooting

See Table 9-2 on pages 240-241 in the

textbook.


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Shop Talk

Starter motors can only operate for shortperiods without rest.

The high current needed to operate the

starter creates considerable heat, andcontinuous operation will cause overheating.

A starter motor should never operate formore than 30 seconds at a time and should

rest for 2 minutes between cranking. Thispermits the heat to dissipate without damageto the unit.


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Starter Relay Testing

The starter relay bypass test is a simple method of

determining if the relay is operational.

Connect a jumper cable around the starter relay.

This bypasses the relay. Then, crank the engine.

If the engine cranks with the jumper installed, the

starter relay is defective and should be replaced.

If the motor still will not crank, check the controlcircuit.


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Shop Talk

In some starters, the solenoid also performs

the function of a relay.

The control circuit is wired to the windings of

the solenoid.

Battery current is routed directly to the starter

motor through the drive solenoid.


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Cranking Current Testing (1 of 2)

The cranking current test measures the amount ofcurrent, in amperes, that the starter circuit draws tocrank the engine. 1. Connect the leads of a volt-amp tester

(Figure 9-20).

2. Set the carbon pile to its maximum resistance(open).

3.Crank the engine and observe the voltmeterreading.

4. Stop cranking and adjust the carbon pile until thevoltmeter reading matches the reading taken in step3.

5. Note the ammeter reading.


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Shop Talk

If the analyzer uses an inductive pickup,

ensure that the arrow on the inductive pickup

is pointing in the right direction as specified

on the ammeter. Then, crank the engine for 15 seconds and

observe the ammeter reading.


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Cranking Current Testing (2 of 2)

See Table 9-3 on page 242 of the textbook.


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Shop Talk

The starter should not be operated if the

voltage at the battery is below 9.6 volts.

Some leasing companies now use a voltage-

sensing module to prevent starter operation ifvoltage is below 9.6 volts to prevent drivers

from destroying starters.


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Starter Circuit Testing


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Warning

When performing this test, do not operate the

solenoid for extended periods of time as

severe overheating will occur.


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Adjusting Pinion Clearance


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Shop Talk

When testing starter circuits, use the OEM-

recommended method of preventing the

engine from starting.

Whenever possible, use autoranging DMMsto perform voltage-drop tests.


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Summary (1 of 3)

The vehicle cranking circuit functions to crank theengine until it can operate under its own power.

A cranking circuit is managed by a control circuitthat uses low current to switch and energize a high-

current starter motor circuit. A starter motor converts the electrical energy of the

vehicle batteries into mechanical energy forcranking the engine.

A powerful electro-magnetic field in the startermotor field coils is used to rotate the armatureassembly in a typical starter motor.


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Summary (2 of 3)

Most starter motors are series wound, so there isonly one path for current flow through the assembly.

This means that all of the current flowed through thefield coils also flows through the armature producing

peak torque at close to stall speeds. A new generation of lightweight, high-torque starter

motors uses planetary gear reduction to multiplycranking torque and greatly reduce motor weight.

Voltage-drop testing should be used to troubleshoota cranking circuit malfunction because it does sodynamically.


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Summary (3 of 3)

A heavy-duty truck starter motor found to be defective should beremoved from the engine and either rebuilt or replaced.

A growler is used to test an armature for shorts: A steel stripsuch as a hacksaw blade is placed on top of the armature duringtesting and if it vibrates, a short is indicated.

A test light is used to test an armature for grounds and opens. Field coils should be checked using a test light.

After reassembling a starter motor, the pinion clearance shouldbe checked with thickness gauges and adjusted if necessary.

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Chapter 09 Bennet