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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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Cranking Systems (2 of 3)
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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Cranking Systems (3 of 3)
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
Starter Motors (2 of 4)
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Reduction-gearing
Starter Motors (3 of 4)
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Reduction-gearing
Starter Motors (4 of 4)
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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.