Transcript Heavy-Duty Truck Sytems Chapter 09

Chapter 9
Cranking Systems
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.
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.
Cranking Systems (1 of 3)
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 two
sub-circuits:
– Control circuit
– Starter circuit
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
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 by
measuring voltage drop.
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.
Cranking Circuit Components
• See Table 9-1 on page 230 of the textbook.
Starter Motor Operation (1 of 2)
Starter Motor Operation (2 of 2)
Starter Solenoids
Override Clutches
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 large
bore, high-compression engines because
they use a planetary gearset.
• This arrangement provides for a reduction
ratio of about 3.5:1.
Reduction-gearing
Starter Motors (2 of 4)
Reduction-gearing
Starter Motors (3 of 4)
Reduction-gearing
Starter Motors (4 of 4)
Cranking Circuit Testing
• The cranking circuit requires testing when:
– The engine will not crank
– The engine cranks slowly
– The starter motor will not turn
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
Troubleshooting
• See Table 9-2 on pages 240-241 in the
textbook.
Shop Talk
• Starter motors can only operate for short
periods without rest.
• The high current needed to operate the
starter creates considerable heat, and
continuous operation will cause overheating.
• A starter motor should never operate for
more than 30 seconds at a time and should
rest for 2 minutes between cranking. This
permits the heat to dissipate without damage
to the unit.
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 control
circuit.
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.
Cranking Current Testing (1 of 2)
• The cranking current test measures the amount of
current, in amperes, that the starter circuit draws to
crank 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 voltmeter
reading.
– 4. Stop cranking and adjust the carbon pile until the
voltmeter reading matches the reading taken in step
3.
– 5. Note the ammeter reading.
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.
Cranking Current Testing (2 of 2)
• See Table 9-3 on page 242 of the textbook.
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 voltagesensing module to prevent starter operation if
voltage is below 9.6 volts to prevent drivers
from destroying starters.
Starter Circuit Testing
Warning
• When performing this test, do not operate the
solenoid for extended periods of time as
severe overheating will occur.
Adjusting Pinion Clearance
Shop Talk
• When testing starter circuits, use the OEMrecommended method of preventing the
engine from starting.
• Whenever possible, use autoranging DMMs
to perform voltage-drop tests.
Summary (1 of 3)
• The vehicle cranking circuit functions to crank the
engine until it can operate under its own power.
• A cranking circuit is managed by a control circuit
that uses low current to switch and energize a highcurrent starter motor circuit.
• A starter motor converts the electrical energy of the
vehicle batteries into mechanical energy for
cranking the engine.
• A powerful electro-magnetic field in the starter
motor field coils is used to rotate the armature
assembly in a typical starter motor.
Summary (2 of 3)
• Most starter motors are series wound, so there is
only one path for current flow through the assembly.
– This means that all of the current flowed through the
field 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 multiply
cranking torque and greatly reduce motor weight.
• Voltage-drop testing should be used to troubleshoot
a cranking circuit malfunction because it does so
dynamically.
Summary (3 of 3)
• A heavy-duty truck starter motor found to be defective should be
removed from the engine and either rebuilt or replaced.
• A growler is used to test an armature for shorts: A steel strip such
as a hacksaw blade is placed on top of the armature during
testing 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 should
be checked with thickness gauges and adjusted if necessary.