Cranking system

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Cranking system

• Cranking circuit
• The starter motor
• Starter drives
• Starter solenoids
• Testing and diagnosis

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Cranking circuit

• Starter motor
• Battery
• Starter solenoid and/ or relay
• Starter drive
• Ignition switch and safety control switches
• Neutral safety switch
• Clutch switch

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Starter motor

• Electromagnetism basics
• A magnetic field surrounds every conductor
  carrying a current
• The strength of the magnetic field is increased
  as current flow is increased

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Starter motor

• Electric motor that converts electrical energy
  (up to 500 amps) to mechanical energy (up to 8
  hp) to crank engine
• Housing contains 4 positive electromagnets
• Current is controlled by a solenoid or relay
• Electromagnets are wired around an iron core
  called a pole shoe

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Starter motor

• The two pairs of shoes are wired in opposite
  directions to provide north and south magnetic
  fields and referred to as field coils
• A shaft called an armature rotates in the center
  of the field coils
• Brushes transfer the current from the field coils
  to the armature
• Current travels from the field coils to the
  armature magnifying the magnetic fields that
  rotate the armature

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Starter motor

• The torque output of the starter is determined by
  the strength of the magnetic fields inside the
  starter
• The torque of the starter decreases as the
  starter speed increases
• Some starters use permanent magnets in place of
  electromagnets
• PM starters are advantageous because of smaller
  size and lighter weight
• Disadvantage is lower torque

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Starter motor

• PM starters do not produce as much torque and
  therefore require gear reduction to multiply
  torque

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Starter motor

• PM starters have a higher armature speed
• Use a planetary gear drive for gear reduction
• Consume less current and therefore allow
  manufacturers to install smaller battery cables
• PMs are more susceptible to damage from impacts

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Starter motor

• Starter drives
• Starter incorporates a pinion gear to drive ring
  gear on flywheel
• Pinion gear ratio is usually 181 (9 teeth to 166
  teeth)
• Normal engine cranking speed is 200-250 rpm
• Starters must have a one-way clutch on pinion
  drive to prevent engine from turning starter

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Starter motor

• Starter solenoid
• Electromagnetic switch
• Contains two windings
• Windings are of two different gauge sizes
• Heavy winding is called the pull in winding
• Purpose is to draw the plunger into solenoid
  which moves the starter drive pinion into mesh
  with the ring gear

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• The smaller winding is called the hold in winding
  and produces enough magnetic force to keep the
  plunger in position

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• As the plunger is drawn into the solenoid, the
  plunger makes contact with a metal contact that
  connects the positive battery cable with the
  motor terminal to operate the starter motor
• The contact bypasses the pull in winding

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Testing and diagnosis

• Voltage drop testing
• Low voltage drop low resistance
• High voltage drop high resistance
• Voltage drop is the drop in voltage that occurs
  when current is flowing through a resistance
• Voltage drop is the difference between voltage at
  the source and voltage at the electrical device
  to which it is flowing

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Testing and diagnosis

• Before voltage drop testing can be done, current
  must be flowing through the circuit
• Resistance is not effective unless current is
  flowing
• A voltmeter tells you the difference in voltage
  from one test lead to another

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Testing and diagnosis

• Starter current draw tests
• 4 cylinder engines 150 to 185 amps
• 6 cylinder engines 160 to 200 amps
• 8 cylinder engines 185 to 250 amps
• Excessive draw may indicate
• Starter motor worn
• Internal engine problems
• High resistance in starter motor
• Starter motor internally shorted

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Testing and diagnosis

• Control circuit
• Battery
• Ignition switch
• Security system
• Neutral safety or clutch switch
• Starter solenoid

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