Logic Circuits and Starter Motor Diagnostics

Digital Logic Circuits

An encoder is a logic circuit with 2ⁿ inputs, where n is the number of outputs. Its main application is to convert input signals, in no apparent order, into binary words to subsequently operate with them.

A decoder is a logic circuit with 2ⁿ outputs, where n is the number of inputs. It performs the opposite function of the encoder; for a specific sequence of ones and zeros (input words), it activates a single corresponding output.

A multiplexer is a logic circuit whose function is to channel 2ⁿ input signals into a single output line of information.

A demultiplexer channels a single input of information into 2ⁿ output lines, based on the combination of selection signals.

Starter Motor Components and Types

The starter motor is divided into the following key parts:

• Casing or inductor
• Rotor or armature
• Commutator side support
• Drive side support
• Drive mechanism (pinion)
• Starter relay

There are several starter motor types, such as the inertia starter motor with a fork gear link, starter motors with a reducer, and sliding armature starter motors.

Starter Motor Circuits

The excitation circuit is governed by the ignition switch to feed the excitation terminal of the starter relay.

The power circuit feeds the starter motor directly through the contacts of the relay.

Common Starter Motor Breakdowns and Causes

• The starter does not work, and no relay movement is heard.
  Causes: Shorted or grounded relay windings, or a jammed relay plunger (nucleus) in the drive mechanism.
• The starter does not rotate, but the relay movement can be heard.
  Causes: Cut armature coils, inductor shunted to ground, excessively worn commutator segments (blades), a defective spring, or dirt/deformation in the brush holder preventing contact between the brushes and the commutator.
• The starter turns with difficulty.
  Causes: Poor contact between the relay terminals, partial seizure of the armature due to dirt or worn bearings, short circuit or shunt to ground in the armature or stator, worn brushes, or insufficient pressure on the brush springs.
• The pinion rotates freely (empty).
  Causes: Broken teeth on the pinion gear or flywheel crown, a damaged freewheel clutch, or a broken gear fork (hairpin).
• The starter motor keeps running after releasing the switch.
  Causes: Stiff relay plunger (nucleus) or a broken return spring.
• The pinion gear does not retract quickly.
  Causes: Broken return spring, or dirt in the drive mechanism and the splines (striatum) of the shaft.
• Excessive starter noise.
  Causes: Wear of the pinion and flywheel crown teeth, excessive clearance in the armature shaft, or a rapid clicking ('machine-gun') noise.