Starting System: Description and Operation

Starting System Description and Operation

Circuit Description

When the ignition switch is turned to the Start position, the body control module (BCM) receives battery voltage on the ignition 1 voltage circuit, and less than 5 volts on the off/run/crank voltage circuit. The BCM supplies voltage to energize the RUN/CRANK Relay 32. The BCM sends a high speed GMLAN serial data message to the engine control module (ECM) that the key is in the start position. The ECM receives the message from the BCM, and also receives a serial data message from the transmission control module (TCM) containing the shift lever position. If the shift lever is in Park or Neutral when ignition switch is turned to Start, the ECM supplies voltage to the START Relay 31. The START relay supplies voltage to the S terminal of the starter solenoid, through the 30 A STRTR Fuse 26, in order to crank the engine.


Starter Operation

The starter motors are non-repairable motors. They have pole pieces that are arranged around the armature. Both solenoid windings are energized. The pull-in winding circuit is completed to the ground through the starter motor. The windings work together magnetically to pull and hold in the plunger. The plunger moves the shift lever. This action causes the starter drive assembly to rotate on the armature shaft spline as it engages with the flywheel ring gear on the engine. Moving at the same time, the plunger also closes the solenoid switch contacts in the starter solenoid. Full battery voltage is applied directly to the starter motor and it cranks the engine.


As soon as the solenoid switch contacts close, current stops flowing thorough the pull-in winding because battery voltage is applied to both ends of the windings. The hold-in winding remains energized; its magnetic field is strong enough to hold the plunger, shift lever, starter drive assembly, and solenoid switch contacts in place to continue cranking the engine. When the engine starts, pinion overrun protects the armature from excessive speed until the switch is opened.


When the ignition switch is released from the START position, the START relay opens and battery voltage is removed from the starter solenoid S terminal. Current flows from the motor contacts through both windings to the ground at the end of the hold-in winding. However, the direction of the current flow through the pull-in winding is now opposite the direction of the current flow when the winding was first energized.


The magnetic fields of the pull-in and hold-in windings now oppose one another. This action of the windings, along with the help of the return spring, causes the starter drive assembly to disengage and the solenoid switch contacts to open simultaneously. As soon as the contacts open, the starter circuit is turned OFF.