Question 9 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is D **Explanation for Option D (Correct Answer):** The significant gap between the start solenoid plunger (C) and the rod that closes the main starter motor contacts serves a crucial mechanical sequencing purpose known as the "pinion shift delay." 1. **Solenoid Action:** When the ignition key or start button is activated, current flows through the solenoid coil (pull-in and hold-in windings). 2. **Plunger Movement (Shift Phase):** The energized solenoid pulls the plunger (C) inward. This initial movement mechanically pushes the starter drive assembly (Bendix drive/pinion gear) forward, causing the pinion gear to engage (mesh) with the teeth of the engine's flywheel ring gear. 3. **Gap Closure (Contact Phase Delay):** The gap exists so that the plunger must travel a specific distance, accomplishing the gear engagement, **before** the plunger finally pushes the contact rod far enough to bridge the main starter motor contacts. 4. **Current Flow (Motor Activation):** Only after the gap is closed and the contacts are bridged does the high-amperage current flow from the battery to the main starter motor windings, causing the motor to spin the engine. This design ensures that the pinion gear is fully meshed with the flywheel **before** the motor begins to rotate at high speed, preventing severe damage to the gear teeth (chipping or stripping) that would occur if the gear tried to mesh while the motor was already spinning powerfully. **Explanation of Why Other Options Are Incorrect:** * **A) Upon starter solenoid coil energization, this gap prevents chattering and associated arcing of the starter motor contacts.** * This is incorrect. Chattering is typically caused by low battery voltage or a failing hold-in winding, not by the initial mechanical gap. The gap's function is purely mechanical sequencing (delaying contact closure) rather than electrical stability. * **B) Upon starter solenoid coil energization, this gap delays the engagement of the pinion to the flywheel ring gear until after the starter motor contacts close.** * This is the exact opposite of the solenoid's function. If the contacts closed first, the motor would spin rapidly before engagement, leading to massive grinding and damage (as explained above). The whole purpose of the gap is to ensure engagement happens **before** contact closure. * **C) Upon starter solenoid coil energization, this gap compensates for starter motor armature reaction to minimize arcing at the brushes.** * This is incorrect. Armature reaction and minimizing brush arcing are inherent electrical design issues solved by brush placement, compensating windings, and commutator design. The mechanical gap is external to the motor's internal electrical operation and has no role in compensating for armature reaction.