A technician is diagnosing a hard-starting issue on a haul truck. The starter motor clicks loudly but doesn't engage the engine consistently. A check of the battery voltage indicates 24.5 V, and the starter solenoid's pull-in coil circuit shows 24 V when the key is turned to 'start'. However, the main contact voltage across the solenoid is only 1.5 V when measured after the click, before the starter motor itself. What is the most probable cause?

Question: A technician is diagnosing a hard-starting issue on a haul truck. The starter motor clicks loudly but doesn't engage the engine consistently. A check of the battery voltage indicates 24.5 V, and the starter solenoid's pull-in coil circuit shows 24 V when the key is turned to 'start'. However, the main contact voltage across the solenoid is only 1.5 V when measured after the click, before the starter motor itself. What is the most probable cause?

Answer options: ✅ Excessive internal resistance in the starter solenoid's main contacts.

• A short circuit in the starter motor armature windings.
• An open circuit in the starter motor field windings.
• A discharged battery causing insufficient current.

Correct answer: Excessive internal resistance in the starter solenoid's main contacts.

Explanation: If the solenoid clicks (indicating the pull-in coil is energized) and there's a significant voltage drop across the main contacts (from 24 V supplied to only 1.5 V reaching the starter), it suggests high resistance within the solenoid's main contacts. This resistance prevents sufficient current from flowing to the starter motor. A short or open in the starter motor would typically result in a different current draw or complete failure, and the battery voltage appears adequate for activation.