Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing a vehicle with an intermittent 'no crank' condition. When the ignition key is turned to the 'start' position, the technician hears a single click from the starter relay, but the starter motor does not engage. The battery voltage is 12.5 volts. A voltage drop test across the starter motor's main power cable (from battery positive to starter motor terminal) during an attempted start shows 0.2 volts. What is the most likely cause of the 'no crank'?
- AA faulty starter motor solenoid.
- BExcessive resistance in the starter motor ground circuit.
- CA discharged battery.
- DA faulty ignition switch.
Show answer & explanationAnswer & explanation
Correct answer: B. Excessive resistance in the starter motor ground circuit.
A single click from the starter relay indicates the relay is energizing, sending power to the starter solenoid. A 0.2V drop on the positive cable is acceptable. This scenario points to a problem on the ground side of the starter motor. If the starter motor isn't getting a good ground, it won't engage or spin, even if power is supplied, and the click could be the solenoid trying to engage but lacking the current flow to fully operate the motor.
Why the other options are wrong
- A. The single click suggests the solenoid is receiving power from the relay, but the starter isn't engaging, which could be a solenoid issue, but an open ground circuit is also a strong possibility for the motor not engaging.
- C. 12.5V is a good battery voltage, indicating it's not discharged.
- D. The starter relay clicks, indicating the ignition switch is successfully sending the 'start' signal to the relay.
Starter No-Crank Diagnosis
Troubleshooting a vehicle that fails to crank the engine when the ignition is turned to 'start', despite a good battery.
- Check battery voltage first.
- Listen for clicks (relay, solenoid).
- Test power (B+ to starter) and ground (starter to chassis/battery).
- Voltage drop tests are crucial for high-current circuits.
Memory trick: Battery, Relay, Power, Ground, Motor.