ASE A1 Engine RepairEngine Electrical Systems (General)Easy
A technician is diagnosing a no-crank condition on a vehicle. With the ignition switch in the START position, the technician measures 12.6 volts at the battery positive terminal, but only 0.5 volts at the starter motor's large power terminal. What is the MOST likely cause of this voltage drop?
- AA faulty starter solenoid
- BA discharged battery
- CAn open circuit in the starter motor windings
- DHigh resistance in the battery cables or connections
Show answer & explanationAnswer & explanation
Correct answer: D. High resistance in the battery cables or connections
A significant voltage drop between the battery and the starter motor's main power terminal, while the battery itself shows good voltage, indicates high resistance in the circuit path, most commonly due to corroded or loose battery cables/connections.
Why the other options are wrong
- A. A faulty starter solenoid would prevent the starter from engaging, but typically wouldn't cause such a large voltage drop on the main power feed to the starter itself unless it was internally shorted, which is less likely than cable resistance.
- B. The battery is measured at 12.6 volts, which is a healthy state of charge, ruling out a discharged battery as the primary cause of the voltage drop to the starter.
- C. An open circuit in the starter motor windings would prevent the starter from turning, but it would not cause a voltage drop across the cables; full battery voltage would still be present at the starter's power terminal.
Voltage Drop Diagnosis
Measuring voltage at different points in a circuit under load to identify excessive resistance.
- A significant voltage drop (typically > 0.2V) across a component or wire indicates high resistance.
- Measurements are taken with the circuit active and under load.
- Common causes include corrosion, loose connections, or damaged wires.
Memory trick: When the car won't crank, check the 'BATTERY' first, then the 'CABLES', 'STARTER', and 'IGNITION'.