Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is diagnosing an intermittent starting issue where the starter motor occasionally does not crank when the key is turned to the 'START' position. A voltage drop test on the starter motor's main positive cable (from battery positive to starter positive terminal) during a 'no-crank' event shows a reading of 0.2V. What is the MOST appropriate next step for diagnosis?
- AInspect the starter solenoid's control circuit for proper voltage.
- BCheck the battery's state of charge and terminal connections.
- CPerform a voltage drop test on the starter motor's ground cable.
- DReplace the starter motor, as it is likely faulty.
Show answer & explanationAnswer & explanation
Correct answer: C. Perform a voltage drop test on the starter motor's ground cable.
A voltage drop of 0.2V on the positive cable is generally acceptable under load. This indicates the positive side of the high-current circuit is likely good. The next logical step in a high-current circuit diagnosis is to check the ground side to ensure it can adequately handle the current flow.
Why the other options are wrong
- A. The diagnosis is currently focused on the high-current circuit, not the control circuit, and the positive side has been checked.
- B. The voltage drop test was performed, implying the battery had enough charge to attempt cranking and a connection was made.
- D. 0.2V drop on the positive cable is not indicative of a faulty starter motor.
High Current Circuit Voltage Drop
Measuring the voltage difference across a conductor in a high-current circuit while under load to identify excessive resistance.
- Typical acceptable voltage drop is less than 0.5V on either positive or negative side.
- Performed with the circuit active (e.g., starter cranking).
- Helps pinpoint resistance issues in cables, connections, or components.
Memory trick: If the positive cable is fine, the ground is your next line.