Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairEasy
A technician is performing a voltage drop test on the starter motor's main positive cable (battery to starter). With the engine cranking, the voltmeter reads 0.5 volts. The starter motor cranks slowly. What does this reading indicate?
- AA short circuit within the starter motor, causing high current draw.
- BExcessive resistance in the main positive cable, requiring replacement.
- CA discharged battery, as indicated by the low voltage drop.
- DNormal resistance in the main positive cable, indicating the problem is elsewhere.
Show answer & explanationAnswer & explanation
Correct answer: D. Normal resistance in the main positive cable, indicating the problem is elsewhere.
A voltage drop of 0.5 volts across the main positive cable during cranking is generally within acceptable limits (typically less than 0.2V-0.5V, depending on manufacturer). This indicates that the positive cable itself is not the primary cause of the slow cranking, and the technician should investigate other parts of the starting circuit.
Why the other options are wrong
- A. A short circuit in the starter motor would cause very high current draw and a much larger voltage drop across the cable, or potentially melt the cable/fuse.
- B. Excessive resistance would be indicated by a voltage drop significantly higher than 0.5V (e.g., 1.0V or more).
- C. A discharged battery would show low voltage at the battery terminals and likely a chatter from the solenoid, but the voltage drop test specifically measures resistance across the cable, not battery charge.
Starter Cable Voltage Drop
Measurement of voltage loss across a cable during high current flow, indicating resistance.
- Acceptable drop for main battery cables is typically less than 0.2V-0.5V.
- High voltage drop indicates excessive resistance in the cable or its connections.
- Measured with a voltmeter while the engine is cranking.
Memory trick: Low drop is 'No Flop', high drop is 'Trouble Top'.