Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairMedium
A technician is performing a voltage drop test across the insulated (positive) battery cable from the battery post to the starter motor's main power terminal. With the engine cranking, the voltmeter reads 0.8 volts. What does this reading indicate?
- AInsufficient current flow through the cable.
- BExcessive resistance in the positive battery cable.
- CA short circuit in the starter motor.
- DA normal voltage drop for the positive battery cable.
Show answer & explanationAnswer & explanation
Correct answer: B. Excessive resistance in the positive battery cable.
A voltage drop of 0.8 volts across the positive battery cable during cranking is typically considered excessive. The maximum acceptable voltage drop for each main battery cable (positive or negative) is usually around 0.2-0.3 volts, with some specifications allowing up to 0.5 volts. 0.8V indicates high resistance.
Why the other options are wrong
- A. Insufficient current flow would usually result from high resistance elsewhere or a weak battery, but the 0.8V drop directly indicates high resistance in the tested cable itself, not just insufficient flow.
- C. A short circuit in the starter motor would likely cause a much higher current draw and potentially a lower voltage drop across the cable (as the voltage would be pulled down at the starter), or blow a fuse/melt the cable.
- D. Normal voltage drop for a main battery cable during cranking is much lower, typically below 0.5V.
Main Battery Cable Voltage Drop (Positive)
Measuring the voltage difference between the battery positive post and the starter motor's main power terminal during cranking to assess resistance in the positive cable circuit.
- Measured with a voltmeter while the engine is cranking.
- Target maximum drop is typically 0.2-0.5 volts.
- Higher readings indicate excessive resistance (corrosion, loose connection, damaged cable).
Memory trick: When volts drop low, resistance starts to show.