ASE G1 Auto Maintenance and Light RepairElectrical/Electronic SystemsMedium
A technician is performing a voltage drop test on the positive side of a starter motor circuit. They connect the positive lead of a voltmeter to the battery positive terminal and the negative lead to the starter motor's battery cable terminal. During cranking, the voltmeter reads 0.8 volts. What does this reading indicate?
- AA short circuit within the starter motor itself.
- BA low battery charge, causing the voltage to drop.
- CExcessive resistance in the positive battery cable or connections.
- DA healthy positive side circuit with acceptable resistance.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive resistance in the positive battery cable or connections.
A voltage drop reading of 0.8 volts on the positive side of the starter circuit during cranking indicates excessive resistance in that circuit. Acceptable voltage drop for the positive side is typically 0.2 volts or less, or up to 0.5V on some heavy-duty applications. Exceeding this indicates a problem with the cable or its connections.
Why the other options are wrong
- A. A short circuit within the starter motor would likely cause excessive current draw, possibly blowing fuses or preventing cranking, but a voltage drop test measures resistance in the cables leading to the starter, not internal starter faults.
- B. While a low battery charge would cause overall system voltage to drop, a voltage drop test specifically measures the resistance within a circuit. The 0.8V reading is the voltage 'lost' across the cable, not the battery's overall state.
- D. A healthy positive side circuit should show a voltage drop of 0.2 volts or less during cranking, so 0.8 volts is not acceptable.
Voltage Drop Test (Starter Circuit)
A voltage drop test measures the resistance in a specific part of a circuit by checking the voltage difference across that component or wire while current is flowing.
- Positive side voltage drop (battery to starter B+ terminal) should be 0.2V or less.
- Negative side voltage drop (starter ground to battery ground) should be 0.2V or less.
- Higher readings indicate excessive resistance (corrosion, loose connections, damaged cable).
Memory trick: High drop means resistance's crop.