ASE G1 Auto Maintenance and Light RepairElectrical/Electronic SystemsHard
A technician is performing a voltage drop test on the negative side of a starter motor circuit. With the engine cranking, the voltmeter is connected between the battery negative post and the engine block. A reading of 1.5 volts is observed. What does this indicate?
- ANormal resistance in the negative circuit.
- BA short circuit in the starter motor.
- CExcessive resistance in the negative (ground) circuit.
- DInsufficient voltage from the battery.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive resistance in the negative (ground) circuit.
A voltage drop of 1.5 volts on the negative side of the starter circuit during cranking is excessive. For a starter circuit, acceptable voltage drop on the negative (ground) side is typically 0.2 volts or less. A higher reading indicates excessive resistance in the ground path, which significantly impedes current flow and causes slow cranking.
Why the other options are wrong
- A. Normal voltage drop for the negative side of a starter circuit is usually 0.2V or less. 1.5V is far too high to be considered normal.
- B. A short circuit in the starter motor would likely cause a much larger current draw, potentially blowing a fuse or causing the battery to rapidly discharge, but the voltage drop test specifically measures resistance in the circuit, not a short within the component.
- D. The voltage drop test measures resistance in the circuit, assuming sufficient voltage from the battery. If battery voltage was insufficient, the drop would be less meaningful, but the 1.5V drop itself points to resistance.
Negative Side Voltage Drop (Starter)
A voltage drop test on the negative side of the starter circuit measures resistance in the ground path between the battery negative post and the engine block. Acceptable drop is typically 0.2V or less during cranking.
- Measures resistance in the ground circuit.
- Connected between battery negative and engine block.
- High reading (e.g., >0.2V) indicates excessive resistance.
Memory trick: Voltage Drop: Power's Path, Check the Math!