Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairMedium
After replacing a starter motor, a technician notices that the engine cranks much slower than before, even though the battery is fully charged and tests good. All connections are tight. A voltage drop test on the negative (ground) battery cable during cranking shows 0.7 volts. What is the most likely cause of the slow cranking?
- AA short circuit in the positive battery cable.
- BThe engine has developed an internal mechanical issue.
- CA faulty new starter motor.
- DExcessive resistance in the negative battery cable or its connections.
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive resistance in the negative battery cable or its connections.
A voltage drop of 0.7 volts across the negative battery cable is excessive (should typically be 0.2-0.3V, max 0.5V). This indicates high resistance in the ground circuit, which restricts current flow to the starter motor, causing slow cranking. The fact that it's a new issue after starter replacement and the drop is on the negative side points directly to this problem.
Why the other options are wrong
- A. A short circuit in the positive cable would likely prevent cranking entirely or blow a fuse, and a voltage drop test on the positive cable would show the issue.
- B. An internal mechanical issue would cause slow cranking but would not specifically manifest as an excessive voltage drop on the negative battery cable during the test.
- C. While possible, the voltage drop test on the negative cable points to a specific electrical issue, not necessarily a faulty starter unless it's a secondary effect.
Negative Cable Voltage Drop
Measuring the voltage difference between the battery negative post and the engine block (or starter motor case) during cranking to assess resistance in the starter's ground circuit.
- Maximum acceptable drop is typically 0.2-0.5 volts.
- Excessive drop indicates high resistance due to corrosion, loose connections, or damaged cable.
- High resistance in the ground circuit restricts current flow, causing slow or no cranking.
Memory trick: Slow crank, volts drop, ground's the path, make it stop.