ASE A1 Engine RepairEngine Electrical Systems (General)Medium
A technician is performing a diagnostic check on a vehicle's starting system. The battery voltage measures 12.6 volts with the engine off. When the key is turned to the 'start' position, the starter motor engages, but the engine cranks very slowly. A voltage drop test across the main positive battery cable (from battery positive post to starter motor B+ terminal) shows 1.5 volts. What does this voltage drop indicate?
- AExcessive resistance in the positive battery cable circuit.
- BA normal voltage drop for a healthy starting circuit.
- CThe battery is fully charged and operating correctly.
- DThe starter motor is drawing excessive current.
Show answer & explanationAnswer & explanation
Correct answer: A. Excessive resistance in the positive battery cable circuit.
A voltage drop of 1.5 volts across the main positive battery cable during cranking is excessive. The maximum allowable voltage drop for this cable is typically around 0.2-0.5 volts, indicating high resistance within the cable or its connections, leading to insufficient voltage reaching the starter and slow cranking.
Why the other options are wrong
- B. A normal voltage drop for the main positive battery cable should be much lower, typically below 0.5 volts during cranking. 1.5 volts is indicative of a problem.
- C. While the initial battery voltage is good, the excessive voltage drop indicates a problem elsewhere in the circuit.
- D. While a starter drawing excessive current could cause a slow crank, a high voltage drop across the cable specifically points to resistance in the cable itself, not necessarily the starter drawing too much. The cable is restricting current flow to the starter.
Starting Circuit Voltage Drop
Voltage drop testing measures the resistance within a circuit by measuring how much voltage is 'lost' across a component or section of wire when current is flowing.
- Excessive voltage drop indicates high resistance.
- High resistance reduces current flow and component operation.
- Typical maximum voltage drop for main cables is 0.2-0.5V.
Memory trick: Slow crank? Check the 'Voltage Drain' before you blame the brain!