Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairEasy
A technician is performing a voltage drop test on the primary side of an ignition coil. With the ignition key ON and engine OFF, the technician measures 12.5V at the coil's positive terminal relative to ground. When the engine is cranking, the voltage drops to 10.0V. The specification for cranking voltage at the coil is typically no less than 9.5V. What does this voltage drop indicate?
- AThere is excessive resistance in the ground circuit of the coil.
- BThe voltage drop during cranking is within acceptable limits for the primary circuit.
- CThe battery is severely discharged and requires replacement.
- DThe coil is drawing too much current, indicating an internal short.
Show answer & explanationAnswer & explanation
Correct answer: B. The voltage drop during cranking is within acceptable limits for the primary circuit.
A voltage drop during cranking is normal due to the high current draw of the starter motor. As long as the voltage remains above the minimum specified threshold (9.5V in this case), the supply voltage to the coil's primary circuit is considered adequate.
Why the other options are wrong
- A. An excessive ground circuit resistance would cause a voltage drop across the ground side, not necessarily the supply side, and would likely be present even with Key ON/Engine OFF.
- C. While a low battery can cause low cranking voltage, 10.0V is above the 9.5V minimum, suggesting the battery is adequate for starting.
- D. If the coil was internally shorted and drawing too much current, the voltage drop would likely be much more severe, potentially below the 9.5V minimum, and could lead to other symptoms.
Ignition Primary Voltage Drop
The reduction in voltage observed in the primary ignition circuit, especially during engine cranking, due to normal current draw and circuit resistance.
- Normal for voltage to drop during cranking.
- Must remain above minimum specification for proper spark.
- Excessive drop indicates high resistance or weak battery.
Memory trick: Cranking voltage always drops, but not too low.