A vehicle with a coil-on-plug (COP) ignition system has a misfire on cylinder #4. The technician performs an amperage ramp test on the ignition coil for cylinder #4 using an oscilloscope and observes a sudden, sharp drop to zero amps during the dwell period, followed by an immediate rise. What does this waveform indicate?
- AA shorted primary winding within the ignition coil.
- BAn intermittent open circuit in the ground control (driver) for the coil.
- CAn open primary circuit to the ignition coil.
- DExcessive resistance in the coil's power supply circuit.
Show answer & explanationAnswer & explanation
Correct answer: B. An intermittent open circuit in the ground control (driver) for the coil.
An amperage ramp test shows the current building in the primary winding during the dwell period. A sudden, sharp drop to zero amps followed by an immediate rise indicates an intermittent interruption in the primary circuit's ground path. The current builds, is suddenly interrupted, and then resumes if the connection is restored. This points to an intermittent open in the coil driver or its wiring.
Why the other options are wrong
- A. A shorted primary winding would cause the amperage to rise excessively high and potentially blow a fuse or damage the driver, not drop to zero.
- C. An open primary circuit (permanent) would result in no current flow at all, showing a consistently flat zero-amp line, not a sudden drop and rise.
- D. Excessive resistance in the power supply would cause the amperage ramp to be lower or slower than normal, but it would not suddenly drop to zero and then rise.
Ignition Coil Amperage Ramp
An oscilloscope waveform displaying the current flow (amperage) through the primary winding of an ignition coil during the dwell period, used to diagnose coil and circuit integrity.
- Shows current build-up (ramp) and dwell time.
- Abnormalities indicate issues with coil, power, ground, or driver.
- Sudden drops indicate intermittent opens.
Memory trick: Amps ramp, smooth and steady. Drop to zero? Ground's not ready!