Coil-on-plug primary voltage and current (2-wire)

The purpose of this test is to investigate the primary voltage and current within a 2-wire Coil on Plug (COP) unit.

Typical connection drawing

How to perform the test

View connection guidance notes.

1. Use manufacturer data to identify COP unit circuit functions.
2. Connect PicoScope Channel A. to the coil supply circuit.
3. Connect the low amp clamp to PicoScope Channel B, select the 20 A scale and zero the clamp before attaching to the same coil supply circuit.
4. Start the engine and allow it to idle.
5. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
6. Start the scope to see live data.
7. With your waveforms on screen stop the scope.
8. Stop the engine and turn the ignition off.
9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note 

The orientation of the current clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

Channel A: Coil supply voltage

The waveform being monitored is the supply voltage to the coil. The supply is at the battery or charging voltage of 12 volts or more. When the coil's primary circuit is switched on, the voltage drops and remains at a low level until the current in the primary circuit reaches the target of 5.5 amps, at which point the supply voltage is allowed to spike for ignition.

Channel B: Primary ignition current

The example waveform above shows the current-limiting circuit in operation. The current switches on as the dwell (ECU controlled ground) period starts and rises until approximately 5.5 amps is reached in the primary circuit. At this point the current is released at the point of ignition. The length of time from the initial switching-on point to the moment the current is released depends on engine speed. The lower the engine speed, the shorter the current ramp; then the ramp lengthens with increasing engine revs.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, ignition coil primary voltage or ignition coil primary current.

Further guidance

Primary Voltage

Historically, the supply voltage was present as soon as the ignition switch was turned to the 'on' position. Modern systems, however, do not provide a supply until the key is turned to the 'crank' position and the engine turns. A simple fault such as a non-functioning crank angle sensor may result in a loss of supply voltage, simply because the electronic control circuits do not recognize that the engine is rotating.

Primary Current

The example waveform shows the current in the primary circuit switches on as the dwell period starts, and rises until a level of 5.5 amps is reached.

As the engine speed increases, the dwell angle expands to maintain a constant coil saturation time and therefore constant energy. The coil saturation time can be measured by placing one time ruler at the beginning of the dwell period and the other at the end of the current ramp. The distance between the rulers will remain exactly the same regardless of engine speed.

GT161