The purpose of this test is to investigate the primary voltage and current within a 2-wire Coil on Plug (COP) unit.
This test involves measuring a potentially hazardous voltage.
Please ensure you follow manufacturers' safety instructions and working practices and ensure the rated voltage for all accessories you are using meets or exceeds the expected voltage.
To avoid possible damage to your scope, you may need to use an attenuator for this test.
Scopes with a 200 V range, such as PicoScope 4x25 models, do not need an attenuator for this test.
All other PicoScope Automotive models need an attenuator on the channel input. You can use either a 10:1 or a 20:1 attenuator provided that you adjust the PicoScope software accordingly. Select from the appropriate Channel Options menu:
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.
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.
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.
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.
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.
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