The purpose of this test is to monitor the primary voltage and current within a 2 wire coil on plug unit.
Connection for diagnostic work will vary dependent on application.
Technicians should wherever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
To avoid damage to your scope, you may need to use an attenuator for this test.
These instructions do not refer to an attenuator as it is based on our PicoScope 4425 automotive scope.
If you are using a previous PicoScope Automotive model, you will need either a 10:1 or 20:1 attenuator and will need to adjust the Probe settings for the relevant channel.
These settings can be found under Channel Options, then:
Use the Waveform Buffer and Zoom tools to examine your waveform.
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.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.
June 22 2017
We’ll get on this straight away. Thanks for bringing it to our attention,
May 08 2017
This guided test is incorrect for Channel A. You wouldn’t connect Channel A to the positive side of the coil to see a primary voltage waveform. You would connect it to the negative side of the coil. All you will see is a slight voltage drop if you remain connected to the positive side.