The purpose of this test is to investigate the correlation between the ignition trigger and secondary HT events in a COP type ignition coil.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever 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):
Uninsulated HT pickups are designed to clip around double-insulated HT cables only – they are not designed for direct connection to a hazardous live voltage. To prevent injury or death, switch off the engine and secure against restart. Clean and inspect the HT lead for damage to insulation and fit only to undamaged fully insulated leads. Ensure that leads are safely clear of hot and / or rotating parts. The engine can now be restarted for the test duration.
Preparation for the test will require disconnection, removal and reconnection of the suspect coil unit.
Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.
The waveform above shows the relationship between the secondary HT on channel A (blue trace) and the digital trigger signal (red trace). When the trigger signal goes high, the coil's primary circuit is closed, causing a current from the battery to flow through it. At the end of the dwell time, the trigger signal returns low, breaking the primary circuit and causing the secondary winding to generate a high tension (HT) voltage.
As the illustration is a dual trace picture, the two traces will be explained one at a time.
The low tension (LT) signal switches between 0 volts and 4 volts. When the trigger signal reaches 4 volts, the coil switches on and the 'saturation' or 'dwell' time begins. As the voltage returns to zero, the current in the coil's primary winding switches off, the magnetic flux in the iron core collapses, induces a voltage in the secondary, and the HT voltage is generated.
The coil switch-on and switch-off times are determined by the vehicle's Electronic Control Module (ECM). The dwell time on an engine with electronic ignition is controlled by the current limiting circuit in the amplifier or ECM.
In a constant energy system, the dwell time is fixed regardless of engine speed. This allows the coil to saturate fully and maximises the strength of the magnetic flux. The dwell angle, measured relative to a complete engine cycle of 360°, increases as the engine speed increases.
The modern engine management system with Coil-Per-Cylinder (CPC) ignition has all the advantages of a constant energy electronic ignition system, with the added bonus of eliminating the distributor cap, king lead, rotor arm and plug lead. Reliability problems from dampness and tracking are now almost eliminated.
Unlike a conventional Distributorless Ignition System (DIS) which fires the plugs with both a negative and a positive voltage, CPC only fires the plugs with a negative voltage, improving plug life and increasing the service life of the plugs.
Inside the coil's primary winding is the secondary winding. This is coiled around a multi-laminated iron core and has approximately 20,000 to 30,000 turns. One end is connected to the primary terminal and the other to the coil tower. The High Tension (HT) voltage is produced by mutual induction between the primary winding and the secondary winding, the central soft iron core serving to intensify the magnetic field between them.
The voltage measured at the spark plug is the voltage required to jump the plug gap under varying conditions. This voltage can be affected by any of the following:
Example pin data for two, three and four pin COP units.
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.
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