Coil-on-plug primary voltage and current (3-wire)

The purpose of this test is to monitor the primary voltage and current within a 3 wire coil on plug unit.

Connection guidance

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:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

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:
• Probe > 10:1 Attenuator
• Probe > 20:1 Attenuator

How to perform the test

  1. Use manufacturer data to identify the function of each of the three coil terminals.
  2. Connect the low amp current clamp to PicoScope channel A, select the 20 A scale and zero the clamp before attaching to the voltage supply terminal of the coil to be tested.
  3. Connect PicoScope channel B to the signal terminal.
  4. Connect PicoScope channel C to the voltage supply terminal.
  5. Connect PicoScope channel D to the earth terminal.
  6. Start the engine and allow it to idle.
  7. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  8. Select Go or press the space bar to see live data.
  9. With your live waveforms on screen select Stop or press the space bar to stop your capture.
  10. Stop the engine and turn the ignition off.
  11. Use the Waveform Buffer and Zoom tools to examine your waveform.

Waveform notes

Channel A: Primary ignition current

The example four-channel waveform, above, shows the current-limiting circuit in operation. The current switches on as the dwell period starts and rises until approximately 11 amps is reached in the primary circuit. At this point the current is maintained for a brief period of time and then 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.

Channel B: Digital switching signal

The low-tension (LT) signal switches between zero volts and about 5 volts. When the trigger signal goes high, it causes the coil to switch on. As the voltage returns to zero, the current in the coil's primary winding switches off, the magnetic flux surrounding the winding collapses, this induces a voltage in the secondary and the coil's HT is fired.

The switch-on (zero rising to 5 volts) and switch-off (5 volts to zero) points are determined by the vehicle's Electronic Control Module (ECM). This interval between these events is called either the dwell period or the saturation time. The dwell period on an engine with electronic ignition is controlled by the current-limiting circuit in the amplifier or ECM. The time taken for the coil to reach saturation is about 3 milliseconds in our example.

Channel C: 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. In the example waveform, the voltage is about 14.0 volts. When the coil's primary circuit is switched on, the voltage drops slightly, and as the current in the circuit increases to the target of 11 amps, the voltage drops accordingly. The final voltage is about 12 volts - 2 volts lower than the original voltage.

Channel D: Coil amplifier earth

The voltage when the coil is disconnected is of course zero volts, rising to about 0.1 volts when the coil is energized. If the circuit is suffering from a poor earth connection, this voltage will be higher, so the lower the voltage, the better the earth connection.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, COP (Coil on plug ignition) primary voltage or COP (Coil on plug ignition) trigger signal.

Example 3-wire COP

Further guidance

Primary Current

The example waveform shows the current limiting circuit in operation. The current in the primary circuit switches on as the dwell period starts, and rises until a level of 11 amps is reached. This current is maintained until it is released at the moment of ignition.

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.

Digital switching signal

The switch-on (zero rising to 5 volts) and switch-off (5 volts to zero) points of the coil are determined by the vehicle's Electronic Control Module (ECM). The time between these points is called either the dwell period or the coil's saturation time. The dwell period on an engine with electronic ignition is controlled by the current-limiting circuit in the amplifier or ECM.


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 earth connection is essential to the operation of any electrical circuit in an engine. As the current increases, so does the voltage drop on any given electrical circuit. An earth return circuit can only be tested while the circuit is under load, so simple continuity testing to earth with a multimeter is inaccurate. As the coil's primary circuit is only complete during the dwell period, the voltage drop should be monitored during this period.

The voltage ramp on the earth signal should not exceed 0.5 volts. The flatter the waveform the better: a waveform with virtually no rise shows that the amplifier or module has a perfect earth. If the ramp is too high, the earth connections need to be investigated to identify the offending connection.


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.

Suitable accessories

  • Premium 6-way breakout lead set


  • 20 A / 60 A DC (low amps) current clamp


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Large Dolphin/Gator Clips


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Guided test: Coil-on-plug primary voltage and current (3-wire)