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

The purpose of this test is to monitor the primary voltage and current within a 2 wire coil on plug unit.

How to perform the test

Channel A - Supply voltage

If there is any doubt as to the maximum peak voltage level being within the scope's input range (refer to scope lid) an attenuator must be used. If using an attenuator the appropriate built-in probe must be selected from within the channel options.

  • Connect an attenuator (10:1 or 20:1) into Channel A of the scope.
  • Plug a BNC-to-4-mm test lead into the attenuator.
  • Connect a Back-pinning Probe to the colored (positive) connection of the test lead.
  • Place a large black clip onto the black (negative) plug on the test lead and clip it to a convenient earth connection in the engine bay.
  • Identify which wire is the supply voltage to the coil pack and back-probe the wire, or make a connection using a set of breakout leads.

Channel B - Supply current

  • Plug the low-amp current clamp into Channel B of the scope.
  • Set the clamp to the 20 amp setting if available and auto-zero it.
  • Place the clamp around the same wire as Channel A. If the waveform is inverted, replace the clamp in the opposite direction.

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.

Coil-on-plug unit

Figure 2 - Coil-on-plug unit

Technical information

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.


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

  • 30 A (low amps) current clamp


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


  • Connector breakout lead kit A


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Small Crocodile/Gator Clips


  • PicoScope Sprung Hook Probe


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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2 comments | Add comment

Pico Mike
June 22 2017

Hi Larry,

We’ll get on this straight away. Thanks for bringing it to our attention,


Larry Dean
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.

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Guided test: Primary Voltage and Current (2-wire)