Coil-on-plug primary voltage and current (4-wire)

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Note: This test is for a 4-wire COP with a feedback signal to the ECU. Some 4-wire COPs have two ground pins and no feedback signal: these should be tested as a 3-wire COP (see 3-Wire COP Test).

Channel A - Trigger signal

  1. Plug a BNC-to-4-mm test lead into Channel A of the scope.
  2. Connect a Back-pinning Probe to the colored (positive) plug on the test lead.
  3. Place a black clip on the black (negative) plug on the test lead and clip it to a convenient ground/earth connection in the engine bay.
  4. Identify the trigger (or 'switching') signal to the coil and back-probe this wire using the Back-pinning Probe, or make connections using a set of breakout leads.

Channel B - Feedback signal

  1. Plug a BNC-to-4-mm test lead into Channel B of the scope.
  2. Connect a Back-pinning Probe to the colored (positive) plug on the test lead.
  3. Identify the feedback signal from the coil and back-probe this wire using the backpinning probe, or make connections using a set of breakout leads.

Example waveform

Waveform notes

Channel A: Trigger Signal

The trigger signal rises from 0 volts to about 4 volts at coil switch-on and then returns to 0 volts at coil switch-off. The time between these events, called the dwell period or the coil saturation time, is determined by a current-limiting circuit in the vehicle's Electronic Control Module (ECM) or ignition amplifier.

Channel B: Feedback signal

The feedback signal tells the ECU that ignition has occurred. The signal is inverted, so a high level (about 5 volts) indicates the inactive state and a low level (0 volts) indicates a successful ignition. The feedback signal on this engine is common to all four cylinders, so there are four feedback pulses for every engine cycle.

Technical information

A 4-wire COP has the following connections (pin numbers may vary depending on the type of coil):

  1. Positive supply - switched voltage from the battery
  2. Trigger - typically a 5-volt active-high pulse from the ECU to the coil
  3. Feedback - typically a 5-volt active-low pulse from the coil to the ECU
  4. Ground/Earth - battery negative

In addition to the trigger and feedback signals shown in the waveform above, it is sometimes useful to monitor the positive supply voltage and current and the ground voltage. For instructions on checking the supply and ground/earth pins, see the 3-wire COP test.

Positive supply voltage. This should be close to battery voltage. If it is low or absent then the ignition system is not receiving enough power and the coil will not function.

Supply current. This can be measured using a current clamp ('amps clamp') around the ignition supply wire, either close to the coil or at the fusebox. A pulse should be seen whenever the coil fires. If the supply and trigger voltages are present but there is no supply current pulse, the coil has probably failed open-circuit. If there is a current pulse but no ignition, the coil output or spark plug may be short-circuited.

Ground. A small pulse will be seen on this signal during ignition as the coil draws current from the battery, but the pulse height should not exceed a few hundred millivolts. A larger voltage indicates too much resistance in the ground wiring, which could cause an ignition fault.


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

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


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


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1 comment | Add comment

August 19 2016

i give thanks to pico

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Guided test: Trigger and Feedback (4-wire)