Coil-on-Plug primary voltage and current (3-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

Channel A - Supply current

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.

  • Plug the low-amp current clamp into Channel A of the scope.
  • Set the clamp to the 20 amp setting if available and auto-zero it.
  • Identify which wire is the supply voltage to the coil pack and place the clamp around this wire. If the waveform is inverted, replace the clamp in the opposite direction.

Channel B - Switching signal

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

Channel C - Supply voltage

  • Plug a BNC test lead into Channel C of the scope.
  • Connect a Back-pinning Probe to the colored (positive) connection of the test lead.
  • Stack the black (negative) plug on top of the black plug from Channel B.
  • Back-probe the wire that the current clamp is connected to, or make a connection using a set of breakout leads.

Channel D - Earth (Ground)

  • Plug a BNC test lead into Channel D of the scope.
  • Connect a Back-pinning Probe to the colored (positive) plug on the test lead.
  • Stack the black (negative) plug on top of the black plug from Channel C.
  • If the coil pack has three wires then the final wire is an earth. A fourth wire may be feedback or a safety earth. Back-probe the earth wire or make a connection using a set of breakout leads.

Example 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.

Coil-on-plug unit

Figure 2 - Coil-on-plug unit

Technical information

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.

Live

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.

Earth

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.

AT162-5

Disclaimer
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

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

    £159.00

  • 30 A (low amps) current clamp

    £259.00

  • Connector breakout lead kit A

    £329.00

  • Back-pinning Probe Set

    £48.00

  • Flexible Back-pinning Probe

    £7.00

  • Small Crocodile/Gator Clips

    £5.00

  • PicoScope Sprung Hook Probe

    £7.00

  • PicoScope Battery Clip

    £5.00

  • Large Dolphin/Gator Clips

    £10.00

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

    £48.00

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

    £179.00

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Guided test: Primary Voltage and Current (3-wire)