Multi-coil-on-plug unit - Primary voltage vs secondary

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

Some vehicles are equipped with a Distributorless Ignition System, where the coils are combined into a coil-on-plug unit that sits directly on top of the spark plugs and houses the coil ignition system. This was commonly fitted to SAAB engines as well as certain Vauxhalls, Peugeots and others, as shown in Figure 1.

  1. Disconnect the multiplug from the coil-on-plug unit and remove the unit from the engine.
  2. Using the HT Extension Leads, connect all the coil outputs to the spark plugs as shown in Figures 2 and 3.
  3. Once all the HT Extension Leads* have been fitted then connect the multiplug back to the coil-on-plug unit using suitable breakout leads as shown in Figures 2 and 3.
  4. An earth cable is supplied with the Ht Extension Lead set. Connect one end using the supplied nut and bolt to the mounting hole of the COP unit and attach the other end to a suitable earth point, such as the COP unit mounting point on the engine. This will ensure that if the coil has a ground return through the coil pack then this is maintained but also should there be any sparks exposed during testing then they can safely disperse to ground ensuring safety to both the user and the equipment as shown in Figure 2.

*Note: the Advanced Kit has four of these leads included as standard. Extra leads can be purchased for working on vehicles such as SAABs, with more than 4 cylinders, but you can only test up to four at one time if using a 4-channel automotive scope.

Channel A - Primary Driver (Digital Switch)

  1. Plug a BNC-to-4-mm test lead into Channel A of the scope.
  2. Plug the coloured (positive) connection of the test lead into the 4 mm adaptor on the breakout lead that carries the primary driver (digital switch) signal to the coil unit.
  3. Plug the black clip onto the black lead and connect this to battery negative or a suitable ground point on the engine as shown in Figure 2 and 3.

Channel B - Secondary Ignition

  1. Connect a PP178 Secondary Ignition Pickup to Channel B of the PicoScope.
  2. Connect the secondary pickup clip to the HT Extension lead and connect the ground clip to a suitable ground point on the engine or chassis as shown in Figures 2 and 3.

With the engine running, a pattern similar to the example below should appear on the screen.

Note: if a secondary waveform cannot be seen then this could be because the output is positive-fired. If the HT pickup is moved to another HT extension lead, the waveform should appear. Alternatively, you can change the settings to look at a positive-fired coil by selecting the channel settings for channel B and changing the probe from "Secondary Ignition Probe (Inverted)" to "Secondary Ignition Probe (Pos)" as shown in Figure 4.

On a 4-cylinder engine with this type of ignition system it is common to have two negative-fired and two positive-fired outputs.

Example waveform

Primary Waveform Notes

Channel A - Primary Driver Digital switch

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 coils 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.

Secondary Waveform Notes

The example waveform is a typical picture from an engine with electronic ignition. The waveform is from the coil-on-plug unit fitted to the Vectra Z22SE engine.

The secondary waveform shows the length of time for which the HT flows across the spark plug's electrode after the initial peak of voltage required to jump the plug gap. This time is referred to as either the 'burn time' or the 'spark duration'. In the illustration, the horizontal voltage line in the centre of the oscilloscope is at fairly constant voltage, but then drops sharply into what is referred to as the 'coil oscillation' period. The 'burn time' is also illustrated in Figure 5.

The coil oscillation period (as illustrated in Figure 6) should display at least 4 peaks (including upper and lower). A loss of peaks indicates that the coil needs substituting. The period between the coil oscillation and the next 'drop down' is when the coil is at rest and there is no voltage in the coil's secondary circuit. The 'drop down' is referred to as the 'negative polarity peak', (as illustrated in Figure 7) and produces a small oscillation in the opposite direction to the plug firing voltage. This is due to the initial switching on of the coil's primary current. The voltage within the coil is only released at the correct point of ignition when the HT spark ignites the air/fuel mixture.

The plug firing voltage is the voltage required to jump the gap at the plug's electrode, commonly known as the 'plug kV'. This is shown in Figure 8. In this example the plug kV is 13.5 kV.

Multi-plug-on-coil example unit

Figure 9 - Example mutli-coil-on-plug unit

Technical information

The operation of the Coil on Plug Unit is essentially the same as any other ignition system.

Distributorless ignition systems are fitted only to vehicles that have an even number of cylinders such as 2, 4, 6 or 8. This is because two cylinders are connected to one coil that produces a spark in both cylinders at the same time. This is commonly known as a wasted spark system. The two spark plugs are arranged so that one is fired on the power stroke of the engine and the other on the exhaust stroke of the opposing cylinder, offset by 360 degrees. After a complete rotation of the engine, the two cylinders are now on the opposite strokes and the two spark plugs fire again but with opposite roles to before.

On a 4-cylinder engine, there are two coils with individual drivers that usually operate cylinders 1 and 4, and 2 and 3. This means there is a dual spark every 180 degrees with one of those sparks wasted on an exhaust stroke of the opposing cylinder which is firing on the power stroke.

The only real difference between COP and other ignition systems is that each COP coil is mounted directly onto the spark plug, so the voltage goes directly to the plug electrodes without having to pass through a distributor or plug leads. This direct connection method delivers the strongest spark possible and improves the durability of the ignition system.

Coil technical information

Primary driver - 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 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, 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 in any electrical circuit, so does the voltage drop. 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.

Primary driver - current

The example waveform shows the current limiting circuit in operation. The current in the primary circuit switches on as the dwell period starts, rises to about 10 amps, where it 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.


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

  • HT Extension Lead


  • HT Extension Lead (set of 4 leads)


  • Connector breakout lead kit A


  • Secondary ignition pickup (capacitive with BNC)


  • 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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Guided test: Primary vs Secondary