Engine Ignition systems

In this image of a typical ‘twin static’, ‘DIS’ or ‘wasted spark’ ignition primary signal, during a no load engine running at idle condition, we can see both the primary voltage (green) and primary current (brown). I used a low current clamp for this and most of the samples in the this section.

Ignition primary profile

Green channel: 20:1 attenuated with a scale for 400 volts
Brown channel: Low current amp clamp scaled for 10 amps
Time base: 10ms/div or 100ms screen sweep. (Non triggered or free run).

This is a close up of the initial firing event, it is completely normal.

Ignition primary firing point

No fuel no load. This sample was captured with the fuel injector disconnected.

Ignition primary profile

No fault ignition, under load. This is a ‘snap’ throttle sample.

Ignition primary profile ‘Snap’ throttle

No fuel condition under load. (Fuel injector disconnected).

Ignition primary profile ‘Lean’ condition

No fuel ignition under load, close up of the spark line.

Ignition primary detail ‘Lean’ condition

This next capture illustrates the use of ‘Multi-strike’ ignition. This is common use on BMW vehicles, which is where this one came from. This is a good sample.

Ignition primary ‘Multi-strike’ profile

The multi-strike count, can be less than above and will drop to single strike at higher RPM. So always compare against all cylinders as a diagnostic approach.

The following is a non start condition, due to some kind of vapour lock issue! It is much as before except the ignition profile shows a weakness. This vehicle was fine after it did start. I’m sure that coil will break down soon!

Ignition primary ‘Multi-strike’ profile

This image shows the same type of multi-strike system, as above, with both voltage and current, taken whilst the engine was idling.
This was also taken from a BMW vehicle.

Ignition primary ‘Multi-strike’ profile

Red channel: voltage via a 20:1 attenuator, triggered at just over 100 volts positive slope.
Blue channel: current via a low current clamp.
Time base: 2ms/div or 20ms screen sweep. (Triggered)
This waveform represents a good working system.

The next set of ignition profiles came from a vehicle with bad starting and poor running. The signal illustrates a weak coil, coupled with bad fuel.

The first capture is of the secondary ignition during a non start cranking condition.
The ignition system is a wasted spark with the transistor built into the coil pack. (So no access to primary available).

Ignition secondary profile

Weak coil non start cranking. Close up of the peak firing line and burn line.

Ignition secondary profile

This was a confusing signal for me, as it has a low firing KV and a direct negative slope for the burn/spark line, with no available energy left over to create the ringing at the end. It represents a weak coil, which was replaced, but did not cure the vehicle’s problem. The vehicle had bad fuel. This is very often a problem you will be presented with, in that you may have more than one thing wrong with the car.

In this next image, you can see a lot more signals in a single capture, including weak coil current. It shows the difference in amplitude between compression stroke and exhaust stroke.

Ignition secondary and primary current ‘parade’

This next image, shows the same vehicle, still with bad fuel but with a new, genuine, coil pack fitted. Look at the current ramps. They’re much stronger and uniform.

Ignition secondary and primary current ‘parade’

You can see in the above secondary signal, the difference between the compression stroke and the exhaust stroke. This ‘wasted spark’ system uses HT leads from the coil pack to the spark plugs. So sampling secondary via one of the plug leads, will show the spark event for both power and exhaust. You can tell between them, as it takes a lot more energy to fire a spark under pressure.

This image shows the same secondary ignition signal from above in close up.

Ignition secondary profile

I find measuring secondary voltage a little hard to get an accurate representation. It seems dependant on the pick up and the HT leads, as some leads insulate or shield better than others.

In the following images, I have captured a primary signal from a COP system, where the coil started good and very quickly started to fail. This is a basic 2 wire COP and the scope probe was using a 20:1 attenuator.

Ignition primary profile

This is the same COP coil several cycles later.

Ignition primary profile

This pattern is very typical of a failing coil, even if there is no misfire present, there will be soon and especially under load. The peak firing voltage would normally be over 300v and below 400v, for a good coil operating in a good environment. If the environment becomes harsh the voltage may increase, or if the coil is bad it may also go higher. The general rule being; if resistance is high, the peak firing is high coupled with short spark line. The spark line would normally be around 30v. If it is higher, that maybe because of high resistance in the cylinder and if it’s lower, it may be sparking outside the cylinder or there maybe low compression.

I’m always reluctant to state values, as things change, especially technology. So just use those figures as a guide and always look for comparisons.

I have included this next image as a nice example of a COP that had been breaking down at high RPM. The coil was causing a misfire at above 4700 RPM.

Ignition primary profile comparison

Look very carefully at the two blue traces.

Blue trace – Voltage Good COP at idle.
Light blue – Voltage Bad COP at idle.
Red trace – Current Good COP at idle.
Light red – Current Bad COP at idle.

The current is perfect and there was no misfiring whilst capturing this signal, but you can see the discreet difference in the voltage signal.

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