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Distributorless Ignition System (DIS)

When you open the bonnet on a V6 Holden Commodore from the late '80s through to the early 2000s, you might notice there’s something missing compared to older engines - there’s no distributor. That’s because these engines run a Distributorless Ignition System, or DIS for short. While it might sound high-tech, the basic idea is actually pretty simple.

Traditionally, ignition systems used a distributor to direct high-voltage spark to the right spark plug at the right time. It worked well, but it had moving parts that could wear out—especially the cap, rotor, and advance mechanisms. DIS does away with all that, swapping the old mechanical system for a more reliable and maintenance-free electronic setup.

On the V6 Commodore—starting with the VN series and continuing through VR, VS and into the early Ecotec engines—the ignition system uses three ignition coils, each firing two spark plugs at the same time. This setup is sometimes referred to as a wasted spark system.

Here’s how it breaks down:

• The engine has 6 cyls, but only three coils.

• Each coil is responsible for a pair of cylinders that are 180 deg out of phase.

• Example

  • Coil 1: Cyl 1 & 4
  • Coil 2: Cyl 2 & 5
  • Coil 3: Cyl 3 & 6

• When a coil fires, it sends a spark to both plugs at the same time. One cylinder is on its compression stroke and ignites the air/fuel mixture, the other is on its exhaust stroke, so that spark is "wasted" (hence the name).

This system is controlled by the engine control module (ECM), which uses signals from a crankshaft position sensor to figure out exactly when to fire each coil. The sensor detects the position of the crankshaft via a toothed wheel or reluctor on the harmonic balancer.

Advantages of DIS

• Fewer moving parts: No distributor cap or rotor to wear out.

• Improved reliability: Solid-state electronics are more durable in the long run.

• Better timing accuracy: Since the ECM controls spark timing directly, it can adjust instantly for things like load, throttle position, and engine temperature.

• Simpler maintenance: No need to set ignition timing manually.

Common Issues

While generally reliable, the DIS on these Commodores isn’t bulletproof. Some common problems include:

• Coil pack failure: Over time, one or more coils can break down and cause misfires.

• Ignition module faults: The module that controls the coils can fail, especially from heat.

• Crank angle sensor issues: If the ECM doesn’t know where the crankshaft is, it won’t fire the spark at the right time - or at all.

These problems often show up as hard starting, rough running, or no-start conditions, and they can usually be diagnosed with a multimeter or a scan tool.

Wasted Spark Overview Commodore VN - VT V6

What Happens When You Swap HT Leads Within a Coil:

If you reverse the leads on a coil (swap the two cylinders it fires), the engine will still run, but you change which cylinder is getting the spark on the compression vs. exhaust stroke. But because the coil doesn't know which plug is on which stroke, it just fires both ends. So what's the difference?

The Key Factor: Spark Polarity

• One spark plug fires normal polarity (centre electrode → ground strap)
• The other plug fires reverse polarity (ground strap → centre electrode)

This affects:

• Voltage required to fire the plug
• Spark energy and duration
• Ignition reliability under compression

The plug with reverse polarity has a harder time ionizing the gap because the electrons flow from the colder ground strap, which gives off fewer electrons, to the hotter center electrode. This can cause: Weaker spark, incomplete combustion and higher hydrocarbon (HC) emissions.

By swapping the HT leads:

• You flip the polarity at each spark plug.
• Now the plug that used to fire in the easier (normal) direction is firing in the harder (reverse) direction.
• This can lead to misfires or less efficient burns, especially under load or at idle