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How the igniter works
in a system with a distributor

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(What about systems without a distributor? See bottom...)

A note before we begin: If you replace your igniter, make certain that the heat sink is correctly installed. Also make sure you use heat-sink compound between the igniter and the heat sink. That compound is the same stuff used on computer CPU cooling fans.

This page is the result of some excellent discussions in the Honda groups, and any information here is distilled from these conversations. I personally know little of electronics, and I owe it to the many posters who kindly offered their skills and knowledge for the greater good of Usenet.

Basically, the igniter unit appears to be a kind of electronic switch. It takes timing signals from the ECU, processes them in its internal Integrated Circuit, then switches power to the coil. Very simple, really. I'm not going to go into the details of how an induction coil system works, except to say that you can apply a small voltage on one side, and get a sinus-clearing (or worse) shock at the other, even though there is no physical connection between the two. Click here for more:

In the Old Days, in the days of the Kettering system, the days innocently free of apocalyptic fears of environmental collapse and death and dismemberment for all, you had something called contact points. These points were simple mechanical connections from one side of a circuit to the other. Points were ubiquitous, found in every vehicle since 1908. Their function was to perform the make-and-break actions required to induce the 20,000 volts that the spark plugs needed to fire. Remember that your battery can only deliver 12 volts, and your charging system 15 volts maximum.

The photos below are of a points set from a '70s Toyota Corolla. If you look closely at the "open" picture (click either for BIGGER ones), you can see a blackened area biased towards the bottom of the contact face. That's wear. Wear means less efficient timing, poorer spark, weaker combustion, and increased emissions, exactly the problems electronic ignition was developed to correct.

Kettering-style contact points  Kettering points -opened up by hand

These days (you DID read the Kettering link above, did you not?), the contact points have been replaced by far more efficient transistors. Honda igniters use a transistor combination called a Darlington pair, a couple of NPN transistors in the same package. There are many other things within the igniter, the exact functions of which is the subject of some conjecture. No one, for instance, has yet been able to obtain a spec sheet on the Control Chip (integrated circuit). Jim Yanik surmises that the IC is "...a full control IC. Probably with circuitry to square up (shape) the drive pulse, and provide enough drive current, and IIRC, the ICs monitored and regulated coil current (that would enable faster switching)."

Collected from GrahamW's page is an actual photo of what an igniter actually looks like inside. You can clearly see the Darlington Pair transistor, the giant black thing near the top. The Control Chip is the smaller eight-legged thing below it:

Igniter innards

From another kind poster comes photos of an identically functioning igniter, but with a different internal construction known as "thick film" rather than the Integrated Circuits shown above. In this case, the igniter is  from an '89 Civic:

Another igniter - inside view    Another igniter - outside view

Now, the schematics...
The schematics have been somewhat simplified in the interest of clarity. Such things as resistors and flyback diodes are not shown, since they are not critical to the understanding of igniter function.
Also not shown is the capacitor (or condenser) which is in the wire from ignition switch to the "+" side of the coil. That condenser prevents backwash HT voltage from making it back to the ignition switch. All vehicles since the advent of the car radio have had one of those.

Step 1
Igniter operation: Step 1
The igniter has battery voltage available at all times when the ignition  is on, or when the starter is operated, but NOTHING can happen until the low-current signal (from the ECU to Terminal #4) is used to switch on the high-current connection (Terminal #2 to ground), thus allowing electricity to flow through the coil to charge it.

The sequence starts when the ECU detects (using the Crank Angle Sensor)  that a cylinder is coming up in position for a spark. The ECU then signals the Control Chip through the white wire, shown here as Terminal #4 at the bottom of this graphic.

It is necessary that the ECU see that the engine is turning. It also uses the Crank Angle Sensor to know that. If it does not receive the correct signal from the Crank Angle Sensor, it will think the engine is not turning and refuse to signal the Control Chip, so you'll get no spark.

The ECU will vary the duration between switching-pulses to the ignition system in accordance with the engine's timing requirements based on RPM, load, etc.  It also varies what used to be called "dwell angle" or "dwell time" on Kettering ignitions, i.e.: it performs coil charge or "saturation" management. The time necessary to fully charge the coil for each spark depends on operation conditions such as engine speed, etc.

The igniter will only cut current to the coil (thus collapsing the magnetic field and generating the HT voltage) once the ECU signals the igniter to do that through the wire from the #4 terminal.

The shop manual warns that the fat white ECU wire at Terminal #4 must never be damaged or disconnected. Doing so will result in damage to the igniter, since the ECU will be unable to switch the igniter with the wire not connected.

Step 2
Igniter operations: Step 2
The ECU has now determined that the rotor is getting close to one of the distributor cap's terminals, so as stated in the previous graphic, it has now sent a signal to the Control Chip through that white wire at terminal #4.

This tells the Control Chip in the igniter to switch the actual coil primary current to ground via the igniter body shell (orange line). Current is now allowed to flow through the primary (12V side) at terminal #2, inducing a magnetic field in the coil core. When the ECU tells the Control Chip to switch off the current, this magnetic field will collapse, inducing a high voltage spike in the secondary winding of the coil (see Step 3). This is the current that fires the spark plug.

In this way, the ECU and the Control Chip only ever handle a small signal current, reducing the heat and power requirements for these components, with the real power switching taking place close to where it's needed.

Persistence of the coil-charge current is on the order of milliseconds,
but that's more than enough to do the job.

Incidentally, this is the time your tachometer receives its signal through its wire (terminal #1).

Another incidental point: Default voltage at T4 is 10V. It appears that the ECU drags that voltage down to 0V to send the signal. How does the ECU know the igniter has failed? A conjecture from an unnamed reader: "I guess it's assuming a certain condition from the igniter, either hard 0V or hard +V, which it could then detect.  My failed igniter appears the same at T4 as a good igniter - it's just that the output is locked hard on.  If there's no detectable change in T4, it won't know what [error] code to set."

Step 3
Igniter operation: Step 3
At the appropriate moment (when voltage across the resistor is correct), the Control Chip cuts its current to the Darlington, which then switches off the coil current.

When the current to a fully charged coil is switched off, it collapses the magnetic field in the coil's core thereby creating the high voltage necessary for sparking.


Step 1 now repeats, and the sequence itself repeats until you shut the ignition off.

Now what happens when it breaks? This.

And what if your car has no distributor?

The concept is identical, except that the spinning distributor has been eliminated. Just as  automotive computers have been reduced in size from the suitcase-sized lumps installed in a few '70s cars to being smaller than paperback books, so too have ignition control electronics gotten smaller.

The coil used to be housed outside the distributor, as did early crank position sensors. Over the years, parts have been consolidated, and eventually the entire system ended up crammed into the distributor housing.

The spinning distributor is prone to, among other things, wear, physical damage, and user ineptness and neglect. Also, since there is a large spark that jumps from the rotor to the cap's terminals, ozone is generated, leading to corrosion inside the unit, possibly leading to the infamous "red dust" bearing failures of some '90s Hondas. The wires get old, leaking current off and weakening the spark.

Driven by increasingly strict emission control demands from governments, most cars had left their distributors behind by about 1998.

In place of this typical Honda electronic distributor assembly, with three Hall-effect pickups...
...newer cars have this, where one toothed wheel and Hall-effect sensor do everything.
Typical Honda distributor signal pickups

Picture from Graham W
Crank pulley & Hall-effect wheel

Meanwhile, the coil itself is now directly on top of the spark plug. In cheaper cars, such as this Toyota Tercel, there is only one coil pack per two plugs, known as a "wasted spark" system. More expensive cars will have one coil pack per plug, with no wires at all. Eliminating the distributor cap, rotor and wires eliminates about 99% of ignition-related problems, and makes it a lot harder for owners to screw things up.

4-cyl "wasted-spark" coil-over-plug ignition

Last updated: March 29/08