I've read and read information on the internet about how ignition systems work, and still have perhaps the most basic question possible.

What kind of signal does the trigger wire provide from an inductance electronic ignition system? Here is a cut and paste from a Wikopedia article that is as close as I could get to answering this question:

Quote: "The control module basically performs the same function as the breaker points in a points & condenser system."

Thus one would think the signal wire would be a make-and-break ground signal. But somewhere I was told, or read that it is an on and off 5 volts, which makes no sense, and must be wrong. OR at least you would think so. But nowhere have I been able to read definitively what it actually does. The module trigger wire goes to the negative side of the primary windings, which would suggest it simply completes the circuit for the 12v that is hooked to the + side. In my so far unsubstantiated logic this ground allows the power to saturate the coil and then, after the proper amount dwell, disconnects the ground thus allowing the primary charge to collapse, thussending 40,000 volts through the secondary spark lead.

Does anybody know for sure if this is how the HEI ignition module works? One of Lou's guidelines is always to work from the most basic principles. In this case I have not been able to find proof or verification of my "understanding" of the most basic principal.

Thanks folks. Sam

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I am pretty sure I am correct about this. The GM 4 pin module switches the groung side of the ign coil which has (should have) full battery voltage. The control signal from the distributer pickup (2 wire magnetic pickup) is a varing voltage sine wave (based on rpm). The seven pin module also switches the ground side of the coil, the same as the 4pin, but gets a 5volt square wave signal from the ECM. The output signal from the distributer is different as it is a 3 wire "hall effect" signal. It must have power, ground applied, and then a signal wire that feeds the ECU. Also different contrlol modules/ECU (different manufactures) might have different perameters,(trigger from a rising signal, or trigger from a falling signal).

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Thanks Charlie, The 8 pin HEI I am currently employing gets its crank signal from the VR feed from the Mopar distributor. This then is fed to the ECU as a digital signal which is timed and returned to the HEI module which then sends the appropriate signal to the Coil negative. This wire, which is labeled C for coil on all HEI modules is the wire in question. What does it do? Does it make-and-beak a ground to the negative side of the coil? It does go to the negative side for sure. Since this is an inductive set up, it seems it must be a simple ground-and-open sequence. All the long winded, and erudite stuff I have read fails to mention that one simple detail. I think they assume it is so basic, why bother.

Sam

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The ignition module grounds (switches "on") the negative side of the coil primary circuit then switches "off", collapsing the magnetic field in the coil & producing a spark. So most all ignitions (points or electronic) operate on the same principle, switching the coil ground on/off.

Points do the switching directly. Mopar electronic & 4pin HEI use the sine wave /AC voltage pattern of the VR pickup to trigger a transistor.

As stated, the 7 or 8 pin HEI has additional circuitry to allow eng computer (ECM) to control timing advance. The mod sends out a 5V square wave (reference ckt) to ECM, which sends back timed squarewave (EST ckt). Another circuit (bypass) controls whether timing is handled by the mod (below 400 rpm) or by the ecm. Last circuit is a ground to ecm.

Many of these HEI with the mod inside the dist had 2 wire VR pickups, while many of the later (external mounted) mod used 3wire Hall effect (digital squarewave) pickups.

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63 Valiant Wagon
225 - 4 bbl

Thanks. That is exactly the precise, and complete info I was looking for. With that, I know for sure the C wire will fire an MSD 6a, because it looks just like points to the MSD.

BTW, I agree. Wagons are cool. I sat next to a small Audi wagon at a light yesterday that almost made me want to buy one. Of course I don't have the 40K it would likely cost. I think it is one of the few real station wagons being made today (if they still make it).

Sam

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Yes, ALL coil-based ignitions will switch the neg coil from "open circuit" to ground.

Another point: The Mopar inductive elec ign dist puts out little voltage "blips" as the reluctor passes the magnetic pickup in the dist. IIRC, the blips are about 50 mV and go positive and negative briefly. This is detected by the brain box (MSD, Mopar ECU, HEI...), which then triggers the coil.

Hall effect is a different dist design used on other makes (and maybe Mopar starting in 80s or 90s?). Our Mopar unit is called an inductive pickup distributor. There are also optical dists, as in the Mallory Unilite.

Lou

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Home of Slant6-powered fun machines since 1988

An inductive pickup is another term for a Variable Reluctance pickup. The voltage blips are AC voltage spikes, they might be a few hundred mV peak to peak while cranking, but voltage will increase as rpm goes up. Even though distributors are pretty much gone from the modern car, you still see some VR sensors in use for electronic control systems.

Hall effect pickups use magnetic field with a steel interrupter to control a digital switching circuit. Mopar, Ford & some GM have used them since the early '80s. Now everyone uses 'em for everything.

Mitsu & Nissan (& GM in their POS Optispark/LT1 dist) have used optical dist pickups. A LED triggers a photoreceptor when a slotted trigger disc allows light thru. Circuitry produces a digital output signal, similar to Hall effect.

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63 Valiant Wagon
225 - 4 bbl

However, an inductance ignition is one where the voltage spike in the coil is caused by the collapsing of the voltage in the primary windings. The seeds of doubt about the HEI are from the fact that with a CD ignition, such as the MSD, the coil is not used in this way. It is only a transformer for the spike of voltage put out by the CD box. The MSD6 puts out a spike of 550volts. The coil multiplies this voltage by the number of windings in the coil. The coil does NOT generate the high voltage in the first place as it does with inductive ignition.

In the CD system the primary windings are not turned off and on by making and breaking the ground connection. This is why the coil gets its own designated connections from the CD box, and does not get wired into the 12V ignition circuit. The spike of 550 volts is produced by the discharge of a capacitor in the CD box. If your coil has 100 windings,which is typical in a stock coil, then the plug has the potential of seeing 55,000 volts, multiple times per ignition event below 3300 RPM.

The disadvantage of the CD system is that the spark is very short, in spite of being high powered. This is not as reliable or efficient as the lower voltage, longer spark of the old inductance style ignition. Making it spark multiple times yields the best of both worlds.

My questions surrounded the GM HEI ignition specifically. It is not an old points style ignition which is clearly inductance style, nor it is it a CD style as the MSD. This discussion has made it pretty clear that the HEI is a solid state version of a points inductance style ignition, and fires the coil in essentially the same way. Now logic can be applied to thinking about it in a circuit. I am always a little dumb founded by electronic black boxes that do their magic in secret. They always complicate trouble shooting, and logic.

Sam

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