Brendan,

The resistor is still in there because of the Duraspark module. I removed the "TFI" from the diagram because of the resistor confusion. If someone was running a completely stock Duraspark ignition and added the GM module, then the diagram is correct. If they have modified their harness for some other coil or MSD, then they will know whether or not they need the resisitor. Make sense?

Later,

I hope I'm not jumping in out of turn, not knowing about the Howell, but this may help.

As far as the MSD vs HEI etc - the HEI & Fords are Kettering ignitions, the MSD isn't. The Kettering types apply power to the coil, charging the magnetic field up - storing lots of energy in it until it's needed. The module stops feeding the coil at the right time, the field collapses and generates the high voltage. The tach signal is based on the "open" or no current side of the primary's square wave.

The MSD, a CD, does just the opposite - it stores the energ in the big internal capacitor, it uses the coil as a transformer. So the "open" side of the primary pulse is AFTER the spark. The tach signal needs to be taken from the "on" side of the pulse if it's to be used for injection triggering, else it's way too late.
And since it's firing multiple times, the first one should be tho only one "seen", otherwise it will probably multiple fire the injectors.
That filter is most likely changing polarity as well, that's why the filter for MSD's is different.

EST - The pickup coil does not move with electronic timing. Yes, the spark release is actually "predicted" in order to advance the spark. It's based on the last several pulses that tells it when the next one should occur (I've heard last 7 pulses.) The RPM is determined from those last pulses from the pickup coil, then a pulse timing circuit "predicts" when the next firing should occur.

If I remember correctly - on the GM HEI (7 pin) that pulse timing circuit is inside the ignition module itself. The EST line only tells it to "predict or not," That's the external brown wire. That's the only thing the ECM has to do with it - EST or not. It's not a pulse, just an on/off line.

On Ford systems that predictor circuit is inside the Computer module, not the ignition module. That "predicted" information is sent to the ignition module in the form of a square wave - that pulse simply triggers the on/off firing of the module just like a direct feed from a pickup coil.

Now how/which signal the Howell needs is beyond me.

But it makes me wonder - if the injection system is based on the tach signal, then it must advance as the R's increase? The faster it goes, the more lead time on the squirt?
If so, basing the tach signal on an immovable pickup coil won't advance it? I wonder how important that lead time is for the injection.

Just some thoughts - hope it helps.

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I wonder how important that lead time is for the injection.

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Since it is a throttle body injection, probably not much.

The GM ECM requires a 1.6v square wave for its tach input(B5). The Howell system uses the GM ECM as it comes from the factory with a modified PROM. The Howell harness includes a tach filter to modify the coil(-) signal into a 1.6v square wave. Howell has 2 different filters that it uses. One is for factory electronic ignitions, the other is for aftermarket CD ignitions. Using the GM module in parallel on a stock system will produce the required 1.6v signal for the ECM to fire the injectors. I am not sure how you would modify the signal on a CD ignition to work with the GM ECM. I doubt that running a GM Module in parallel with an MSD box would work.

Later,
Tom

I wouldn&amp;#8217;t call it Kettering unless you where using points and a condenser.

With electronic ignition I would call it inductive storage.

so you got inductive or capacitive. easy.

Inductive storage - probably a good name to use - for the electronic "Kettering's."
The only real difference between points and an electronic type is just the switching device. A set of points vs. a transistor.

The CD's a whole different animal. And a mutiple spark CD's different yet!

Kettering got us away from the constant spark system - like was used on the Model T's.

He He - sometimes I wonder if making it complicated with electronics is "Better?" Just the opposite of KISS.
In the "old days" a piece of gum, a hunk of wire, a matchbook cover almost always got you going again.

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The only real difference between points and an electronic type is just the switching device. A set of points vs. a transistor.



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points don't switch current. don't forget the condenser. it has a very active role in the behavior of the ignition. its the only reason the spark doesn't jump the points instead of the plug gap and......

when the points open current flows into the condenser. until the condensor is charged.then flow back into the coil. then flow back into the condensor. its an oscillator circuit. its damped by parisitic resisternce.

Just to clarify:
When the points open they shut off all current flowing from the battery, through the coil, then through the points. That time during which charging current was flowing is called Dwell time. That current flowing was the current that built up the magnetic field.

Once the current is stopped, the magnetic field starts to fall. Yes, the induced voltage in the primary would jump the points again unless the condenser was there. The magnetic field's induced energy goes into the condenser.
That's known as Spark Time.

Then once the field's collapsed, (at which time the secondary's spark goes out,) the condenser, being charged, tosses the energy back into the primary again. It oscillates back and forth between the condenser and coil primary, slowly diminishing. The voltsge induced in the secondary during those oscillations is not enough to fire the plug agan.
That's called Intermediate Time.

The next step is close the points again to start the process all over - Dwell Time again.

The electronic ignitions do the same thing - but instead of the points doing the switching by opening and closing, a transistor does it by turning on and off.
The electronic type still needs something to trigger it, so either points are still used, a magnet with a coil and reluctor, a photo-cell/LED, Hall Effect device etc..
Some computer driven modules get the information from a crank and cam sensor, then send it through the computer to the module, but it's still the same principle.

Charge coil, dump it.