On the DSI ignition the coil is not energized with the key in the run position if the distributor is not turning, on the DSII ignition the coil is energized with the key in the run position irregardless if the distributor is turning or not, i don't see how this is an improvement over the earlier system, am i missing something here?

 

The reason this is a problem is because shutting off power to a charged coil after the motor comes to a stop can cause the coil to spark. What this means is that under some isolated conditions, if the motor were to shut off and the key was left in RUN, it could try to start itself again without you ever having touched the key. This is a problem referred to as "start on the spark." Cutting power to the coil is essentially the same effect as the module firing the coil on purpose. Both use the same method. When you shut the truck off normally by taking the key out of the ignition switch, this can't happen because you're cutting power to the coil completely, so no current is flowing at this time. It's only a problem if you have power to the coil, but unground it which was the case in the DS I design.

The reasonining behind this is a derivation of Faraday's law. Voltage across a coil is proportional to the derivative or rate of change through the current flowing through it. If current is flowing through the coil and is all of the sudden shut off, the rate of change is said to be very high (it has to change very quickly to go from on to off instantly). This creates a large voltage spike, which appears across the plug gap. This spark can start the engine in some cases.

Note that this is essentially how the module fires the coil under normal conditions. The coil "charges" (builds up energy in a magnetic field) when the module pulls the green wire low. It's essentially grounding the coil so that current flows through the primary winding. When the Hall Effect sensor in the distributor signals that the coil must be fired, the module open-circuits the green wire, which essentially turns off the coil. Current goes from on to off very quickly, which creates a voltage spike, just as before. This is how the spark is created.

Breaker-point ignition systems work the same way, except it is mechanical breaker points that ground and open-circuit the lower end of the coil primary winding instead of a power transistor within the module.

 

I think i'm following what your saying fmc but wouldn't the same circumstances be in play with the DSII which keeps the coil energized when the key is left in RUN?

 

It's not a problem with DS II because the coil is energized for as long as the key is in RUN. Because the coil is kept on the whole time, the field never collapses and the spark does not occur. If for some reason the coil was un-grounded with the key in RUN and the motor stopped, the coil could spark and cause the problem. This doesn't happen with DS II. With both designs, you do cut power to the coil when you remove the key to shut off power to the truck, but because there's no power on the BATT terminal of the coil to begin with, a magnetic field isn't present anyway.

With ignition coils - applying power to them, or keeping power to them isn't what makes them spark. It's cutting power that does the trick. It's due to the rate of change business I described earlier. Ignition coils are a form of inductor, which inherently "charge" relatively slowly. When you first apply power, the coil charges exponentially. In this case, the rate of change of current is relatively low, so there is not a noticeable voltage spike. When you continuously maintain current (as is done in DS II), current is the same, therefore the rate of change is zero. Therefore, there is no voltage spike. It's shutting off the coil that causes a voltage spike - going from continuous current to effectively an open circuit causes a very large voltage spike. It's the same reason the switch on a space heater will spark when you shut it off, but not so much when you turn it on.

 

That makes sense to me now, thanks for the clarification and the detailed explanation, tried to send rep points but it said i need to spread them around.

On a side note, how common is this "start on a spark" phenomenon?

 

Glad I could help. It's not a very common problem on an absolute scale because DS I was fairly short-lived (only one or two model years); DS II was around much longer. Also, my understanding is that many replacement DS I modules manufactured today are actually DS II modules with a note that says to use them for DS I systems as well. However, I can't guarantee this notion because aftermarket ignition modules are reverse-engineered from the originals, so it's hard to say what they do or don't do. But if it's true, then it's reasonable to assume that there aren't many factory original DS I modules on the road today.

The question then becomes if you do happen to have an original DS I module, how likely is it that you'll see a "start on the spark" condition? That's tough to answer. If there is a situation where the key is in RUN and the motor is running (like normal) but the motor cuts out with the key still in RUN (stalling, etc), the motor could immediately try to re-start itself once the module senses the motor has stopped, and inadvertently fires the coil. However, if the motor died, it died for a reason and it's unlikely that a single spark immediately after stalling would re-start it. However, it is possible.

One case in particular where I have known it to happen with DS I is if you shut the motor off, wait a few seconds, and turn the key back to RUN, the motor can start back up without the starter ever having been engaged. This is because the motor is hot, and wet with fuel and a single spark is all it takes to start it back up.

 

To my understanding DSI was only around for 74 and 75, 76 went to the "wide cap" DSII ignition, i've had no success in aftermarket modules for my 75 (DSI) none would work, i've had to source NOS modules from ebay, even though as you stated the aftermarket modules include a note stating they are "retrofitted" to fit earlier applications i haven't found this to be the case, at least not in my situation.