Duraspark Ignition Box

What you write is correct for DC or constant current AC circuit. However, the coil circuit is reactive, changing with time under the influence of a switched inductance. A meter, especially a digital meter, will time-average a changing voltage using its internal algorithm for such measurements. While the voltage across the resistor will drop, the purpose of the resistor is to limit current.

There is a big transistor in the module that takes the place of the points. Following the signal from the distributor, the transistor opens and closes to switch the coil. There will be a big inductive kick when the coil switches - here's a typical trace of the primary and secondary voltage:



The primary is the blue trace, and the red is the secondary (shown in thousands of volts). The points open at zero. Notice the big inductive kick at this point which trails off to where the spark ends at 1.1 ms. The inductor (coil) then recharges, oscillating back to the a voltage near 12V.

Now, with the points closed and the engine off, the resistance of the primary is in series with the ballast resistor, and there will be a voltage drop. That big transistor that takes the place of the points should be open if there is no signal from the distributor ... maybe not, I don't know. Ideally, it would be open if there's no switching signal from the distributor.

That big transistor also has an internal resistance which you can calculate from the transistor's characteristics. That resistance will always be in the circuit, and it's what makes heat when the module switches. The amount of heat is dependent on how much current flows from emitter to collector (through the transistor). The ballast resistor is in the circuit to limit the amount of current in the circuit ... Ohm's law!

Sorry if I'm flogging this... but I get started on an explanation and have to complete it.

 

I understand, but I suggest you don't go quixotic... the problem may be the crap quality of the modules you are getting, in combination with a hot coil and maybe slightly higher operating voltage in the rewired harness (better wiring -> less resistance -> higher voltages). In that case, you'll never be able to resolve the issue.

 

NOS type module (not aftermarket) installed.. still running 12.5 volts to POS coil terminal through resistor...
had some (2-3) ignition shut downs this AM, but no fried box...
I think there might be a thermal overload in the factory units that the aftermarket lacks..
only a few seconds in-between shut down and re starts..

I remember way back on my YJ when the Duraspark was acting up...
I poured a bottle of water over it and it was good to go... really plays to the whole "they don't build them like they use to) These were my symptoms when I first noticed this problem and coincidently I was also running a factory box..

I have a meter with me today going to try a handful of resistors and possibly hook 2 up in series..

ordered a resistance wire.. wont be here till tomorrow, but perhaps that's a better "value" that the ceramic variety..

HEI does seem like the long term plan..
but its 85 and sunny and the really cool custom hardtop is already off for the season

 

Something else to consider...

You wrote you were using an Accel coil. The resistance of the coil is in the circuit and is added to the resistance of the ballast resistor. I presume the Accel coil is a "hot" coil, which means it has more windings and likely a higher resistance than the factory coil. It also means that it has a higher inductance, which makes the inductive kick of the coil switching that much higher, maybe significantly higher. This could be what's killing your modules. It shouldn't ... the OEM modules are used with the Ford EI coil, which is a similar high voltage design. But the aftermarket may not be building up to the OEM spec - they may be using a transistor with a lower voltage rating.

The HEI module is meant to be used with the HEI distributor, which has a similar "hot" EI coil.

 

See my post above - I don't think a voltage measurement while running will tell you anything. If you ground the coil lead: ignition switch -> ballast resistor -> coil -> ground, and measure the voltage at the positive side of the coil, that will tell you the DC voltage drop across the ballast resistor. That leaves out the junction resistance in the module, but it should not be a lot, ca 0.6V at whatever the current is (barrier potential is not current dependent).

 

swapped resistor with a new Standard unit.. 9.8 at the coil...
still need a bypass wire for hot starts even though it doesn't seem to have a horrible effect.

preliminary drive at lunch successful... no apparent issue and its 85 deg today..


borrowed time or solution fix......

 

Im running my 360 as if it was a 1991 Grand Wagoner.
Stock distributor, coil, module.. only "upgrade" was to a larger cap and rotor

factory harness must have resistance.. or like you said a less robust electrical system

plus the factory units were just plain better

 

really thanks for the help..
unsure if our of the woods.. but on a path at least..

 

Checked with a friends 86 CJ and 10.1-10.2 volts at hot idle...

Going to install the resistor jumper tonight and install a new box...

 

just to circle back..

took a resistor from a 64 CJ5 and it seems to be doing the trick..
Voltage at coil ~ 9.5v..
could prob play with the timing a bit.. but running well..

 

little more update...

had another box failure..
decided to play around with placement in the engine compartment since heat seems to be this things worse enemy...

Driver side box was mounted on firewall.
its a bit busy over there with the coolant and washer bottle, charcoal canister, etc...
Tub is aluminum so there should be a adequate "heat sink" effect, unless airflow is restricted and the V8 manifold is shedding too much. (plus exhaust runs on that side of the vehicle)

I mounted another box on the inner passenger side fender.
Much less equipment on that side of the truck.

first test yesterday on 95 deg heat was promising..
Un connected (not running) box on drive side firewall was 123* deg
box on passenger side fender that is currently powering the vehicle was 106* deg.

that's a significant difference especially since there was no load on the hotter box..
Time will tell

 

That area of my fender gets splashed pretty good when just driving down a wet road.

 

Might keep it cool.... duraspark isn't afraid of water... just heat

 

another 1hr drive last night.. cooler temperatures, but Ignition box temps remained constant on passenger side, while drive side inactive box got hotter @ 135 deg

 

more updates to come on this.. but not currently speaking about it as not to jinx it

 

little update on the ignition box gremlin issue...

make a very long story short..
The ignition module ground (black wire in harness) grounds through the distributor.
The distributor is in fact grounded via the distributor hold down clamp...
I created a secondary ground from the vacuum advance screw to the engine block.
4 tanks of fuel later and no issue...

note to self: don't powder coat distributor hold down clamps... :

 

Glad you have it working now. Bad ground at the distributor to engine would mean weak signal response at the module but should not cause damage to module internal parts or over heating. Bad ground means less current flow.

 

another little update on this...

After about 2 years of monkeying around with this I decided to try something different..

I installed last night a Chrysler ignition box.. specifically the Orange Mopar performance one..
Very clean, simple 2 wire installation using my current stock Duraspark distributor..

Engine fired right up and idled nicely..
took out for a spin last night and have been commuting in it today..
Jeep runs like a raped ape now..
snappier throttle response and easily bolted on 20 HP

time will tell..