Re: One Spark Key OFF -> ON
I forgot to explain the "why."
Normally when you turn the key on, the module goes into the "on" mode - just like the points were closed. The coil charges up like that. The "on" mode of the module is essentially at ground potential, so current flows through the coil, charging it. When you turn the key off, the module goes "dead" or "open" or "off" - current no longer flows through the coil - magnetic field collapses in the coil, creating the secondary spark. (Normally running the module's switching from "on" to "off," creating the sparks.)
That's why I suspect the pick-up coil - or the wires from it. The module is going to default, "on." Usually a burned module is "open"or "off." But, it could be shorted, so that's a 90% rule.
Checking the pick-up coil - making sure the pulses are getting to the module, and voltage is there, == bad module.
The TFI system has a higher available voltage, about 40KV - a higher amperage output than conventional systems (thicker spark,) and at the lower voltages required by a plug (8-18KV) it has a much longer spark duration. A typical conventional ignition, (points or most electronic ignitions) have a spark duration of about .75 milliseconds. The TFI, Duraspark II, and the GM HEI typically have a duration of about 2.5 milliseconds. The longer burn time does a better job of getting a fire started where it counts.
The TFI module -- and the Duraspark II and HEI systems - have a variable dwell and a current limiter in it. At lower RPM's the coil initially charges up for a few milliseconds, then the current limiter in the module lowers the current through the coil - to just enough to maintain the magnetic field without it collapsing. Current for the first few milliconds is about 12 Amps, then it drops to about 4 mps. And, since the Dwell at lower speeds is far less -- the duration that all this is happening, the duty cycle on the coil, is low. Dwell at idle is about 8 degrees, increasing to about 30 at 2500 RPM.
The reason for all this is to charge the coil as much as possible without overcharging, and without overheating the coil. Remember - "Variable dwell and current limiter."
The plain old original Ford Module (Jeep), the DuraSpark, had neither.
When the key is on, current flows just like the points were closed. The signal from the pick-up coil tells the module to "open", to stop current flow for an instant. Default is always "on."
The TFI coil primary has a lower resistance than the original Ford/Jeep coil, so it will draw a little more current than the original. That alone puts a strain on the module. But that's if the original ignition resistor is retained. The coil output - max voltage, is about the same as the original coil, since it's turns ratio is very similar. The max voltage is about 28-32 KV, same as original. I know some folks say otherwise, but it's very easy to measure the max available voltage with a scope.
Now - if you want the TFI coil to put out more voltage, and more current, and a longer spark duration (more power,) it can, but the ignition resistor must be removed so the coil primary can "see" a full 12 volts. (Energy in = energy out.)
But -- then you are straining the module -- you are asking it to carry almost twice as much primary current as originally designed for.
Remember the variable dwell? At lower speeds the dwell is still 30 degrees on the module, where the TFI module would only be 10 degrees. That means this excessive current is there 3 times longer!
And -- the current limiter - the TFI module had one, the Duraspark does not. The TFI lowered the current after the coil was charged, but the Duraspark continues hitting it full blast! That's really straining both the module and the coil - the coil wasn't designed to carry that much current for so long, the module wasn't designed to carry that much current ever!
Try it - key on, engine off. Wait a few minutes, note how hot everything gets. Now try it with a stock TFI system on a Ford, cool. Look at them with a scope.
Yup, lots of folks get away with it. Some win in Vegas too. Someone proved you can hijack an airplane and jump out with extorted money and a parachute over a mountain too.
If I had on, I'd sure carry a spare module and coil, and good walking shoes.
Check for pulses to the module from the pick-up coil.