The project is an old Sears garden tractor, early '70s, that has lots of attachments including one with which I mow the lawn (which is now knee-high). Tecumseh single OHV, 14hp, 20A alternator. Tecumseh is out of business; somebody is said to be trying to get it going again but so far parts are NLA, including the magneto/CD ignition module which died.
The popular fix for those in the know is to forget the mag and adapt a Chrysler 4-pin ignition module and an automotive coil. The fellow who is most into this, Ed Stoller, has been experimenting with this for some years and makes, bench-tests, and sells his own pickup/sensor. This consists of a small ceramic button magnet bonded to a soft steel bolt with about 400 turns of fine wire, one end attached to ground, the other to one of the 4 pins on the Chrysler switchbox. The second of the two pickup connections on the Chrysler box is not used with Stoller's system, but it works fine on the few hundred conversions that have been done, and so far as I know it has nothing to do with my own issues.
Stoller's pickup-coil was designed to make use of the peculiar Tecumseh flywheel. Tecumseh press-fitted two 3/8" steel dowels partway into the periphery of their flywheel to trigger their mag/CD ignition (the windings for the mag being located under the flywheel, as are the alternator windings). The leading pin, that is, the pin that passes under the pickup first, sits up higher than the second pin, and one sets the gap to this pin with a feeler-gage in the usual way. This first pin is for starting, and fires the plug at about TDC. Once the engine starts and spins up past about 550rpm, the pickup is then for some reason able to sense the second pin, which is pressed into the flywheel farther (so, more gap), and which fires the plug at the running advance of about 17 degrees BTDC, I believe.
Now to my own mystery problems:
Rather than spend cold cash on a 4-pin Mopar set-up, I dug into my old dusty boxes and got out a never-used example of the earlier 5-pin ignition, complete with a new harness and a used ballast resistor, and also a good, oil-filled coil. This happens to be a mid-'70s "blue box," which was represented to me at the time, by a dealership, as the NASCAR version, which they said put out the same spark as the standard box but had components that would stand up better under heat.
Now, the ballast resistor for the 5-pin box was a special item with two resistors, four male connectors. This seems to get some people baffled and worried for no good reason. One of the resistors is the actual 1.2 ohm ballast resistor that protects the coil while the switch is in the RUN position, and is bypassed while in the START position, the normal deal. The second resistor is not a ballast at all but some sort of surge-protector for the circuitry in the module, and it is used whenever the switch is on. The later 4-pin ignition simply put this surge-protection inside the module and eliminated the "extra" outside resistor. I can think of no reason why a 5-pin box can't serve my purpose just as well as the favored 4-pin box (Ed Stoller's set-up also uses a coil with an internal resister, but I don't like them as well and don't mind the slight additional wiring complexity).
BUT, BBUUUUUTTTT, something is amiss! I'm sure I must have something connected wrong, but I'll be danged if I can see what it is!
On the tractor, I have an on-off 12V/20A switch for RUN, which takes 12V from the battery positive post, through the switch, through a fuse-holder with a 10A fuse in it, and on to the 12V input pin which is at the apex of the five pins in the module.
To run the starter motor, I'm using a 3-post type of solenoid; two heavy posts for the big cables on either side, and a single small post in front. On a 3-post solenoid, this small single post is always hot; by connecting it to ground through the START circuit, it makes the contact between the big posts. However, in my case I am not using this post at all, because this is one of the fine old solenoids that has a mechanical button on top which I push with my thumb to start!!
At the power post of the starter I've connected my wire that bypasses the ballast and sends 12V direct to the coil.
I should also state in advance that all wiring and connections are new and good, and that all of the grounding from the case of the module to the frame AND direct to the engine block (AND direct to Ed Stoller's ignition pickup), and from the block to battery negative post are excellant.
NOW . . . for some reason, when I switch on the RUN circuit, and then put my thumb on the starter, the engine fires right off, and it will run along just fine, at low rpms, as long as I hold my thmb down and have power to the starter. When I let off the starter . . . silence. This is ordinarily a sign that you have an open ballast resistor, thus no connection to the coil in the RUN mode. But I have checked the resistor with an ohmmeter and not only have continuity but both resistors read about the same ohms as a new off-the-shelf resistor. In case the surge of current might be causing the resistor to open in a way not testable with the ohmeter, I simple ran a jumper-wire across the resistor. But the symptoms did not change.
The second clue (and here is where i'm hoping one of you will have the "aha" moment that saves my project): I keep blowing the fuse in the RUN circuit. As I understand it, the module only ought to be drawing about 3A normally, and a 5A fuse should be appropriate, but I am blowing 10A fuses. WHY??? I wouldn't have thought I could fry something in a never-used module, but maybe I did when I was sorting out the connections, so, does anyone know if this box has a failure mode that would, say, create a dead short through the 12V input pin to ground under load?
The pickup and the ignition do work. The engine is trying to run. What have I done?
Thanks for your patience
. Eagerly awaiting your solution 
--Smitty
)