Pertronix Ignition Quit Again

A clamping diode would burn out too unless it was current limited. A limiting resistor would essentially take away any clamping properties. A varistor might work if it is close enough to the Pertronix that the feed wire would have a bit of resistance itself. You don't know what your peak voltages are unless you have a meter with a peak/hold function. In my distant past at work I had a high quality Fluke that did this. It costs a pretty penny. To be honest I'd replace the alternator. If there are any over voltages, the alternator would be the source.

 

ETA: Are you taking the voltage for the ignition switch from inside the box before your ammeter? If so, move this wire to the battery.  Fuse this wire right after the battery. The battery will act as a large voltage dump, which might cut down on your spikes.

 

I asked if the coil was replaced. No reply. A coil can ohm good until it heats up or may otherwise have an intermittent short.  New, less than a year old, works fine otherwise... none of these mean it's a good coil. The problem could be one of many things.  Unless you have a peak hold meter or an oscilloscope, you won't see those peaks. Sometimes the problem rears its head when things get hot. My problems usually reared their heads because of the cold. At least the equipment I had was usually small enough that I could put it in the freezer for a couple of hours. Sometimes you can CAREFULLY warm up a coil with a torch, held a long ways away from the coil. The cheap/easy fix is to replace the alternator with a used 3-wire from a junkyard, a good used coil (Beware, some automotive "12 volt" coils were really 6 volt coils fed with resistance wire), and hook the ignition switch directly to the battery. To replace the Pertronix now would be dooming the unit to the trash can.

 

ETA: Go to a junkyard and pull a 3-wire of the same frame size you have now. I'd shoot for a CS121 or CS130. Stick with an early 80s GM that does not have a 130D alternator. Pull the connector too, cutting the wires a good 6 inches from the connector. Wire this alternator as a one-wire, but remember which wire is the "L" terminal. Start the tractor, rev it a little, then touch this "L" wire to that BAT terminal on the alternator. The alternator should start charging and will keep charging after you remove this wire. While at the junkyard, find a good old real 12 volt coil and replace yours. Run it a few weeks like this, remembering to touch the "L" wire after you start the tractor. If everything keeps working, check back for instructions on how to wire that "L" terminal. It's not that hard.