No Spark - Farmall 100

Attempting to start a 1955 Farmall 100 that hasn't run in a year or so. I'm not getting a spark when cranking. I have 6 volts up to the plug when the switch is on and 3
volts when the switch is OFF. I put a meter on the coil and get 1.4 ohms across the +/-, and 7000 ohms between the + and coil tower. According to a coil troubleshooting
site those #'s are good. I'm troubled by the 3 volts I'm reading when the switch is off. Thoughts?

I assume you meant to say coil not plug in sentence #3. The issue with a modern volt meter is that they will read voltage when not connected, and give a false impression of very small volts with no current. would temporarily ignore the 3 volt reading.
The supply to the coil is good. The small terminal connected to the distributor is the next test point. Using that same meter, tour on the ignition and check voltage at that terminal. If there is 6 volts there, shut off the ignition and remove the distributor cap. (do not disconnect the plug wires) Pull off the rotor and using a good flashlight, look at the points. They are a cam operated switch. They have a pair of round contacts. most engines stop with the points closed. use a small screw driver to lift the movable point a tiny bit (1/16") the surface probably needs to be cleaned. Use a emery board or folded fine sandpaper to polish them. Clean them with a folded dollar bill to be certain they have no grit between them.
If they were open when you looked at them, bump the starter with the starter rod to move it to closed and then clean them. Put the rotor and cap back on and check voltage at that same terminal. It should be zero with the points closed. See if it starts. Jim

I did all you recommended but no change. I just tried to get a spark by connecting a lead from the coil tower directly to the spark plug, and grounded the plug to the neg battery terminal and cranked. Nothing. Regardless of what my ohm meter read on the coil test, it must bad. Wouldn't you say that would be the next logical thing to rule out?

If the points are opening and closing. Indicated by putting that volt meter on the coil terminal connecting wire from the coil to the distributor, and turning the ignition on, then opening and closing the points with a plastic knife. If the volt meter shows 6v then no volts, then 6volts, the points are working, and I suspect the coil is bad. Jim

Very simple no brainier test is this. Pull the distributor cap, rotor and if it has one the dust cover. Turn it on and open the points by hand being careful because you may find the spark if you do it wrong. But when you open and close the points by hand you should see a spark ever time. No spark at the point means bad or corroded points that need to be cleaned or replaced. Any time an engine sits for a long time the point will corrode so then need to be cleaned. I have over the years gotten a many tractor super cheap because they did not run and a good many where winched on the trailer but drove off a couple hours later all because of the points

I replaced all the ignition parts and now have a very weak spark. I also now have 5.5 volts when the switch is off verses the 3 earlier. The ignition wire from the switch is so brittle, I'm going to replace it....I'm also going to try and find an analog volt meter. A technician at a tractor parts counter confirmed what you said about digital meters on some older models.

The switches are also suspect. if rusty inside, they can leak (as found) but the low voltage is also suspect of switch issues. To assess that, just jumper from the bat negative to the coil, bypassing the switch, to see if that hot wire lets it run. Jim