When the starter motor is engaged, the system voltage drops to about 6 volts, this is due to the high amp draw on the 12 volt battery. The resister drops the normal running voltage to 6 volts. Ford did this so a 6 volt coil can supply the 20,000 volt spark needed to fire the spark plugs. Two wires are attached to the start switch, the one that goes to the start terminal has the non-resister wire, the second wire with the resister goes to the run terminal on the start switch. Some time during the mid 1950"s Ford stopped using the ceramic resister and used a resister wire which is a little hard to identify in the wiring. All this was to keep a constant voltage at the coil during engine start. If the resister wire is broken or shorted the engine will start only when the key is in start mode, then will dye in the run mode. If the non-resistor wire is broken the engine will not start except if the key is back to run mode when and if the coil fires as you let off the start mode. So the moral of the story is both wires must be intact. Also, using a newer model 12 volt coil will burn you points in a very short time.
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Today's Featured Article - Oil Bath Air Filters - by Chris Pratt. Some of us grew up thinking that an air filter was a paper thing that allowed air to pass while trapping dirt particles of a particles of a certain size. What a surprise to open up your first old tractor's air filter case and find a can that appears to be filled with the scrap metal swept from around a machine shop metal lathe. To top that off, you have a cup with oil in it ("why would you want to lubricate your carburetor?"). On closer examination (and some reading in a AC D-14 service manual), I found out that this is a pretty ingenious method of cleaning the air in the tractor's intake tract.
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