The oil pressure switch only runs the oil light. Has no effect on starting or ignition.
The transmission switch only affects the starter solenoid circuit.
The S terminal on the solenoid goes to the coil side of the ignition resistor. The coil should be 6 volts not 12. The resistor cuts the voltage down, but the S terminal on the solenoid applies full battery voltage while cranking (which may be 9 or 10 volts when its really cold) for better ignition while starting. The coil is also supplied through the resistor from the ignition switch. That circuit is missing if it stops when you release the start button or start position (different vintages are different, some have a separate button, some start on the ignition switch).
It doesn't matter which end of the wire between the coil and the resistor that the wire from terminal S connects to. My 4020 has a ceramic resistor mounted next to the coil so that wire is short.
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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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