Farmall H 12 Volt Conversion.

this will help

Its best at 12 volts to use a solenoid with a 12 volt rated coil. Otherwise, she may overcurrent and overheat, but could work a long time fine depending on how long its activated and its design.

Now, you ask about 3 or 4 terminals. On many older solenoids, ESPECIALLY the 6 volt units, they had the big in (from battery) and out (to starter) terminals plus a single small terminal possibly labeled S or possibly no label. Thats the activation terminal which leads to a push to start or a start switch which is HOT to operate the solenoid when in the START position.

On many 4 terminal solenoids ESPECIALLY the older Ford types mounted up near the battery, they had the 2 big in n out posts plus 2 small terminals labeled S and I. If you have one of those, the S is the activation terminal which wires to the start switch to operate the solenoid while, the I Ignition terminal is hot ONOLY when cranking and can be used to by pass an ignition ballast if the auto or truck or tractor had one. If you have no ballast then you just use the S terminal.

An electric solenoid has to have a starter with a drive gear (Bendix) that engages into the flywheel (mechanically or electrically) when she cranks, while many mechanical activated starter drive gear mechanisms use the lever action to push the drive gear into place and then the electrical switch closes to spin the starter motor and engine. Im just not sure what you have there so I cant fully give you an answer, sorry.

John T

If yours indeed does have a 6 volt solenoid, it's good idea to replace it with a 12 volt. (Feeding 12 volts into a 6 volt solenoid may cause it to overheat and/or bang itself to death internally)

When wiring the solenoid up, simply connect the wire from the starter pushbutton to the solenoid "S" terminal.

You can leave the solenoid "I" terminal empty. It is provided on some 12 volt systems to bypass the ballast resistor and increase voltage to the coil while cranking. However it's really only needed on older (60's era) cars with high compression V8 engine. My experience is the ballast coil bypass is completely unnecessary on low compression tractor motors...

The most common "4-wire" auto supply starter switch is commonly known as the "Ford fender-mount type".

If this is what you have, "S" is START, and "I" means "ignition resistor bypass".

If you are NOT using a system to bypass the ignition resistor (if used) during cranking, simply connet the "start" pushbutton to the solenoid's "S" terminal, and IGNORE the solenoid's "I" terminal.