Tractor solenoids connect the starter directly to the battery during the starting procedures. Starters require a significant amount of current to operate--current that is too large to send through a standard ignition switch. Doing so would burn out the ignition switch within seconds. Solenoids solve this problem by using a low electric current to activate a high-current switch.
Identify each terminal on the solenoid. Solenoids usually will have three or four terminals. There are two large terminals and one or two small terminals. The two larger terminals handle the large currents needed by the starter. The one or two small terminals handle the small signal current from the ignition switch.
Connect the large red wire that runs to the starter to one of the large terminals on the tractor solenoid. Some terminals may be labeled. Connect the second large red wire, which runs to the battery, to the second large terminal on the solenoid.
Inspect the small terminals. If the solenoid has one small terminal, make sure it is connected to the ignition switch. The housing itself will be used as a ground when the solenoid is secured to the engine bay. If the solenoid has two small terminals, connect one terminal to the ignition switch and the second terminal to the engine frame for a ground. Follow the wiring instructions included with the new solenoid.