IH 2500 ignition wiring

You need to find the hot wire that has voltage. Use your volt meter and set it on dc voltage. Once you find this wire connect it to the No1 terminal. Do not connect any other wires at this time. Using your volt meter again turn the key to run position and check for voltage at the other terminal by holding one the test leads from your meter to No1 terminal and then touch each of the terminals. Should only have voltage on one of those terminals 2 or 3. If you do find voltage present, turn the key to the start position and check for voltage on the other terminal. If you do have voltage here when the key is in the start position and there's no voltage present when you release the key. That's the terminal that feeds voltage to your starter solenoid. You will need to trace the wiring. You should have one wire from the switch to the the tractor's ignition and one to the starter solenoid. Those solenoids usually have the letter s stamped by the small terminal. Hal
PS: Is that a new switch?