Correct Starter Relay

In the tractor relay power is hooked from the terminal block (power with key on side) to the "I" terminal. Goes into relay wire windings and out the "S" terminal which gets hooked to the starter button on the trans. When you push the trans starter button you complete the circuit. Power flows through the relay heating the wire windings causing the plunger in the relay to move connecting the battery to the starter.

In the car relay the starter button goes between the terminal block and the relay. In this set up you hook the wire to the "S" terminal. This is hooked to the wire windings in the relay and then internally to the relay case for the ground. The "I" terminal gets hooked to the coil bypassing the external coil resistor. When you push the starter button it sends power to the "S" terminal; through the wire windings and then to case ground. This heats the wire moving the plunger to connect the battery to the starter and the "I" terminal. This starts the car and at the same time sends the full 12v's to the coil to assist in starting.

NAPA has a relay catalog with wiring diagrams to show you what the circuit looks like.

The NH sounds like a good price. Might take your ohmmeter with you and check the circuit to insure that it is what you want.

Mark