A few people have asked me to explain how the different starter relays work so here goes.
While they make many starter relays we will only look at the 3 used on the Ford tractors. The first thing you need to remember is electricity needs a ground. With out a ground there is no path for electricity to flow so you have no electricity. We also need to look at how a starter relay works. In the relay there is a little piston or plate. If we wind wire in one end of the relay in a coil and then apply flowing electricity to it we can create a magnetic field or a magnet to move the piston or plate. We can then use a spring to hold the piston in the neutral position. When we apply electricity to the wires the magnetic field over powers the spring moving the piston to make contact and when we remove the electricity the spring pushes it back to neutral.
So now we have a few options. We can change how the electricity gets to the relay; or we can change how the electricity gets to ground.
Lets start with the 3 post relay on the early hundred series. X00 tractors. In this relay the small post is hooked to one end of the coil windings and the large battery post is hooked to the other end. So the small post has power 24/7. Since we have a wire hooked from here to the starter button it also has power 24/7. All we need to do to make this relay work is apply a ground. This is done with the starter button making contact with the trans when pushed down. This is why this tractor will crank without the key on. It will not start because no power is getting to the coil but it will crank.
Now we move on to the 4 post relay used in the X01 tractors. Since we now have tractors that run on diesel we need to change how the electricity gets to the relay. We need to have it keyed so the tractor can not be started without a key.
In this relay each end of the coil windings are hooked to a small post on the relay. So we run a wire from the key to one of the small post; and then a wire from the starter button to the other small post. When we turn the key on electricity runs threw the relay to the starter button. It gets its ground the same way as before by making contact with the trans when pushed down.
Now we move on to the 4 post relay used in cars or 3 cylinder ford tractors built after 1965. Most if not all of these systems are 12v and we will see how that helps later. With this relay we do not use the starter button on the trans so we have changed the path to ground. In this relay one end of the coil windings is hook to the small S terminal and the other end is hooked to ground. So if we use a dash starter button or a key with a start position the motor starts to crank the second we apply power. This is because the ground path is always there via the relay mounting bracket. Now you can see why this relay will not work in the older tractors with the starter button on the trans.
The small I post is not used for cranking in this relay. It is hooked to the starter side of the relay and gets power from here only when cranking. If you look at the ignition system you will notice most have a resistor in the wire going to the coil. If we hook a wire from the I terminal directly to the coil bypassing the resistor we can apply a full 12 volts to the coil only when cranking. This helps with cold weather starting.
I hope this helps everyone understand these starter relays.