Starter relay question

[Charles in SC]

My relay has two large and two small terminals. The small ones are labeled S and I. The S terminal connects to the S terminal on the key switch, but I do not know where to connect the I terminal. Maybe it does not get used. This came to my attention because there is a diode that connects to the S terminal on one end and I realized I did not have anything that said what the I was for.
Thanks in advance!

[HFS]

I'm sure someone else will chime in - but the short version is, for your particular installation the "I" terminal is not used.

This style contactor is generally known as a "Ford" type and was designed many (many) moons ago when conventional ignition systems used a coil and a distributor to make and distribute spark energy. During the "start" cycle the coil needed a source of energy to concentrate and send down the line to the distributor - the way the coil got the energy was from the "I" terminal that became "hot" when the contactor was energized. The "I" terminal was connected to the coil and provided battery energy to the coil during the start sequence - all other times the "I" terminal was open.

HFS

FWIW - This contactor gets its ground from the steel butterfly shaped mounting bracket. Some people have mistakenly connected the "I" terminal to ground thinking that terminal was used to complete the circuit - not! What happens is that the small copper wafer inside the contactor that connects the copper plunger to the "I" terminal is instantly vaporized when the contactor is energized the first time - the wafer acts like a fusible link. It happens so fast it is a no harm no foul situation. Thereafter, the "I" terminal is forever out of the loop because it is no longer connected to anything. As long as the mounting bracket is still in contact with a grounded surface the contactor will operate as designed. If the contactor is unbolted to whatever is supporting it
(ground plane) and the start switch engaged - nothing will happen, even if the "I" terminal is still connected to ground.

[bruceh]

Go to the Vertical Power web site and download their VP-X installation guide.
They also have a Contactor Wiring guide document.

They contain excellent information about how to wire these up even if you aren't using the VP-X system.

[GeneL]

The "I" terminal was used with old point ignition systems. These old systems actually ran on 9 volts, in a 12 volt engine. The 9 volts was obtained by using a ballast resistor. When the starter was being cranked the battery voltage drops by around 25% (3volts) giving 9 volts, the "I" terminal was wired directly to the ignition, bypassing the resistor, giving the full 9 volts to the ignition during the start cycle= happy starting.

[Pat Hatch]

You can also use the "I" terminal to support a "Starter Engaged" light on your panel should you want to do that. This is just assurance that the starter disengages when you release the "start" button. Starters have been know to remain engaged and you would want to know that. Of course, this would be caused by the relay sticking in the "on" position. Happens very rarely, but it happens.

[Charles in SC]

Thanks a bunch! This brought back some "old" memories of working on Fords years ago. Yes, I am old enough to not consider them old.