I am in the process of rewiring under the hood. I ran into something that I cannot get my head around so I am going to through it out there for an expert.

The starter relay has four posts. One is the hot lead from the battery. One is the lead to the starter motor. The S terminal (Red/Blue wire) is the start signal which is 12V when the ignition switch is turned to start/crank. The I terminal (Brown wire) is 12V when the ignition switch is turned to run.

My question is what does the I terminal do and why does it need to be connected? What would happen if the I terminal was not connected to the switched voltage? The relay will pass voltage to the starter from the battery when the S terminal receives 12V. The I terminal is not for the +coil wire because the coilis connected via another harness, which is connected to the tach and indirectly to the I terminal via some jumpers in the car. So again, what is the I terminal for?

Thanks for helping me. It is late, and I just can't figure it out.

 

The "I" terminal wire powers the coil with a full battery voltage when cranking. This by passes the resistor wire so when starting if the battery voltage drops you still get a hot spark to start the engine. I hope I worded that clearly enough.

 

67coupeis correct

 

+1. the original ignition setup is with resistance, so you only get ~7.5V at coil.
For cranking the engine this will put the full 12V on the coil.
i made a little diagram in a different post if it's any help

https://mustangforums.com/m_4648142/tm.htm

 

Nice diagram.

 

Question...The brown wire when it is not connected to the I terminal is already hot (not sure now with how much voltage. Should this be 7.5V when the key is run? So if the relay is supposed to feed full 12V voltage, wouldn't that create a + to + connection? Is that right? Does this allow full 12V to the coil during cranking and then the I terminal goes dead dropping the coil voltage back to 7.5V? Why do you want the coil to run 7.5V normally but 12V during cranking?

Question 2...With an aftermarket full MSD setup withthe coil voltage coming thru the MSD, whichgets power from the battery lead when run voltage is applied via the key? So under this setup, you do not need the I terminal because the coil wire is not longer connected via the orginal location?

 

I guess this makes sense. When you thow in MSD ignition, dizzy, coil, tach converter...it all gets confusing and seams to me the I terminal is no longer needed. Does that sound right?

 

So, is the I terminal needed when you run a MSD iginition system?

 

hiya. sorry for late reply
From how I understand this sytem, this seems correct to me.
Normally those setups require full 12V on start and run. So for this people usually splice to bypass the resistor to feed 12V to the module in the distributor. And do the same for coil if needed.

I believe you have it understood smack bang (at least that's how I understand it as well). I'll check on the diagrams later just to make sure