GOVTMOD's dumb electrical question of the day; solenoid "I" post as power source?
 

I'm putting a power control box in my truck with a key on relay.

I figured it would be OK to hook up the relay to the "I" post of the solenoid. The box is going pretty much where the solenoid is right now and I'll in turn be relocating that down the fender shirt but it will still be right there.

I just wanted to check if there would be any issues. If there are, where would be the best place to hook it up? I'd rather not have to run it all the way from the ignition switch.

Maybe time for my dumb question, what is a "power control box?" not familiar with this term.

Under hood fuse and relay box, most new vehicle have them. Some don't have in cabin fuse boxes anymore.

Oh. My solenoid only has 3 wires, it has battery in, starter out, and the ignition that runs to it from the switch. So there would be no appropriate connection there for a relay switch that was turned on by the switch, as its only hot when the ignition is turned forward to start.

More often than not newer vehicles have two... one in the cab and one under the hood.

So, I understand you want to run a signal wire to the "I" post of the starter solenoid to engage the relay (a continuous duty solenoid) of the power distribution box, right? It'll work.

Does it need to be constant hot with the ignition on or just hot in "start"?
The "I" terminal is only hot when the key is in the "start" position.

Hot only when key is on. Which I thought the "I" was. I'm putting in a MK VIII fan and I'll be running the temp switch ground through it. Since I'm putting some headlight relays right next to it I figured I'd hook them up through it too.

Do you think it'll need to be fused?

Isn't the I terminal electrically the same as the + post on the coil???

The S terminal is for "Start" and is energized by the ignition switch.... Or am I reading these schematics incorrectly?

Hello GOVTMOD,

Yes, the fan and headlights relays need to be fused.

Hello HIO Silver,

Yes, the 'I' terminal can be connected directly to the + post on the coil, and will only have current at start up.

This connection is for ignition systems with ballast resistors, or resistor wire, whereby the voltage to the coil is +/- 9 volts, but at start up, the coil needs +/- 12 volts.

Yes the "I" post should work fine. There is only a very small electrical requirement to trip a relay. I would fuse it just to be safe. A short there would probably fry your ignition switch wiring as well as the coil wiring. No fun.

If you need power with the key in the ON position the connection to the "I" terminal WILL NOT work!! The "I" terminal ONLY sees power with the switch in the START position. You will need to find another "keyed on" source.

Thats not true, unless its a latching relay, a standard automotive relay requires current to stay on to keep the relay switched.

You guys aren't going to make me go out and check myself are you? There's like 6" of snow out there, no joke and it's not letting up. No room in the garage either.

Seriously, I do appreciate the input even if it is contradictory. I do know that I don't have to have a full 12v to run the relay as I've actually been testing them with a 9v battery. The terminals are spaced perfectly for 85 & 86. I do need to find out if it is actually hot with the key at "on" and whether it will cause problems anywhere else if I borrow the current.

That sucks about the snow, glad I'm in Fla.
The "I" terminal on the starter solenoid is hot ONLY when the key is in the start position. It is dead when the key is in run.
The "I" terminal shoots a full 12V to the coil when starting. When you release the key and it returns to the run position the "I" terminal will be dead.
If you need a constant 12v source when the key is in the run position, the "I" terminal will not work.

EDIT: Thinking about it :-huh. The "I" terminal will have voltage to it when the key is released but I wouldn't recomend using it as a power tap.
When you release the key the circuit inside the solenoid opens and stops feeding 12v to the coil, BUT the circuit will now backfeed FROM the coil. It will only be ~6-7 volts though AND it will place an additional load on the ignition circuit, something that generally isn't desired as it could lead to mis-fire or other ignition problems :-arrgh.

OK the plot thickens, I can't think of anything else under the hood that is key on only. I guess I'll run a line from in the cab. No biggie I'm going to have run some other wires from the cab I'll just add that one to the list.