Ford Solenoid Polarity

I am working on a 1989 Ford F700. I believe that the solenoid is bad on it.

Looking around for wiring diagrams, most of them show battery and starter connections to the solenoid opposite of what I have on the truck.

Does it matter which of the big posts the battery and the starter connect to?

How is the “I” terminal connected internally inside the solenoid? I would not want to supply constant 12V to the ignition coil. At least one diagram on the web ( shows the “I” terminal bonded to one of the big posts, which would make the big posts non-interchangeable. But I hope that the diagram is wrong.


If you look at the wiring diagram, the I terminal goes to the ignition coil.

This done so when the engine started cold, the coil gets full battery voltage.

Once the engine starts, and the ignition switch is allowed to go to the run position, the solenoid contacts open up, and the coil is supplied with voltage thru a resistor wire.

If this wasn’t done, and the coil was always connected to the battery, the secondary ignition components would fry as soon as the charging system came on line.


That is my understanding too. I think the main reason for applying the full battery voltage to the coil during start is because during the start the starter motor draws large current and the system voltage drops to 8-9 volts. So with the resistor, the coil would get maybe 5 volts.

But my question is about something else. Does the “I” terminal have a “preference” for one of the big posts versus the other? Can I swap the battery and starter cables around on the solenoid?



If they’re switch around, the ignition coil would always be connected to the battery.


Ok. Does my picture show that someone has crossed the cables? Because that is opposite of everything else I have seen.

The solenoid has a layer of grime on it, so it has been used for a while.

If the solenoid mounts on the right side fender well near the battery, then it’s labeled backwards.


The solenoid is on the right side fender. But on this vehicle, the battery is mounted on the frame. Both the battery and the starter cables go in the same direction from the solenoid, so they can be connected either way.

I hope that the solenoid is made like in this picture, so the “I” terminal is only energized when the solenoid is energized, regardless of how the battery is connected. But I do not know this.

All I know is, every Ford I’ve owned/repaired with this type starting system had the battery connection where they’re calling it the starter connection.


I just did a quick test on a new solenoid, and there is no electrical continuity between the "I’ post and either of the big posts when the solenoid is de-energized. So I think it makes no difference how the battery is connected.

Yeah, your diagram seem to show that the “I” post is only contacted when the contact washer hits the battery and starter posts:

You can test it out using a battery and some jumper wires to cause it to close if you want to do an experiment. Then you’ll know for sure which current paths are open, and which are closed when the relay is activated. Coincidentally I had a problem with this part on my Ford truck a couple of weeks ago when replacing the battery. Apparently the ground for the coil is through the connection where it is mounted to the body. The mounting screws had worked loose over time, plus there was a little rust in that area, and what with the jiggling of the wires during the battery install, it wouldn’t activate reliably. I cleaned and tightened that ground connection which fixed the problem.

I’m pretty sure on mine the terminal labeled “starter” (on the left) in the photo above is connected to the battery on my truck. On my truck the one on the right goes to the starter. The solenoid is located on the passenger side. I can’t speak to whether it is advisable to switch the battery and starter post function. There may be subtle differences about how the two connections are configured inside, to minimize electrical spikes during opening and closing for example.

I was thinking something similar. Since this is a direct current relay, the contacts may be designed to work one way so as to minimize arcing and make them last longer.
However, there are no markings on the solenoid to indicate battery or starter connection, so who knows. If this was important enough, I would think they would mark those big posts.
As is, I am operating strictly on assumptions, from the pictures I have seen on the web, to guess which side should be the battery and which side should be the starter.
If someone knows of specific guidelines on these connections, it would be good to see them.

I’ll take a look at my truck next week and see if those big posts are marked or not on mine.

You’re way over-thinking this…it simply doesn’t matter.

The “S” terminal gets it power from the ignition and activates a plunger solenoid. The plunger has a metal top that makes contact between the battery cable and the cable going to the starter (the two big ones). This provides the power to the starter. Connecting the “S” and “I” incorrectly will not provide power to the coil that activates the plunger. (I think that’s how it works with this type of solenoid regardless of brand or model car).

I hope the “I” and “S” terminals are marked.
Update: “insightful” is correct, it does not matter which side on the large post the battery and starter cable goes. What does matter is the “S” and “I” connections. I just found this link that explains it much better:

Interesting. This video shows the “I” terminal permanently connected to the starter terminal. There is no such connection on my solenoid.

I do have the “I” and the “S” terminals marked on the solenoid, but no markings for the big posts.

not all solenoids have an external “I” terminal.

Then why does the photo of the solenoid you posted shows the “I” terminal? Are you saying that your old solenoid does not have one but the new one does?

My solenoid does have an external “I” terminal. However, when the solenoid is de-energized, the “I” terminal is not connected to any other solenoid terminals nor ground.

Now in the video they clearly show a permanent connection between the “I” terminal and what they call “TO STARTER”. It is always connected, whether the solenoid is energized or not. This would make the big posts non-interchangeable.

When you turn the key to “start” the plunger pops up and completes a path between the battery and the starter. The “I” terminal is on the same path/circuit as the battery and starter cable. It only matters that you correctly wire the “S” and “I” terminal.