Starter Relay Connections Question

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Sep 16, 2011 | 07:03 PM

LARIAT 85

Thread Starter

Cargo Master

Joined: Dec 2004

Posts: 3,362

Likes: 22

From: Florence, SC

Starter Relay Connections Question

On the starter solenoid or relay located on the passenger's side inner fender, does it matter which end the positive battery cable connects to, and which end the starter cable connects to?

It looks like this:

Sep 16, 2011 | 07:41 PM

HIO Silver

Fleet Owner

Joined: Dec 2009

Posts: 20,678

Likes: 82

From: NorCal

Yes it matters.

The brown wire's terminal (I) provides power to the coil when the ignition switch is "on". The red-blue wire's terminal (S) traces back to the ignition switch where it gets the signal to engage the solenoid.

If only the heavy gauge cables are reversed, then it won't start because the starter side (right) is only hot when the solenoid is engaged...

Sep 16, 2011 | 08:01 PM

fmc400

MSEE

Joined: Apr 2004

Posts: 10,386

Likes: 35

From: Austin, TX

The LARGE terminals (battery, and starter) CAN be reversed. All the solenoid does inside is short the two large terminals with a plate when the solenoid is engaged; it doesn't have any sense as to what side is what. If you're asking because you want to install the solenoid upside down, then that's no problem.

The SMALL terminals (S, I) can NOT be reversed - but I don't think that was your question.

To give you a better sense of what's going on, look at the photos in the following post (courtesy ctubutis): https://www.ford-trucks.com/forums/1...l#post10715598

The solenoid in those pictures does not have an "I" terminal, but it shows how the battery and starter terminals get shorted together.

Sep 16, 2011 | 08:54 PM

LARIAT 85

Thread Starter

Cargo Master

Joined: Dec 2004

Posts: 3,362

Likes: 22

From: Florence, SC

I knew the "S" and "I" terminals can't be reversed. And they are even marked on the solenoid, so I figured that much.

Fmc400 is correct. I was speaking of the LARGE terminals, on the far ends. My cables will fit better if the starter cable connects on the left side, and the battery positive connects on the right side. That is opposite of what I see on all the other vehicles I have seen, so that was the reason I asked.

Sep 16, 2011 | 09:08 PM

LARIAT 85

Thread Starter

Cargo Master

Joined: Dec 2004

Posts: 3,362

Likes: 22

From: Florence, SC

Does anyone know why some models have this round starter relay instead? Is there a difference between the one below and the first one I posted that is more of a square shape?

This round one is the relay my truck came with, but the large terminals are just too close together. After nearly jumping out of my skin for about the third time this year because I accidentally bridged the two terminals and the starter began spinning, I would like to switch to the other style shown in my first picture. Will there be a problem doing that?

Sep 16, 2011 | 09:08 PM

fmc400

MSEE

Joined: Apr 2004

Posts: 10,386

Likes: 35

From: Austin, TX

I figured as much. Also, for completeness, I beeped out a spare solenoid I have that does have a hot-in-START "I" terminal, and I verified that the "I" terminal is not hard-tied to the large lug that usually gets connected to the starter cable. If it was, then you wouldn't be able to turn the solenoid around because then the ignition coil (which taps into the "I" terminal for the start bypass) would be permanently connected to the battery. But since it's not hard-tied, you should be good there too.

It wouldn't hurt for you to check yours, however - in case for some reason ours are different. The one I checked does look exactly like the one in the first picture however, but it wasn't Motorcraft. There shouldn't be any differences, though - otherwise you wouldn't be able to call mine an OEM replacement. However, the "I" terminal should be isolated like I found with mine. The starter terminal should be isolated from everything once the solenoid is disengaged because of back-EMF from the starter shutting off.