Sorry in advance for the long post. I have posted this in the 73-79 truck forum, but wanted to get as much advice as I could.

I have a 74 F100 4x4. I just swapped in a 390 into the truck. The 390 came from a running parts truck I have.

So here is the problem. When I first did the swap 2 weeks ago the truck fired fine 3-4 times. Then the starter stuck on. There was a thread at this time that said if you yanked off the 'S' wire (I think that is what it was) and the starter stopped it was a solenoid problem. So I yanked the 'S' wire off and it stopped. I had another soleniod from my parts truck (exact same soleniod) that I swapped in. It started fine with this solenoid for 10 days. Now it's back to doing the same thing, except yanking off the 'S' post will not stop the starter. So back to the original thread 2 weeks ago and it said if pulling the 'S' post off does not shut the starter off then it's an ignition problem. So I pulled the ignition switch out and swapped in the ignition from the parts truck. Did the same thing. I pulled both ignition switches out and bench tested them using the 1974 Shop Manual procedure and they both come up just fine.

So yesterday I went wire hunting, to make sure all the wires still looked good. When I swapped the engines I went thru the engine compartment and fixed everything in there that looked bad. So yesterday I pulled the dash completely out and traced wires around and there is nothing bad under the dash either.

This has got me completely stumped and I don't know where to go now. I've been told by a friend that it's a problem with the starter. I don't see how/why it would be since the starter wont disengage, which means it's still getting power, which leads back to the soleniod. So that means in the last 2 weeks I have 2 perfectly good soleniods that have gone bad. I would go get a new soleniod, but want to make sure I have the problem fixed before I burn another one up.

Could this be a bad ground for some odd reason? I removed everthing from the engine bay and painted it before I replaced the engine. Or is there something else that I am missing. Any help would be appreciated.

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I believe you have it backwards. The "s" wire is the small 12v signal that comes from the neutral safety switch, and this activates the coil in the solenoid, which pulls in the big connection to the starter from the bat +.

So if you jerk the "s" wire off, and it stops cranking, that means the solenoid is ok, it's just the signal "s" wire is getting 12v from somewhere other than the ignition/neutral safety.

If you jerk the "s" wire off, and the starter keeps turning, then that means the solenoid contacts inside are stuck or welded together, and that's usually the solenoid's fault.

If you are at that point right now, I would put the other solenoid back in. If you run into the problem again, and jerking the "s" wire stops it, then I would look carefully at the wiring down at the tranny if it had or has a automatic. It's very easy for the neutral safety switch wiring to get melted against the back-up light wiring, since they are bundled together. The back-up light wiring has a hot in run wire that if it gets against the neutral safety wire, will cause this problem.

 

I called the local Ford dealer and they are getting me a new solenoid tomorrow. I haven't had good luck with Napa electrical stuff, plus the parts guy cut me a deal, so it wasn't much more expensive then the cheepy Napa one.

As far as which wire is what, I may have it backwards. From what I recall there is an 'I' and a 'S' post. I was thinking the 'I post went to the alterntor or something and the 'S' went to the coil or something. I went thru my wiring diagrahm last week to make sure the small posts weren't flipped for some reason and they weren't.

I may look into the neutral safety switch. I haven't had back-up lights since I have owned it. So they may have a problem. That was actually next on the list of things to do, after I got it running down the road.

 

Just looking at the schematic in my shop manual. Just had a dumb thought. If I pull the fuse for the back-up lights, that should also kill the wire to the N.S.S.? or is it tied to something else. This is just a temporary band-aid to rule out that the N.S.S. is the problem.

 

That's a good idea, good thinking. That would kill the 12v to the backup circuit, and if that is where the stray 12v is coming from, then the problem would go away and then come back if you put the fuse back in.

The "s" terminal is the start terminal, and that's what makes the solenoid activate and send power to the starter.

The "i" terminal is just a contact inside the solenoid. Voltage goes out on this terminal. All it does is send 12v to the coil + during cranking. The coil is usually fed through a resistor wire in the harness which drops the voltage to the coil to around 9v. To make the engine start quicker, especially in cold weather, they send a full 12v to the coil during cranking for a hotter spark. This 12v comes from the "i" terminal on the starter.

Some of the later models did away with the "i" terminal, and instead added a contact in the ignition switch "start" position. Then when TFI came along, they did away with the resistor, so there was no need for any of this at all.

 

I took a look at the N.S.S. last night. It was good, but one of the screws holding it on was gone, so it wasn't working because it was just rotating on the other screw. I took it out, cleaned it up and put it back in there and it works fine. Thank God.

I got the soleniod from my local Ford dealer yesterday. While I was there I was talking to some of the mechanics about the problem. They said checking the typical stuff like the soleniod and ignition. Then one of the guys asked me how the ground to the was. I told him it was a new grounding strap and both ends where good. He then asked me how the grounding was at the soleniod. I told him probably pretty bad since I had removed it and painted the inner fender behind it. He said that a bad ground at a soleniod will cause it to go bad.

So I have the new soleniod in and cleaned the fender behind it. So hopefully my problem is gone. I fired the truck up last night 5-6 times over a 2 hour period with no problems, knock on wood.

Thanks for everyones helps. If it goes bad again, I will be back.

 

You should be good to go now. The key to the problem was no ground to the starter relay mounting bracket.