Greetings,

Been having some trouble with the some of the starter wiring on my truck. I have identified the problem thanks to search, I just wanted to double-check with some of the great minds here before tearing into the harness to make sure I'm going about it the right way.

The wire feeding the S terminal on the starter solenoid still has voltage when the key is returned to the run position which I believe is the cause of my starter failing to disengage (disconnecting S and jumping it from the battery works just fine). The starter solenoid has been replaced (which fixed a slow crank issue) and the ignition switch has also been replaced which fixed a "nothings happening" issue when I attempted to start the truck. The truck has been parked minus battery for a couple months so it is very strange to me that these issues manifested "all of the sudden" but that is neither here nor there at this point. I assume my next move is to take apart the wiring harness in order to identify the source of the short? I can't honestly think of any other way to find it but that is why I'm asking. All too often I've dived into a project and found out there was a better way to do it had I only asked first so, here I am. I have measured continuity between the I and S wires, not sure if there is supposed to be. The truck fires up if I jump the solenoid with S disconnected but without knowing more about the source of the issue don't see that or running a new wire as a viable long-term solution.

If it matters (it always seems to) - 75 F100 4x4 w/ FE power and duraspark (green grommet)

 

First of all, do you have a ground-strap cable running from your engine block to the firewall of the cab? It's pretty common for unknowing previous owners to omit the 'body' ground when throwing together a vehicle under their shade tree. My truck was missing the cab ground when I got it, and it had a wicked-bad starter run-on problem. Running a ground cable fixed it.

Perhaps you could try unplugging your ignition switch, and testing it independently to see if the Start terminal is still passing current when released to the Run position.

Another remote possibility is that if the truck calls for a black grommet DS box, perhaps the wiring is different, and that difference is enough to manifest this problem with the blue grommet box.

The "I" post on the solenoid should only be hot when 12V+ is being fed to the "S" post.

So you're saying the truck never had this problem, but then it sat for a time, and then the problem started occurring?

 

I don't know the answer to this, but will find out. What I do know for sure is I can hook up the battery and put the key all the way to the run position and everything is fine. When the key is turned to the start position I get 12v on the S wire (disconnected from the solenoid) as I would expect it to, but when the key is returned to run I still have voltage even when the key is turned off. At this point I can disconnect/reconnect the battery and the voltage is gone until I attempt to start the truck again. This is also true if, leaving the key in the off position and feeding the S terminal 12v directly from the battery, the wire stays hot until the battery is disconnected. I have to disconnect the wire to get the starter to quit. With the wire disconnected and feeding 12v to the S post, starter engages/disengages as it should.

I can try this but the only point that it gets voltage is after the key has been put into the start position or the solenoid is jumped even when the key is off, before and after replacing the ignition switch (although I didn't test it while it was disconnected all together). The rest of the time it's dead.

I'm headed into my 4th year of ownership. I have had problems getting spark in cold temperatures and/or have had trouble with not getting spark until the key was returned to the run position (once that I know of). I've never really bothered to try and get the no spark issue fixed because my defrost dampers are also in need of repair among other things so I just don't drive it when it's cold out (I'd like to fix it but have other projects ahead of it). As long as the temps are above 40~ish it's been dead reliable, and still starts/runs just fine provided I disconnect the wire going to the S terminal so the starter can disengage.


I haven't checked the I post, I stopped at the point the S post was constantly receiving 12v. I do have continuity between the I and S through the wiring harness and don't think I should, I'm not measuring it through the solenoid.

Yes. Typically the truck sits in my driveway but it was parked down the road in a limp wristed attempt to sell it. I drove it down there, pulled the battery and there it has sat. Rodents are present and very much a possibility here although I haven't seen any evidence to suggest that as a cause there are mice around here. Frankly I think the truck is just pissed at me for putting a van where it used to be parked (just for the motor, different project).

 

To find your cab ground, check around your transmission-to-block bolts. It's usually an uninsulated flat braided cable running from a transmission bolt to a self-tapping bolt on the firewall.

If your negative battery cable goes straight to your engine block, then you need a ground from the block to the cab.

So when the starter is running away on you, pulling the wire off of the "S" post will stop it?

And there should be a wire connected to your "I" post I believe. The "I" post sends 12V+ through that wire to the ignition, so that the coil will get 12 volts while cranking the starter. Once the key is turned back to Run, the coil receives an attenuated voltage via the resistor wire.

I wondered about rodents, the SOBs...

And I can tell you that one time I put a DuraSpark II on my old '68 Cougar. I thought I could be lazy and source my 12V+, while cranking, straight from the "S" post. That got me a starter run-on condition as well. After wiring it to the "I" post, it worked perfectly. (Might've needed a new solenoid, as the original might've not had an "I" post. Some of the older ones didn't.)

 

I will check these to make sure since I don't know the answer currently.

Yes.

There is a wire connected to I and a different wire connected to S (although now I'm questioning if they're separate, maybe I just assumed they were). These wires have continuity between them when disconnected from the starter solenoid. Should they? It has been my understanding that I would only get signal through the solenoid AFTER the S is provided 12v (in fact you made mention of this earlier). I have not thought to check for voltage on that wire on it's own to see if it's getting voltage from anywhere else but I will check tonight.

Small price to pay to live in a rural community I guess :/.

Unfortunately I don't have any of the background on how this generation of DS ended up in this truck (turns out I do, it's the stock one and I'm an idiot), my understanding is it should have a different module than it does being a 75 but unless I'm color blind it's most certainly green. That doesn't mean somebody didn't do something wrong but it would be pretty strange for it to have worked 3+ years by accident haha. Going to take this one step at a time and sort out the issues as they come. Right now I think my objective is to figure out why I'm getting signal when there shouldn't be.

Edit: Amended my previous statements about being DSII because I was totally wrong and have no idea how or why I thought this.

 

Glad I came back to your answers before I posted my long-winded dissertation on testing ignition switch connectors and such.
Got a couple of questions/answers/comments first.

Easy enough to do luckily. Most of the time there is a very visible steel braided wire between the engine and a visible point on the firewall. Doesn't hurt to really hang over the fender and take a deeper look, but with any luck you'll see one near the top center of the firewall.
While you're at all this looking-for-grounds stuff (which we should all do with old trucks anyway) you can check for an additional one coming off of the battery negative directly to the body/fender/wheel well near the starter relay or battery bolt-down bracket.
If you don't have one, it never hurts to add one. Ford often integrated it into their main ground cable in the form of a clamp to the body, but that always gets removed when changing to an aftermarket battery cable. It's a rare original cable that's still in service after all this time.
Besides, as we like to say, you can't have too many grounds. And with battery cables, "overkill is just enough" too!

You're correct. Completely separate circuits, and both should always be separate except and only during START.

Now that's a big no-no! No continuity in any way that I can think of. Any electrical experts out there know if a meter will mistake this due to some common ground (not literally) with the ignition switch?
The Brown "I" wire has only it's connection to the starter relay and the engine connector at the back of the engine where the Red w/green wire comes out and goes to the coil positive.
The Red w/blue "S" wire is only connected to the starter relay and the ignition switch, with a break at the neutral safety switch.
So this is a big clue that something is amiss. Test it again with the ignition switch disconnected and you may have your answer.

The "I" wire sill show 12v (or slightly less) any time the key is in the RUN position because it's getting feedback (inaccurate description, but works for us in this case) from the ignition coil's positive (resistor) wire. It "pushes" voltage from the starter relay to the ignition wire when in START, but still shows voltage at the wire when the key is in RUN simply because it's hard wired to the ignition circuit. But you should never see any voltage at all at the "I" post on the relay unless the relay is energized.

Yep, still have some work to do. But you're narrowing it down. Another member is going through the same process right now, but for the opposite reason. He doesn't get any power to the ignition module. Still has to put on his Sherlock Holmes hat and pipe though, just like you do.
Maybe yours was an early '75 Federal truck and did not have the Duraspark ignition? I thought all '75's did, but I believe I found here on the forums that some did not. Unless that was for '74 and I'm not remembering it correctly.
But if that's the case, then maybe someone simply added it later, using the most common (and at the time, least expensive) module and a home made, or store bought harness. Might be as simple as that, or they converted the old one to the newer one. Either way though, like you said, it worked for this long. Be odd if that was just a fluke. But you definitely have an issue.

Good luck!

Paul

 

Oh, and I meant to ask too, what starter type you have? This is not likely related to your particular issue, but many people have starter run-on issues when installing a modern PMGR ("mini") starter on their engines.
So is yours the standard motor style, or is it one with it's own solenoid mounted piggy-back on the starter motor?

Thanks

Paul

 

Perhaps another clue, I have had a little trouble with the NSS in the past and have neglected to mention it. Nothing I considered serious in that a little wiggle of the shifter would clear it up, but another thing I haven't really inspected very closely. Does the NSS only open/close the circuit to the S terminal? I'll have a look at it in any case.


Excellent.



That may be. I was told that it had a 390 swapped in from a passenger car of some variety (I've never bothered to verify because it doesn't effect my usage but some evidence to support it), perhaps they changed that out at the same time.

Looks like a stock style starter to me, not the mini starter. I am so far relieved that nobody has brought up the specter of the starter itself being a possible culprit since the truck also has headers.

Thank you both for your insights on this. Will do some follow-up with your recommendations and have more information to share in the near future, although it might not be until next week. Pretty busy schedule between now and the end of the weekend, but with any luck I'll have it back in my own driveway pretty soon.

 

I just bought a 74 250 that had no battery or solenoid. When I started figuring out what wires go where I had the same continous starter issue. Mine came from a wire off the positove of the coil going to the solenoid. I had put it to the same terminal as the ignition. Put the key to run and it energised the solenoid. I ended up just pitting it on the other terminal of the solenoid.

Not sure if that's where it goes, but thats where it is now

 

That makes sense. Mine started doing this after 3+ years of reliable service. Just about have the space freed up in my driveway to get it back home so I can really tear into it, probably won't be until next week(end) though.

 

The suspense will be bugging me. My top three culprits are:

#1. The ignition switch, my favorite candidate.

#2. Possibly the DS II box - it's the only other component I can think of which has access to both the start and run circuitry.

There's also the DS II specific wiring, which if it was added by a P.O. might be a problem. But still, why would it have worked for the years leading up to this...

Lastly and leastly at #3, is the fender-mounted starter relay. But if you say that pulling the start wire from the "S" post stops the starter from running on, then that probably rules out the ol' solenoid.

 

The issue was present before/after the replacement of this switch but I know better than to rule it out. Just so happens I have a second BNIB switch (non OEM) I can throw in, but also want to run the same test as before without it attached to see if that does anything different.

I wonder if I could disconnect the box and perform the test I have been just to see if the voltage remains on the S wire. What puzzles me is how the voltage remains even after the key is shut off, and how it doesn't return until after the battery is reconnected.

The solenoid is also a new Motorcraft unit, read enough bad reviews of the crappy aftermarket versions I tried to avoid all that.

Honestly this has been weighing on my mind as well. The only reason I haven't wanted to work on it is I don't want to disable a 3rd vehicle in my driveway in the event I end up having to get into the harness to look for a short and things get out of hand (I'm famous for this) but I can still check many of the things on this list without going ham, you know probably. Might give it a look tomorrow.

 

Not totally sure, but I think some parts stores might be able to test the DS box, although it would probably be an old-time independent store, and I would want to find a store clerk who knows what they're doing.
You might even be able to find specs online, which enable you to do some rudimentary testing on the box with a basic multimeter.

Oh yeah, and the body ground! Look for that first, and if it's missing, then that might be the whole problem. In the absence of a suitable cable, you can use some jumper cables between the battery negative and the cab sheetmetal, to test before going and buying a proper cable.

 

Small update on this. I haven't actually checked anything but needed the truck this weekend for a time sensitive matter (surprise gift from my girlfriends kids for Mom's day). I disconnected the S wire from the solenoid, turned the key on, touched it to the post and got the truck started. Once it was running I disconnected the I terminal and left them off, just used the key to kill the truck. Kind of unorthodox and slightly more difficult to get started at times but it worked just fine. So I got that goin for me, which is nice.

Be a little while before I can properly troubleshoot the matter. Vacation next week, short week this week, still no vacant space in the driveway. Was a lot busier this weekend then I expected to be.

 

Thanks for the update. Such as it is anyway.
The cool part is that you're driving it again. The un-cool part is that eventually you'll get tired of the new routine and dig back into it.
Been there, and definitely done that!

Good luck.

Paul