Hey guys, first off, I own a '79 F150 Ranger w/351m. It's a great truck, and I love it dearly. I've done all the work on it myself since I've owned it. I've run into a problem that has stumped me and several mechanics. Here's the problem:

When I start the engine, it fires up great, but the starter continues engaging after the engine is running. The only maintenance I did barely before this problem occurred is clean some of the wiring harness in the engine bay and repaint the driver side wheel well (where the coffee can sits).
I've replaced many parts and disconnected many wires trying to solve or at least pinpoint this problem. Here they are in chronological order:
1) Swapped to a 302 solenoid to see if it was a bad solenoid. Truck started right up and starter continued to engage. Swapped back original solenoid.
2) Installed a new solenoid just to double check. Same engaging problem. Returned new solenoid.
3) Checked several wires connected to the solenoid and swapped them places (one by one of course). Problem continues to exist. Put all wires back in original spots.
4) Installed brand new starter. Problem continues to exist. Sent the starter back and got another new starter for peace of mind. Problem continues. Sent second new starter back and installed a THIRD new one. Problem is STILL there. Definitely not a starter problem, lol. New one is still installed.
5) Installed brand new ignition switch. Problem continues.
6) Installed new ignition tumbler. Problem is not as bad, but still continues. If you lightly bump the key to the "start" position, it'll fire up without the starter continuing to engage, although if you hold the key on "start" for more than a second, it'll continue to engage.
6.5) Reinstalled the relatively new ignition switch and sent brand new one back.
7) Upgraded ground cable from the intake manifold to the firewall.
8) Rechecked wiring from ignition switch to the solenoid and all wires are clean/good. All insulation checks out good.
9) Checked all fuses powered by the battery. All checked out good.
10) Replaced all wiring to the neutral safety switch.
11) Reinstalled brand new ignition switch.
12) Gone nuts. Absolutely nuts.

 

Do you suppose the starter gear may be to close to the ring gear causing it to bind enough that it can't return. Seems you have eliminated everything else. Good luck and welcome to the site.

 

No, I don't believe it has anything to do with the gears. It's strictly power, and I have proof of that:
While the starter was constantly engaging with the engine running, I unplugged the red 12 Ga wire from the neutral safety switch on the solenoid and it stopped engaging. As soon as I put the wire back on, the starter did not engage again.

Is it a possibility that the wiring inside the neutral safety switch could be corroded and is causing a short? It's $75 for a NSS :/

 

NSS switch can be carefully taken apart for cleaning and new grease. Just need to bend up some tabs and drill out a couple small rivets with 3/32 bit. I recently had mine apart due to backup lights not working. It was beyond repair so I completely eliminated it and put in a toggle switch for the reverse lights. You should try turning over the engine by shorting out solenoid with a screw driver to see what happens. Might give you some new ideas. I'll think some more on it today.

 

Try pulling the ignition start wire off the soleniod after it starts. See it the starter stops running.

 

The starter stayed engaged

 

Are you sure you don't have the start wire and the ignition wire crossed at the solenoid ?

 

You could be on to something.