Hey everyone, first time poster, long time viewer. The problem I've been having with my 1978 Ford F-150 400 is this: my ignition switch pretty much broke in half so I disconnected the wiring harness, bought a new switch, and reinstalled the harness without switching any wiring around. The harness matched the new switch identically and plugged in with no problem, but as soon as I turn the key right to the on position, the starter begins to run. I replaced the starter solenoid just in case, but I'm still having the same problem. I get power in the acc position no problem, and I can turn the key to the on position and get power if the "S" post on the solenoid is disconnected, without the starter firing of course, but no way to start the truck that way unless I bridge the + and the S post with a screwdriver. I'm not really sure what I should try, I'm more knowledgeable about the mechanical side of trucks than I am about the electrical systems sadly so replacing parts is easy, but wiring them is not. Any help is appreciated!

 

Welcome to FTE

1973/79 F100/350: There are THREE different ignition switches, did you get the correct one?

1973/76 is one type (6 blade terminals), 1977 is another (6 blade terminals with 2 locking bars).

D8TZ-11572-A .. Ignition Switch (Motorcraft SW-1494) / Marked: D8TB-11572-AA / Obsolete ~ Available NOS & from auto parts stores.

1978/79 F100/350, Bronco & Econoline.

 

Did the old switch do the same thing ? If not, the new switch may be defective.

 

This is the one I bought, since it was identical to the one it replaced: http://m.autozone.com/ignition-tune-up-and-routine-maintenance/ignition-switch/duralast-ignition-switch/ford/f150-1-2-ton-p-u-2wd/1978/8-cylinders-s-6-6l-2bl-ohv/146035_0_0/?location=

I know auto zone isn't good for much of anything, but my grandpa told me where he got the ignition from and I bought the same one.

 

The old switch didn't do the same thing, so that's a possibility. I'll take it back to have them replace it and see if a new one does the same thing.

 

Went back to autozone, exchanged the switch for a new one and I still have the same problem. Gonna look into getting the motorcraft switch D8TZ-11572-A, but I noticed this while I was looking at the wire going to the "S" post, looks a bit shot. Think this could be causing my symptoms? I'm gonna replace it regardless of course.



This is the wire that goes to the S post on the solenoid.

 

Certainly won't hurt to fix that up.

Take a look at this post, has some good info on the wiring. https://www.ford-trucks.com/forums/1...-starting.html

 

Thank you Kegger, that will be helpful for sure. I was able to get the motorcraft ignition switch and installed it, but the problem still didn't go, so I'm gonna rule out ignition switch. The truck runs perfectly fine when I get it running, so I'm guessing it's just a wiring issue somewhere from the switch to the solenoid. Thanks for the suggestions so far guys, I'll get this hammered eventually.