'79 F250 SC 4x4, 400 auto.
Issue I'm having is inconsistent starting. Seems that when the switch is mounted in the dash it only tries to catch as I let off the start position. Take the switch out and let it hang and it actually starts.

So would my problem be more likely in the harness or in the rotation part of the switch.?

I'm scratching my head on this one...

 

I'd start by replacing the switch they're cheap and easy to get.

 

So I finally had an afternoon to go put the new switch in, I hooked up the batt, plugged in the new switch and it popped right off and idled. Didn't seem to want any accelerator so I think the carb needs work too but when I mounted the new switch in the dash it was right back to the not quite catching thing.

 

Sounds like a problem in the connector

 

Update: new switch, carb is rebuilt, new pickup coil in dist, new icm & it runs. Still gotta put the switch back in the dash as it got late & cold working on it, so hopefully the weird issue is fixed with pickup coil.. vac adv is shot so I'll swap that over the weekend hopefully.
Even with the vac adv not working the truck felt stronger when I backed it out & turned it around.

making progress, I do plan to plug the old icm in to test it out, it was popping some so i just threw it on to see if it'd clean up but may've just been timing since we jad the dist out.

 

When the ignitions were just using points instead of electronics, this issue of starting only after you let the key back to ON from START was two things failing.
One was the ignition switch, were the power to the ignition coil was interrupted by turning the key to the START position AND the Brown wire on the "I" terminal of the starter relay/solenoid no longer doing it's job.

When the switch fails like that, the I wire is kind of a backup plan. Giving power to the coil while the starter is cranking.
When the ignition fails to spark while in START both of the circuits are compromised.
With the newer ignitions there are also circuits to the ICM that are part of the equation, so that means more contact points in the ignition switch and more wires involved, with more connections added to the mix to become possible failure points after this many years.

So it's not always the switch, but seems to be most of the time.
Now, if you check for voltage at the Brown wire and the I terminal of the relay (separate tests) you can narrow it down to find out if it's the circuit to the coil or the circuit to the ICM that's causing the mischief.

Paul

 

Thank you for the explanation. Are those tests both performed under cranking?

 

Yes, but while you're at it, might as well check a couple other things at the same time.
Measure voltage at the battery just as a point of reference. Fully charged should be 12.6v or so, but anything over 12 is good for now. This is not critical, but it's something I do whenever I'm chasing electrical gremlins in case it helps me find an excessive voltage drop/loss along the way.

With the key ON, pull the Brown "I" wire off of the starter relay. Check for voltage at the wire.
Normally there would be between 6v and full battery voltage, depending on other conditions. That's because the Brown wire shares a connection/splice with the Red w/green wire going to the ignition coil.
In your case, we're sure you have some voltage on the ignition coil wire because the engine runs when you release the key back into the ON/RUN position.
If no voltage on the Brown wire then the connection has been broken somewhere between the connector at the back of the engine and the starter relay. That's test one.

Test two is to disconnect the Red w/blue stripe "S" wire on the starter relay so that the starter won't crank when you turn the key to START.
Have a helper hold the key in START and re-check for voltage on the Brown wire. If it is as we suspect, the voltage will go away. If it does you have an issue with your ignition switch, at least as far as the ignition coil is concerned.

Re-connect the Red w/blue S wire for test three. Leave the Brown wire disconnected.
With your light or meter on the "I" post of the relay, have the helper turn the key to START again. Be aware that your starter will crank at this point, so keep hands, shirt sleeves, beards, pony tails and other appendages out of the way of spinny thingies! If you don't want this to happen you can leave the battery connected but disconnect the starter cable.
With the key in START and the starter cranking, or at least the relay energized, you should now see battery voltage on the "I" post of the starter relay. If you do, then all is well with the relay.
If you don't, then the relay is bad as well.

You can do some of these tests over on the ICM's wires as well, but that's fodder for another discussion.

It's late, but I think I got all that correct. I'll re-check in the morning to be sure, in case nobody else has found fault with those tests first. We're just trying to determine if BOTH the ignition switch and the starter relay have failed in their duties.
If neither one fails the tests, then it's probably on that other circuit between the ignition switch and the ICM. But at some point, the wires may share a common source, so I'll need to check some diagrams to determine that.

Good luck. And have fun!

Paul