Ignition Problem?
 

So my truck will start fine but as soon as I let off of the key and it returns to the "ON" position it immediately dies. It will stay running so long as I hold the the key on the "START" position.

'74 Ford F-100 302 points style ignition.

Replace the ignition switch.

Check for voltage at + side of coil with key in the "RUN" position to rule out a bad resistor wire.

Excellent advice from both people. Do the check that MH describes first, and if it's fine, suspect the ignition switch as 77393 points out.

On a points setup, the voltage you see at the coil positive terminal with the key in RUN depends on how the points landed the last time the engine shut off. If they are OPEN, you will see the battery voltage. If they are CLOSED, you will see about half that.

So I took both of your advice. 12v on the + wire of the coil with the key in the "Run" position. So I replaced the ignition switch and still having the same issue. What else can I try?

OK, didn't see that coming. So the issue is that the truck cranks over and starts with the key in START, but as soon as the key springs back to RUN, the engine shuts off.

This is points ignition; the only place to look for power is the positive terminal of the coil. You verified the circuit is not interrupted with the key in RUN.

Something is just not adding up. Are you sure that the action of returning the key to RUN is what actually kills the engine? The motor may be starting, but just can't stay running because the choke isn't closing - so maybe it just happens to be stalling about the time you release the key. Are you having to coax it to get it started in the first place? Is it actually starting, or just kicking over and coughing such that it "acts" like it's starting? I highly suspect that something along these lines is what's really going on. This would make the problem NOT an ignition issue as was originally described to us.

If you're 100% confident that the truck really is starting, and it's solely the act of releasing the key from START to RUN that shuts it off like a light switch, then about the only thing you have left to check is the ballast resistance. If the ballast resistance is too high because the resistor wire has deteriorated somehow (I'm reaching here), it will still show 12 volts at the coil with the points OPEN, but not allow enough current to flow through the primary to create a spark in RUN. You could also do an old school spark test by manually working the points and checking for spark, but if the truck is really starting with the key in START, then that test wouldn't give you any new information. The only difference in the ignition system between START and RUN is how the coil gets powered - everything else downstream is the same.

I thought for sure that would have fixed it. Follow FMC400 on his advise to get it running, he is pretty sharp on these trucks.

I am absolutely sure this is the problem. I held the for about 6-7 seconds to "start" and it starts and runs without a problem but as soon as it springs back to "run" it dies immediately. Ballast resistance? Isn't that one of those ceramic things with the spring thing in it or does my truck even have one of those? If so, where is it located?

Got it. So is it safe to assume the starter is grinding for those 6 to 7 seconds the engine runs with the key in START?

On to trying to solve this problem. The ballast resistor is a long length of wire bundled in the dash harness. When the key is placed in START, full battery voltage is applied to the ignition coil. When the key is returned to RUN, the coil is powered through the ballast resistor wire instead. Between the lowered battery voltage during cranking (key in START), and the voltage drop of the ballast resistor with the starter let off and battery voltage back up (key in RUN), the voltage at the coil stays relatively constant regardless of whether the key is in START or RUN.

You can effectively measure the ballast resistance by looking at the voltage divider at the coil. Turn the key to RUN and verify that you still have 12 volts at the coil. Now manually force the points closed (or short across them). The voltage at the coil should drop to about 6 volts. Just be careful when you let them open (you may get a spark). This indirectly measures the resistance of the ballast without having to probe at both ends with your multimeter.

I'm going to be honest, I have no idea what you just said. What is the voltage divider? How do I force the points closed or short them across? You want to measure what with my multimeter? Sorry I'm an idiot.

You didn't happen to change the coil did you? A temp jumper from battery to + side of coil just to see if it runs will narrow it down.

The temp jumper had no effect and no I haven't changed the coil.

Would it be too big a stretch to look at a ground that is closed through the starter but when starter is kicked out the ground breaks and the ignition circuit opens?

Not sure if its possible on that model, but you did replace the ignition switch and not just the tumbler where the key goes in, right?

I have drawn a very simple ignition diagram to help you understand what is happening in your ignition system. As you stated that the truck runs in the start position. This means that the ignition system is working ok, so the problem has to be in the ignition switch or the resistance wire to the coil. (It Can’t Be Anything Else) as you can see by the diagram the only difference between run and start is the resistance wire. You stated that you put in a new ignition switch, so that eliminates the switch. This only leaves the resistance wire as the culprit. You stated that you ran a jumper from the battery straight to the coil and it still didn’t run. You must have mistakenly put the jumper on wrong because this is an impossibility. If it runs in the start position then it would have to run with the coil connected directly to 12 volts. I hope this helps. Good luck solving your problem. I spent nearly 50 years in the automotive electrical industry and believe me I have seen some weird and perplexing problems.