While at a photo stop on Rt66 this year my 72 mostly stock 4-banger would not crank. Battery is fully charged, alternator light goes out when running, starter, alternator and battery each have 15K miles since new. Headlights do not dim when attempting to crank. FLAPS 12V "hot start" relay installed. No idea how old the ignition switch is.
I push started and it fired right up. It cranked with the key after lunch and when stopping for fuel on the way back to Idyllwild. The next morning it would not crank. I checked and wiggled the wires on the starter and relay, tried it again and it fired right up. Started no problem 3 more times on the way home. I tried a restart after shutting it off in my driveway and nothing. Came out the next morning and it fired up.
I bought a proper Ford starter relay which I will install using Mark Henry's procedure but I'm not sure that will fix the problem. Once or twice while starting it seemed like the starter didn't engage until the key was turned a bit further than normal but I could just be imagining that. I have a new ignition switch I could install but I would really like to isolate the problem first. And I don't want to fiddle with it at WCR.
Any ideas on what/how to troubleshoot before I start throwing new parts at it?
TIA

Get a volt meter and connect it to the yellow wire going to your "Flaps Hot Start Relay". This should be the wire coming from the ignition switch.
Then you have to try starting it until it acts up and you can see if you are getting voltage out of the ignition switch.

If you find it fails to crank but you are getting voltage to the hot start relay hook the it is the Hot Start relay or the wiring or started down stream of the hot start relay.

If it fails to crank and you don't get power to the hot start relay it is the ignition switch or the wiring upstream of the hot start relay.

You can then move your volt meter upstream or down stream to the next component in the circuit and test again until you identify for sure what it is.

If you replace any part in the system with an intermittent problem you run the rick of the appearance of fixing the system when in reality it is just not failing after your replacement. Which based on my experience means it will fail at a very inopportune time (like on the way to the airport or out in the middle of the mountains or desert)

Good luck
Jim

You should always start with cleaning all grounds, but the reason I don't like the relay is I've seen the exact issue you're having. Switching to the solenoid cured it.

Another thing that could be happening is your ignition switch is falling apart, just holding on by a thread before it craps.

I had a crack Ignition Switch that worked sometimes but not always, and was not going to heal it's self. Actually the instructions in Pelican's Tech pages makes it easy to replace the Ignition Switch if followed.

Same here, Michael, replaced the ignition switch ... the old one looked to have some burn marks on it, as well as cracks in the plastic.

I was able to remove the 'hot-start' relay after installing the new switch.

I assume that when you discovered wiggling the wire conn's at the solenoid you repeated this when it would not crank in the drive; yes? If so, I'd throw a starter switch at it. That Ford solenoid will pay for itself. Kent

I did not have to wiggle the wires during the drive home...it cranked as it should every time.
I said I had a Ford relay but it is actually a Ford solenoid as described in Mark's write-up. Hopefully I can get the damn thing to act up while I'm troubleshooting and before I leave for WCR...otherwise I'll throw all my new parts at it and buy a AAA membership before WCR.

This is probably not what you have going on, but I had a starter problem on a Chevy many years ago where every once in a while it would not start. It turned out to be a flaky or open winding on one of the commutator contacts. So it was like a roulette wheel and sometimes the motor stopped on the bad contact. Tapping the starter would usually be enough to get it to contact again. If everything else fails it would be good to go through that starter and solenoid and ensure the contacts, brushes and commutator check out ok. Or replace it with a known good one to rule it out.

Don't forget the ground strap on the transmission. It grounds the entire engine/transmission to the body. I had this issue. An easy check and fix.