70 Bronco Wont Start After Warm Up

[tonytony9]

That’s a good call to move the relay. And doing so will most likely necessitate at least some new battery cables.
Never hurts to renew, and upsize the battery and starter cables.

Some tests you can run, just in case it is still doing this when you get back on the road, driving it in the heat.
Carry some cool water with you and the next time it won’t start, dowse the starter relay. Give it a moment to cool, then try to start again. If that fixes the problem, then perhaps you need a more robust relay.
If that doesn’t fix it, then move on up the chain to see what else might be causing the issue.
Something between the ignition switch and relay. Perhaps one of the wires is compromised.
Follow the red with blue stripe wire as far as you can and see if it has any kinks, tears, cuts, or otherwise compromised points.

The water trick works like a charm. Thank you, this has helped me a few times in a pinch the last few days.

I am going to start with the relay and the cables. And continue down the line if necessary.

Can you clarify what you mean by a more robust relay? Are there specific relays you would recommend? I picked up a Motorcraft at the autoparts store for around $40.

 

[tonytony9]

Oh, and another thing…
Since you have a 70, that presumably has not been rewired(?) you need to go through and do some general maintenance that is not normally required on modern vehicles with modern connections and fuse panels.
Separate the two rectangular firewall connectors with all the wires coming and going, and make sure that all the connections are clean, tight and rust free.
Apply some dielectric grease and reinstall them.
Next go to the fuse panel and remove all five fuses. Clean them up, and reinstall them.
If the fuse contact points need cleaning, do that too as well.
Glass fuses build a crust of oxidation that, over time will reduce the ability of the electrons to pass through.

Awesome advice, yes, original wiring. I will do this today.

As a longer term project, I was considering buying a painless wire harness and redoing all the wiring. Yay or Nay for this idea?

 

[gr8scott]

Yay or Nay for this idea?

Yay all the way. New wiring is one of the best things you can do for your Bronco.

 

[m_m70]

As a longer term project, I was considering buying a painless wire harness and redoing all the wiring. Yay or Nay for this idea?

Yes

 

[DirtDonk]

The water trick works like a charm. Thank you, this has helped me a few times in a pinch the last few days.

Nice! Now you’ve narrowed it down and can move forward.

Can you clarify what you mean by a more robust relay? Are there specific relays you would recommend? I picked up a Motorcraft at the autoparts store for around $40.

As you suspected, I simply meant better quality. There’s some pretty cheap crap out there these days.
You may have done that already by choosing Motorcraft over the $10 no-name brand house brand specials. Many of those have lifetime warranties, but that warranty doesn’t do you any good when you’re stranded in the middle of nowhere, half a state away from the nearest store that can warranty it for you.

While it is possible that your relay started out wimpy, it’s also possible that it just got old earlier than it’s normal time, due to the heat.
For a specific brand, I happen to like the not very reasonably priced Volvo/Penta marine rated versions. They’re probably 80 or 90 bucks apiece by now. Maybe more!
But a Marine rating usually gives a part a little leg up on the competition in the longevity in the saddle department.

 

[73azbronco]

check the battery as well, low voltage can cause the relay to not work. How old is the battery? They are usually lasting less than 3 years, one is not unheard of.