Total electrical power loss

[Millersir]

I know this issue is next to impossible to troubleshoot via the Internet but I'm at my wit's end. I installed a new intake manifold and carburetor Saturday. I let it sit until last night around 6pm to test out the new upgrades. The truck fired right up, sounded great, no leaks. I took a spin around the block 3 times (2miles) shifting up and down through the gears either no issues. I get back to my driveway, down shift into first to pull up the small hill that is my driveway and boom, everything died. No lights no spark, nothing.

I thought perhaps that I had a loose battery negative as I disconnected the neg to work on the truck Saturday. Ground connection is good. I can get the truck to crank if I us a jumper wire from the pos on the battery to the S post on the starter relay. My battery is reading 12.67V. My fog lights work because they are directly connected to the battery and a toggle switch.

I had a similar issue a few months ago that ended up being a fuse link. To fix the issue a few months ago I just cut out the fuse link so I don't think that can be the problem again.

When I go to get readings from my fuse box in the glove box(neg lead from meter to bare metal, POS lead to metal on each fuse, I'm getting 0.36v which I don't know if thats how it should be but that seems really low for a 12v system...

Is there anyone that's seem a similar issue? It's acting like a fuse link is blown, which I already cut out a few months ago. Trucks been driving perfectly fine for months without it. Thanks for any insights that you guys can lend!

Not sure if it matters but I have a 71 wagon with a 5.0L from an 86 mustang GT. Edelbrock performer 289 intake and 1406 carb is what was installed Saturday.

 

[Steve83]

If something caused the fusible link to burn, and you only repaired the link withOUT fixing the electrical problem: yes, I'd assume the link burned again.

Find the problem & fix it. The link is burning to keep the truck from burning.

 

[Millersir]

I cut out the fuse link off the positive side of the starter relay a few months ago. How can I tell if this is the same issue if there is no fuse link off the positive side of the starter relay?

Any help is appreciated.

 

[Rustytruck]

Turn the key on and check for power on long stud on the back of the ignition switch.

Check the wires under the dimmer switch. The power there is live all the time.

If you have a later model Bronco the wire that use to be your fuse able link has a plug in connector with a couple of other wires in it close to the firewall in the engine compartment. Check that plug if you have it.

Bad news for bypassing your fuse able link. If you burn out a fuse able link and then bypass it, then there is still a short or malfunction strong enough to burn down your whole truck. Its just going to move to another weak spot and hope it isnt your wiring harness under the dash.

 

[Millersir]

Well. Who wants a bronco? I'm done with it.

 

[Millersir]

I got the truck running by cutting back the black fuse link wire then soldering in an in line 15amp fuse. Now when I turn the truck off the 15amp fuse blows. What could be causing this? Anyone see this before?

 

[Rustytruck]

Check the wiring circuits for anything that is live when the key is off.

Headlights are live, High beam switch is live, horn is live, flashers are live, brake lights are live, alternator is live. Maybe cigarette lighter. And who knows what else that may have been added by the owners.

 

[Brent13]

Electrical problems are not fun. I had them all the time during the first 6 months of ownership. I said screw it, and ordered a Painless harness. Replaced the old factory harness, and haven't had a problem since. If this is the first electrical issue you have encountered, maybe chase it down and fix it. If this electrical issue is one of many, you may consider a new harness. My factory harness was so hacked up, no way I was comfortable sticking with it!

 

[DirtDonk]

Your fuse should be more in the 60 amp range Millersir. That wire handles all the current that the truck needs except for the starter, and also handles the full charging capability of the alternator to charge the battery.

So if your alternator is an upgraded one, it might just be too powerful for that wire and needs to be handled differently.
Do you know what alternator it's got? Is it the way you bought it, or have you modified it?

Do you remember the circumstances under which the original link burned out?
I'm not sure why the fuse would only blow when you turn the key off yet.

Paul

 

[Millersir]

Thanks for the suggestions fellas. I ended up just putting a 30a fuse in the in-line fuse wire I installed. I know it's not the right thing to do, but it's at least getting me to work until I can chase down the underlying cause.

When I got the truck I did install a new alternator, but I'm not sure I know how to tell if it's 3g or whatever. I got it at AutoZone for like $35 plus core. It's a duralast, if that helps.

 

[Millersir]

Originally the link Burt out after I installed the new alternator, voltage regulator, and battery. It sat for 3 years before I started using it as my daily driver around 3 months ago.

 

[DirtDonk]

...When I got the truck I did install a new alternator, but I'm not sure I know how to tell if it's 3g or whatever. I got it at AutoZone for like $35 plus core. It's a duralast, if that helps.

Sorry, not really. Duralast is the brand, so it could be any model under that name.
But the price indicates perhaps just a stock replacement. And if you went in and asked for a replacement unit for your year, you would normally have received a 1G of up to about 65 amps output capacity.
A picture would tell many tales though. So if you have a chance, that would be a good next step.

Paul

 

[swc1293]

I may have the same issue. I took my 77 for a ride yesterday and everything worked fine.
I tried starting it later in the day, no luck, no power to anything. Battery is fully charged. Does anyone have a wiring diagram that may help with this issue. I bought a shop manual on cd but it does no have the diagram I need. Any help would be greatly appreciated.
Thanks

 

[Steve83]

Click this & read the caption:


https://www.supermotors.net/registry/media/879816 (for phone apps)

 

[swc1293]

Thanks Steve83...I appreciate it!

 

[DirtDonk]

Steve's got the most up-to-date diagrams I've seen around. Should be able to help you there.
But to narrow it down, total power loss can only come from three (well four) places:

1. Battery cable(s)
2. Fusible link on Black (sometimes Black w/red stripe) wire at the starter relay/solenoid.
3. Battery itself.
(#2.5 the bullet connector under the dash on that same Black wire)

The fact that you can measure power at the battery is a good thing to start with, but you can't count on it 100% either. How old is the battery?
A battery can have 12v and still not have the current to crank an engine. However, that same battery will almost always be able to turn on the radio or headlights, so you can usually go back to #'s 1 and 2 pretty quickly.

The Black wire's fusible link (a short section of wire and the connector at the battery side of the starter relay) is designed to self-destruct if it's been over stressed.
If you were messing about at all under the dash, that same wire has a connector adjacent to the back of the ammeter in the cluster that can pull apart. Or after years of service can get so rusted up that it just stops functioning. Usually that manifests itself in other issues prior to failure, but it's still something you can check by pulling the connector apart and checking/cleaning the contact points before putting it back together.
This Black and Black w/yellow wire is virtually one continuous wire from the back of the alternator, into the cabin and through the harness, to the fuse panel and all points needed, then back out to the battery at the starter relay.
It literally powers the whole truck and charges the battery while the engine is running, and powers everything by the battery in turn when the engine is off.
Obviously then, if anything happens to that wire you lose it all. Same for the negative battery cable especially, but also the positive.

If the positive cable has failed, this could explain why it worked at one point and you were able to keep driving (engine running from the alternator and ground cable) but when you tried to re-start got nothing.

Hard to tell when a battery cable is bad unfortunately. Sometimes you just have to replace them as your "testing method" because even relatively new ones can fail internally.

Good luck.

Paul