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Hello all. Recently I picked up a Tuff Stuff 7078 3 wire 70 amp alternator for my 1968 Mustang 289 V8 Coupe. After reading through many threads there seems to be somewhat of a gray area of what to do with the battery wire. Some people say the stock wiring harness should be thick enough to support the 70 amps. Some say that the battery wire should be upgraded to something thicker. The instructions provided with the Stuff Stuff alternator recommend a thicker battery wire.

I have a thicker wire to run for the alternator battery post. My question is how do I run it? The 3 prong stock connector seems to have the battery wire in the center post. Would I just not plug this in and run the thicker wire straight to the battery? If I do this, would it be bypassing the regulator? I was trying to trace where the battery wire went on a schematic but it seems is routes back to the ignition. My worry is that I'll send to many amps to the wrong place and burn something up. Do I leave the existing battery wire from the 3 prong connector AND add the beefier wire on the same alternator battery post?

Happy to provide any information to help me get this sorted. Thank you.

Alternator being used:
 

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This seems like a good thread for @Yadkin to chime in on. Hang-on for a bit, and let’s see what happens.
 
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The original alternator was probably 40 amp at best, maybe less, so yeah you are creating the potential for a problem. Run the battery wire to the regulator in the stock fashion but put a 40 amp resettable breaker in line, and in an accessible area.
 
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The original alternator was probably 40 amp at best, maybe less, so yeah you are creating the potential for a problem. Run the battery wire to the regulator in the stock fashion but put a 40 amp resettable breaker in line, and in an accessible area.
What is confusing is that there is this three prong loom that connects into the alternator. From the connector I think this goes to the regulator (please correct me if I am wrong). The regulator has 4 pins and I am not sure how I'd wire into that. From there I'd have to go back to the battery with another thicker gauge wire? Online threads make it seem like they go straight from the alternator to the battery.

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The 4th pin on the regulator is to a "dummy" light on the dash that you probably don't have.

Regarding the harness, there are lots of wiring diagrams for this car online so just search for one. The battery terminal is the large wire and carries current, so that should be fused (with a resettable breaker). Best to do it in that harness because it splits into three soon after the connector.

The only reason to go back to a thicker gauge wire is if you have added accessories. If that's the case then you should build your own sub-harness and use it to feed a new fuse panel, then feed your old fuse panel from that, using a fuse sized for the wire that feeds it.
 
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The 4th pin on the regulator is to a "dummy" light on the dash that you probably don't have.

Regarding the harness, there are lots of wiring diagrams for this car online so just search for one. The battery terminal is the large wire and carries current, so that should be fused (with a resettable breaker). Best to do it in that harness because it splits into three soon after the connector.

The only reason to go back to a thicker gauge wire is if you have added accessories. If that's the case then you should build your own sub-harness and use it to feed a new fuse panel, then feed your old fuse panel from that, using a fuse sized for the wire that feeds it.
The tech at Tuff Stuff said I should be able to run a fused wire straight from the alternator BAT post to the BAT side of the solenoid. Is there any negatives to taking this route? He also said that if I do this, to not connect the smaller gauge battery wire that's already in the 3 wire connector.
 

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Same difference. But I prefer using a resettable circuit breaker placed at a location where you can access it. I have been stranded before with a broken fuse wire, or "fusable link" that I could crawl on the ground, see and touch with the fingers on one hand, but not service. A breaker that can be reset could be the difference between waiting for a tow truck or just a minor inconvenience.

40 amp resettable breaker: https://www.amazon.com/OUHL-Trollin...=circuit+breaker+40+amp&qid=1617668850&sr=8-6

Keep in mind that we've been assuming 40 amps. Best to measure the size of the power wire and use the proper size breaker to protect it. For a 12 gauge wire use 40 amps. 14 gauge use 30. 10 gauge wire use 50 or 55.

Regarding leaving the small wire disconnected, as you can see from the diagram they go to the same place, so I can see why a tech may suggest that. However the purpose of the wire is to sense the voltage close to the battery so that the regulator knows what to do, and it will be different than in the power wire that is under load. Best to connect it as the engineers at Ford wanted it for best control of the circuit.

Wiring diagram:
 
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Same difference. But I prefer using a resettable circuit breaker placed at a location where you can access it. I have been stranded before with a broken fuse wire, or "fusable link" that I could crawl on the ground, see and touch with the fingers on one hand, but not service. A breaker that can be reset could be the difference between waiting for a tow truck or just a minor inconvenience.

40 amp resettable breaker: https://www.amazon.com/OUHL-Trollin...=circuit+breaker+40+amp&qid=1617668850&sr=8-6

Keep in mind that we've been assuming 40 amps. Best to measure the size of the power wire and use the proper size breaker to protect it. For a 12 gauge wire use 40 amps. 14 gauge use 30. 10 gauge wire use 50 or 55.

Regarding leaving the small wire disconnected, as you can see from the diagram they go to the same place, so I can see why a tech may suggest that. However the purpose of the wire is to sense the voltage close to the battery so that the regulator knows what to do, and it will be different than in the power wire that is under load. Best to connect it as the engineers at Ford wanted it for best control of the circuit.

Wiring diagram:
I think I get it now. Sorry for my ignorance, still learning about old cars and electrical things aren't my forte. So because the stock harness can only support the ~40 amps given the stock accessories, I'd want a 40 amp resettable breaker for that circuit to effectively limit its draw to 40 amps. After doing some research in the past week or so, I've come to see that alternators don't force all 70 amps out. It'll just make a little more if its needed by a component? I was worried about having 70 amps sent to the ignition or something lol.

The resettable switch would be like a house breaker where you just flip it back on and you're good to go?

I would like to go the resettable breaker route. Is this an option: Keep the stock smaller gauge battery wire plugged into the alternator with the heavier gauge wire on the same post as well? They'd both be going to the battery in some way. If I take that route do I have to fuse both or just one? I just don't want something burning up. Apologies for all the questions
 

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Even some seasoned mechanics get intimidated by electrical circuits and shouldn't. It's a large percentage of what goes wrong with old cars and for newer ones quite a bit higher than that. It's far easier than pounding out seized suspension components with the car on jack stands.

The alternator will only put out what the regulator calls for. If you battery is drained this breaker may blow. And again, if you have add-on accessories then you need to go another route, which is to add a new panel and back-feed your old one with a fused circuit.

A resettable breaker is exactly what it's name implies. I gave you an Amazon link.

The ignition circuit normally isn't fused in these old Fords, and this was done for dependability. Ford Inc. would rather have you burn a wire than get stranded on a railroad track. Instead of a fuse or breaker they just increased the wire size to well above what the circuit normally uses.
 
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I may be ignorant on this too. But since I’m considering a larger alternator too, I have a question. Is this going to create an issue with the amp gauge. I have read where it does. But for 20 years on my last vintage car I had a 120 Amp Powermaster alternator and never had an issue with the amp gauge.
 

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It doesn't matter the rating on the alternator because it's only going to put out what the regulator calls for. You should protect your OEM wiring by using a breaker on that circuit.

Remember, a fuse (or breaker) protects the wire. Otherwise the wire becomes the fuse.

Regarding the ammeter, it's only going to report what is going through it (or going through its shunt).
 
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