As the title states my 2014 mustang will randomly lose all electrical power. When it is working the dash and lights will randomly turn off and reset while driving. Usually only once during that drive. While idling it will sometimes kill the engine. If it decides to keeps its power will usually start again. I have replaced the alternator, the fuse that connects to the positive battery terminal and the battery is fairly new. When it looses power everything is off. No dash or lights. When it decides it has power it will usually start. Sometimes it will power on and everything shuts off as soon as I try to start it. Everything seems connected. I've checked the power relay fuses and they are fine. I've taken it to the dealer the first time it happened when it just lost all power at a stop light. They claimed corrosion on the battery terminals. Worked fine until a month later and wouldnt power on again. I used to be able to reset it by taking off the positive battery connection and reconnect it and get it running again. Yesterday doing that did nothing and it stayed dead. It decided to have power this morning and does currently but I can't say it will stay that way. If anyone has an idea please let me know.

 

Hm.

Ok, well, if you're saying *everything* goes off, and you actually mean everything electrical, then it's a fairly few things to check.

First, the battery terminals are the obvious thing, and you've already sort of checked the positive. But I didn't see that you looked at the negative. It could simply be loose. And/or you may need to clean them up a little (I'd use scotchbrite these days, actually), and then reattach. May be all it is. Also, read this, in case it helps: https://themustangsource.com/forums/...4/#post6917899

After that, you chase the wires from the battery. I'd start with the grounds from the negative terminals:
http://iihs.net/fsm/?d=711&f=010%20-%20Grounds.pdf

This took me a second to figure out, but there's information here that's valuable. The battery on the upper left (as the page is oriented in the link) shows the negative cable is split, and lead to two mounting locations. The first is G106, and the second is G107. Below each of those you see 151-1, 151-2 and 151-3. Those are page indicators. And here they are:
http://iihs.net/fsm/?d=711&f=151%20-...0Views.pdf&p=1

Page ahead to the 5.0L on the next page. You will have to look, but the 3.7L V6 page shows G106 is on the back of the engine. For the 5.0L, it's located on the right side of the engine block. G107 is on the right shock tower just in front and probably below the battery somewhere on both cars. That's pretty much it for the negative terminal. You have an engine block ground in G106, and a chassis ground on G107.

As far as the positive side:
http://iihs.net/fsm/?d=711&f=012%20-...g%20System.pdf

As you can see there's only three places from the positive terminal. First, the left one is going straight to the battery junction box (the fuse panel in the engine compartment, right front) and through fuse F17, which is the alternator (nee generator) fuse. You'll want to check that one and the connector it's in... even underneath, btw, it could be corroded under there. That wire, after passing through the fuse, continues to the alternator. It's one of the three in the quick disconnect on the back of the alternator.

The middle one goes to the 200A fuse on the side of the battery, the double one that Ford added (the other, 100A, goes to the power steering rack.) They're called 'circuit breakers', but they're just big fuses:
https://lmr.com/item/LRS-14526AA/mus...-breaker-11-14

Anyway, that middle one goes 'straight' to the back of the alternator, and is the bigger cable that connects to the stud with the ring connector and is fastened by a nut. You may need to check the alternator connections, is what that boils down to. Maybe even the alternator itself, but I'm pretty sure that is going to be a low-chance issue.

The third wire goes directly to the starter motor, where a solenoid is used on the starter to make it work. The starter is grounded through the engine block. I wouldn't think that'd have anything to do with it, but...

That's it. Any of what I just said is the likely candidate...

...unless it's your ignition switch cylinder, that is. Where you stick your key and turn. It's entirely possible it's bad and could cause the car to go off.

And even then... it could be something else. Bad battery junction box. Maybe the smart junction box (kick panel, passenger side.) Or something else, I don't know. Electrical gremlins can be... fun.

I have a suspicion it's the negative terminal/grounds, myself, but I wouldn't be surprised it wasn't the ignition switch either.

I hope I helped in some way. I'm sure I forgot something, or didn't think of it, or... In which case I hope someone else pops in and says something.

I wish you good luck in finding it, and also welcome to the forums!