Was driving and the electrical went haywire almost stalling, everything turning on and off, temp and fuel gauge pinned then stalled out and I was able to coast to the side of the road. Wouldn’t start but even with the key taken out of the ignition different electrical items were turning on and off. Tried rolling up the window and put the key in and it slowly pulsed up with manual help.
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Your car’s battery voltage may be too low. That confuses the car’s computers, no telling what they’ll do when confused. That’s the first thing to check imo. A fully charged battery measures 12.5 volts or more. Best time to check battery voltage is after car has been sitting unused overnight. My car’s batteries measure 12.6 volts first thing in the AM, pretty consistently.
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Dear George, had the battery on a tender as it had died a few months ago - way too early. So battery appears to have been fine. Do you have a 2nd thing to check when this is the case? OR could it be that the battery really has gone bad and after a few hours off the tender, it’s causing this malfunction? Best, JF
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If you are in the US most Autozone stores will check your battery for free . Other places do that also .
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Sounds like the alternator failed and you continued to drive until the battery completely discharged.
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Hello, Itsfair. I’m the Car Talk moderator lackey. I really loved this post for many reasons. Not because you had trouble; sorry to hear that. But because many of the answers were of the “Don’t panic” variety. Coincidental to your post, Car Talk just put up this feature story. If you have some time to burn, maybe it will be helpful. I vote alternator based on my past experiences that were similar. Thanks for your post. After it is sorted out, please circle back and tell us what happened. Best of luck, Most Common Causes of a Car That Stalls - Car Talk
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Tester
Agee that the BCM or GEM in Ford-speak may have failed. It performs thenBody Control Module function on your Mustang.
Or the GEM blew a fuse for some reason. Check for blown fuses. Start with the underhood fuse box.
Hi, well here’s what’s been found: the previous owner had an after market batter harness which had started to short . As you guys pointed out, it was killing the alternator (which finally died) and I lived off the battery for only so long. The tender I was using was only prolonging the death of the alternator. So now replacing the alternator and the main cable between the alternator and the battery. Should be fine. I’ll test it taer today.
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Ooh, I hope it was choc. chip cookie. 
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