My 'alternator' failed in spectacular fashion the other month. Whilst my GT is nearly 5 years old, it's only travelled 26K km (not miles).
Whilst driving, I had a big variety of errors coming on, starting with 'high oil temp' warning, then 'low voltage'. At one stage, I tried to nurse the car home and ALL the dash gauges went limp, where the tacho froze at about 3000rpm and the speedo at about 50km/h, no dash lights (and no odometer
). It all sprung back to life for a short period, before the car went into complete limp mode and 'died', leaving me stranded.
I have a voltmeter in the car, and I could see it drop below 12V, despite driving, so for the last kilometres, I was surely relying only on battery power.
Anyway, it was the voltage regulator that failed. This is integrated (ie: bolted onto) the back of the alternator. The only way to properly diagnose my problem required removal of the whole unit, to be checked by an autoelectrician.
Because I was stranded, I took the option of being towed to the dealer 20km away, rather than my home 120km away. Realistically, it would have been a biatch to DIY. The labour times alone - with a hoist and experienced mechanic - was around 8-9 hours for removal and fitment (4-5 hours each way), requiring disassembly of the driver/offside front suspension and dropping the subframe, for proper access. Plus money for the autoelectrician to diagnose and replace the voltage regulator. Fortunately I have a very good rapport with the dealer/service department, so I wasn't totally caned by the cost, but it was still A$1100 for the whole exercise (ie: they probably only charged me for half the normal labour costs).
Typically, a failing alternator or regulator don't 'recover' in my experience. They fail, and continue to fail. So if that's your problem, you know about it.
Whereas, a battery can definitely lose power/voltage due to extremely cold weather. And should recover. So if you now see 14V with the engine running, chances are it's the battery, and not the alternator/voltage regulator. *fingers crossed*