Alternator stopped working then came back

2005 Mustang Gt with month old alternator. Yesterday, Stopped at train and started to lose electric power. Battery gauge all the way to zero. Car died and wouldn’t start. With a jump, the car started and runs fine now. Battery has 12.24V, and Alternator is putting out 13.8V. Any guesses at what happened?

Wiring is good.

The ECU regulates the voltage out of the alternator dependent on the power demand of the vehicle.

Brain fart?


I guess that means the battery posts and connectors a clean and tight including the other ends .

Battery posts clean and tight and alternator has internal voltage regulator.

Which the ECU controls.


I guess I’d put a charger on the battery to see if it takes a better charge before you ruin the new alternator. If the alternator is putting out 13.8, I’d bet you need a battery.

Or that’s all the voltage that’s required for the vehicle as measured by the ECU.


I’ll charge overnight and see what it takes in the morning.

Where’d that alternator come from? Many aftermarket remans are junk.
My first suspicion is poorly seated brushes.
An old serpentine belt that slipped when it got a little wet is my second.
The battery at 12.24V needs a long (6+ hours) slow (5- amps) charge at ~14.5V. Then test it.

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AdvanceAuto, yes their alternators may be junk. The one before this was dead out of the box. I charged overnight with a 2 amp battery tender. I’ll see what happens. The battery is about 3- 4 years old. Going to check manufacturing date to see actual age.

image Belt is new also. It was a very hot day when car died sitting at idle for train. 5-7 minutes. But water temperature was only upper 190’s. The car is extremely well kept in heated garage looks like new. Also has Edelbrock supercharger, headers, and cam installed. I keep track of all engine parameters with Interceptor gauge mounted in dash. Checked all wiring last month when received bad alternator.
Battery was fully charged this morning at 12.6V, after starting, measured 14.1V. Battery is Durolast Gold, made 10/16. Will take for drive now and see what happens.

After 10 miles, the alternator was at13.8V the whole time and at idle after drive. Battery was at 12.8V after shut down

Re the belt: somehow after reading the original post and the replies “train” mutated into “rain” in my ageing head, and I thought of the belt getting wet.

13.8V from alternator is OK in hot weather.

It might not be Your ageing head. A while back, I bought 8 rolls of toiletpaper, but when I got home with them, they had mutated into 4 kitchen rolls. I don’t know what’s going on nowadays.

Once my battery’s contacts rattled off; once the alternator belt started slipping. The 31-year-old alternator keeps on working.