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My car died while I was driving it, but I can put the battery on a charger (or from another car, with jumper cables) and get it running albeit for a short period of time.

I put a multimeter on the batter and I'm getting 12.5v while the car is off or running.

replaced the alternator, still only getting 12.5v.

I'm checking at the posts, at the cables going onto the posts, and even at the alternator (although the alternator shows 0, when off) itself.

I'm stumped!

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3 Answers

So looks like I made a dummy mistake.

Although I believe I had check the belt tension, it was simply too loose and was not spinning the alternator.

With the belt correctly tensioned I am getting the correct 14.5v to the battery with the car running.

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I know that some vehicles ( Dodge diesel at least and maybe more) the alternator output is regulated by the ECM (engine control module). It looks at several signals to adjust the output,voltage,load,etc. If you don't find an open wire as @Larry has suggested, have the alternator checked. Most parts stores will check them for free. There is always the chance you got a defective unit. If it tests ok you will need a wiring diagram to start tracing the circuit.

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Alternator output is determined by a Alternator Voltage Regulator located inside the Alternator assembly. In most cases it uses a 3 wire plug (+/-/ref) to signal to the ECM on it's output. Higher end vehicles use this same setup but on a single wire LIN-bus (1 wire with bi-directional signal) to perform the same task. –  cinelli Mar 22 at 15:12
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If you have 0 volts at the alternator output terminal when the vehicle is off then you most likely have a blow fusible link. You could also have a broken wire. You should have battery voltage at the output terminal even with the car off.

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The cause of the blown fusible link is what I can't seem to stop thinking of. Hopefully this was from the alternator that was already replaced? Maybe you could elaborate on how to inspect the wiring for a short in case the fuse blows again upon replacing it. –  cinelli Mar 22 at 15:08
I'll have to double check this, it may have simply been that my battery is also going, so at that point my battery was in and out of working when I checked the voltage from the alternator when the car was off. –  Joel Barsotti Mar 25 at 21:04
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