2018 Mustang GT - Alternator Dead?

[carpenj]

I had to jump start my car back to back times a couple days ago, so I took it to Auto Zone for a new battery. They did their tests and came over to tell me the battery may or may not be dead, but the alternator definitely is. I was pretty shocked (and frustrated since my bumper to bumper warranty expired just 8 weeks ago), and I'm not even sure how much I trust a random Auto Zone employee who isn't really a mechanic etc.

What would you guys do? Go ahead and drop a new battery in and see how it does, or is there a chance a dead alternator could harm the new battery? I have an appointment scheduled at the dealership Monday but if it IS the alternator, I'm not sure the car will even make it that far.

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[spedy7]

If you have a DVOM/volt meter you check the voltage at the battery terminals with the car running. North of 13 volts is a good sign the alternator is doing its job. Roughly 3 years is the average life of a lead-acid battery, at least out here in the desert.

What made them say the alternator was bad?

 

[carpenj]

If you have a DVOM/volt meter you check the voltage at the battery terminals with the car running. North of 13 volts is a good sign the alternator is doing its job. Roughly 3 years is the average life of a lead-acid battery, at least out here in the desert.

What made them say the alternator was bad?

They ran an alternator test with a similar meter to what you're talking about. I decided to drive it to the dealership yesterday afternoon so if it did break down, it wouldn't be during the Monday morning rush hour. I watched the voltage gauge and it held at 14.5v the whole trip. I told the dealer tech that and he said he hasn't seen an alternator fail that soon before and it wouldn't be worth the $50 to even test. They put a new battery in and it's running fine, voltage holding steady. I just went out and did what you said with my volt meter, all still looks good.

Lesson learned, don't let oreilly/Auto zone even test anything. Didn't cost me anything but a small bit of frustration at least. Cost them the sale of a battery though.

 

[Jackson1320]

It’s the alternator

 

[Cobra Jet]

Usually the voltage regulator on an alternator will fail before the actual alternate will be deemed junk. I know with the older Mustangs, the voltage regulator was a service part - as was the alt brushes to it.

I have not worked an S550 alt yet, but can someone cofmirm if the voltage regulator is a service item separate from the alternator?

 

[carpenj]

It’s the alternator

I'm curious what makes you say that, given that it was producing consistent voltage with a completely dead battery and a brand new battery? I usually avoid anything remotely electrical-related, I have no knowledge in this area.

 

[Jackson1320]

S

I'm curious what makes you say that, given that it was producing consistent voltage with a completely dead battery and a brand new battery? I usually avoid anything remotely electrical-related, I have no knowledge in this area.

Start the car and remove the negative battery cable and you will know

 

[carpenj]

S

Start the car and remove the negative battery cable and you will know

I'm not sure how that's different than me driving 20 minutes with a completely dead battery to be honest but either way, I'll run it until it stops working rather than risk damaging something to test and see if it will eventually stop working.

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