Help. Battery? Alternator?

[4RrDriver]

2004 4.0. 137k miles. Battery is about 2 years old AGM high-powered. Truck was parked for 2 weeks at very cold temps. Came back to start it at about -5c / 23f. Started up perfectly. 2 short trips and started great each time.

3rd trip and it started beeping for a while. No dash lights. Then VSC light, then battery light, ABS. reved it up and thatbkufbrve helped, mightve not, truck returned to normal. Then beeping and lights again. Eventually died. And won't start. Put a booster pack on it for 10 mins, limped home. Repeated.

I'd normally think alternator. But it's odd that a full battery with a dead alternator had no battery light come on. Normally, with no charging (even with full battery) the battery light comes on if alternator isn't pushing volts.

Before I spend $ on an alternstor, should I be think g something else?

Any hp is appreciated. I'm stranded a few hours form home at a buddy's.

 

[ArthurKotb]

Can you take the alternator off and have it tested? It it's bad you can just buy a new one from them. I've had problems with a diode going out in an alternator and not charging the battery. I don't know why the alternator light isn't coming on.

 

[4RrDriver]

I'll have it tested. Seems odd for factory alternator to fail at 137k miles, but def possible.

I charged the battery for an hour at high power on a smart charger. Truck started, all was normal. Multi meter showed initially 17v at battery, I should've tested battery before starting, that voltage could've been left over from the smart charger doing some magic, not necessarily from alternator - although if it was from alt it's too high and maybe that's a clue...

I turned on all electrics to load up the alt, after a 10 mins it started beeping, VSC, TRAC lights come on, and then eventually a few mins later the battery light comes on. At that point with engine running, the battery shows 11 volts and steadily decreasing.

In theory, even with a bad battery, the 130 amp alt should be able to run the whole truck. It just odd that the battery light doesn't come on for a while. In theory it should immediately come on if the alt isn't pushing 14-ish volts, regardless of the battery state. Perhaps there's a voltage controller ormsomemother component that's failed?

Before I spend $300 on an alt and an hour outside at 12f, I'm not crazy to say there's a high probability it is the alt? These symptoms don't point to something else I'm missing?

I didnt have time to search the forum, I'm a bit stranded. Really appreciate any advice. Thanks!

 

[ArthurKotb]

I've never seen a charger go up to 17 volts. It sounds like the voltage regulator to me, which may be incorporated into the alternator. My 2012 Corolla's A/C compressor went out a couple months after the warranty, not even 40K miles (all city driving).

The battery is used to start the car. Once started, the alternator provides all the electricity for all things electric. You can even take the battery out once the car is started (but don't. It needs to be in the circuit). If the alternator goes out the car will use the battery to continue running, but it will eventually die when the battery does. I had a brand new Olds Vista Cruiser have the alternator go out on a 400 mile trip, in the middle of nowhere Indiana. A friend had his Pathfinder's go out on a return trip from fishing in Canada. We had to use the boat battery to get him home. So it does happen.

 

[Bluesky 07]

FWIW, my alternator failed a couple of months ago and the battery light didn’t come on. Replaced it and the battery and all has been well.