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Old 06-26-2022, 08:58 PM #1
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Charging system mystery

Here's a good one that I'm having trouble figuring out...

08 Limited with V8. Did the third timing belt at 297k. Water pump, tensioner idlers too. Got it all back together, start it up and it runs fine but the battery light is on. Checked voltage, at the battery, it's running at 12.5.
No big deal, right? Check the battery terminals, they are fine. Go underneath and pull the connecting plug on the alternator, put it back on. Light goes out and STAYS our for the next four thousand miles. It's charging at approximately 13.7 volts.

Fast forward to yesterday. Son drives the truck, no lights or issues. I start it up the next day, battery light is on. Same drill as last time, checked the terminals and the plug. Start it up and it's still on. Put some dielectric greas on the plug and the light goes out. Drove it to work (30 miles), no light. Started it up after work to go home and the lights on. Crawled under the truck and wiggled the plug, pulled it off and replaced several times but the light stays on. Drove it home, didn't act like it was getting low.

Did the plug routine again, no luck, light stays on. So I got a wild hair and cycled the ignition key between off and accessory rapidly three times. For some reason, the light goes out and has stayed off the few miles I've driven it.

So far, the light has only come on when I start the truck. Hasn't come on (or gone off) while driving.

I know it's a long shot, but has anyone had something like this happen? Normally I would think something is going out in the alternator, but the cycling the ignition thing has me wondering if it's something else.
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Old 06-26-2022, 09:17 PM #2
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Possibility it's the ignition switch going out.
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Old 06-27-2022, 10:10 AM #3
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How many miles on the alternator?
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Old 06-27-2022, 10:13 AM #4
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Quote:
Originally Posted by AuSeeker View Post
Possibility it's the ignition switch going out.

Hmmm...curious. But the ignition switch would have no bearing on whether the alternator is charging or not, correct? When the battery light is on, it's only running at 12.4-12.5 volts. When the light is off, it's at least 13.7 volts.

Is there something in the ignition switch that would affect the alternator that way? I'm thinking there's a diode or something in the alternator that is on the way out, and me cycling the ignition switch somehow cured it for a bit.

But I honestly don't understand it. Weird.
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Old 06-27-2022, 10:15 AM #5
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Quote:
Originally Posted by Seymore Butts View Post
How many miles on the alternator?
Luckily, the guy I bought it from (235k, original owner) kept great records on the truck. He replaced at 174k. I did notice when I did the timing belt that it had a "remanufactured in China" sticker on it. I'm leaning that something inside is going out. Just weird that I ran four thousand miles on it without a hitch after the light originally came on.
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Old 06-27-2022, 12:15 PM #6
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Quote:
Originally Posted by crimedog View Post
it had a "remanufactured in China" sticker on it. I'm leaning that something inside is going out.
I think you found your problem.

Proper alternator voltage for the 4th Gen is between 13.2 and 14.2

Your observation of 12-point-something volts is indicative of charging system failure and assuming that your battery is in good condition and you have clean reliable connections including the body ground wire, the alternator is the primary suspect.

OEM rebuilt unit from my local dealer is about $280.
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Old 06-27-2022, 02:46 PM #7
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Quote:
Originally Posted by Seymore Butts View Post
I think you found your problem.

Proper alternator voltage for the 4th Gen is between 13.2 and 14.2

Your observation of 12-point-something volts is indicative of charging system failure and assuming that your battery is in good condition and you have clean reliable connections including the body ground wire, the alternator is the primary suspect.

OEM rebuilt unit from my local dealer is about $280.
I'm going to run it down to autozone tomorrow and see if their machine will tell me anything, but I think you are right. The rebuilt one on the truck has about 130k on it and I'm probably real lucky that it has lasted this long.

Appreciate the help.
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Old 06-27-2022, 03:56 PM #8
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I'd suspect the rebuilt alternator also. When the brushes get worn down these things have odd behaviors. If you were to watch it on an oscilloscope you may see lots of drops the computer isn't catching.
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Old 06-27-2022, 07:30 PM #9
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Quote:
Originally Posted by crimedog View Post
I'm going to run it down to autozone tomorrow and see if their machine will tell me anything, but I think you are right. The rebuilt one on the truck has about 130k on it and I'm probably real lucky that it has lasted this long.

Appreciate the help.
If that alternator checks out, I would make sure you have a good, tight connection on the wire that runs from the alternator to the battery. Then I would do a voltage drop test on that wire....would be nice to do it when you are seeing the 13.5v and then again with the 12.5v.
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Old 06-27-2022, 09:36 PM #10
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First thing is do the basics, check battery terminal connections even under the ends. There may be corrosion and you can't see it. Happened to my dad on his Lexus RX. Would not charge, voltage was low, would not jump off a booster pak. I took the cable ends off the battery and corrosion was everywhere. Cleaned everything and it started right up and got 13.5v with my tester.

As far as I remember when an alternator output goes below 12.8 the diode trips the light( some manufacturers have different trip points ). So your alternator may have a bad windings, brushes or a bad voltage regulator.

Like I said do the basics first, it might be that simple.

Good luck.
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Old 06-28-2022, 07:07 AM #11
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I just had an issue where I thought the battery was dead, the truck wouldnít start. As soon as I turned the key to the on position all the lights on the dash would cut out and there would be a buzzing sound. I thought for sure the computer was shot or something like that. I tried the easiest solution first by cleaning the terminals and positive and negative clamps and I havenít had an issue since.


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Old 06-28-2022, 09:17 AM #12
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i would just replace the alternator with aftermarket and besides, you don't want to get stuck somewhere and pay more for a tow. also, check your battery for age. note that if you are in a place where it's hot all the time, the battery's life is a little bit short, maybe 3 years.
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Old 06-28-2022, 12:18 PM #13
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Verify that the vehicle is giving proper charging signal to the alternator, I have seen that 4-wire plug and wiring have issues.
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Old 06-28-2022, 02:02 PM #14
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How do you verify that?
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Old 06-28-2022, 02:19 PM #15
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Quick update:

Started it this morning, battery light on. Instead of running to have alternator tested, I took the battery terminals off even though they looked good. Wire brushed the hell out of them and put back on and tightened.

Started truck and no light! Laughed at myself because I thought there was no way it was the terminals. Let it charge (14.1 volts) for about 20 minutes and shut off.

Just went out and started to run some errands. Light is back on! Ugh. Going to let the engine cool down and then pull the ground connection off of the alternator and wire brush it. Going to inspect the 4 wire connector too. Then if that fails, off to Autozone to have the alternator and battery tested.

This thing makes no sense. Driving me nuts. Thanks for the suggestions though.
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