Battery not charging...alternator bench test good?? (1 Viewer)

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OK battery went dead couple days ago. on my 81 FJ40. Thought it was battery, but after putting in on charger it tested good...held 12.6 volts. So decided to test alternator using battery cables with a multi-meter. No change in battery voltage after starting. So got to be alternator right? So I ordered a replacement. I removed the old one and as an afterthought I decided to have it bench tested at auto parts store. It tested good. So now I'm wondering what is going on. Need some help from some experts (Coolerman??) Do I change alternator, battery, etc.? Is there a fuze or connection I'm not aware of. BTW, the alternator is original to the truck and has never been changed..gotta love quality parts. Thanks in advance.
 

hobbes

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You might check for an alternator grounding issue. The alternator grounds through the mounting bracket, through the starter to frame ground strap, to the frame. A bad ground connection in any of those areas will cause issues. On Coolerman's recommendation, a few years ago I added a battery negative ground line direct to alternator bracket. It's not required if all the other ground area's are good, but its nice insurance. Photo below.

My first check would be to make sure you have a good alternator and battery grounding. If not make sure the alternator/bracket, the bracket/engine, the starter ground strap, and battery ground strap mating surfaces are all clean, rust free, and tight.

If you grounding is good, then the electrical guru's will need to jump in.
IMG_6218.jpeg
 
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Check the regulator if you did not change it out also. Then, test for continuity between the positive post on the alternator and battery posts.
 
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Check the regulator if you did not change it out also. Then, test for continuity between the positive post on the alternator and battery posts.
Regulator is part of the alternator just like picture above...not sure how to check it. Wouldn't bench test check for correct output voltage?
 
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Regulator is part of the alternator just like picture above...not sure how to check it. Wouldn't bench test check for correct output voltage?
Nevermind, I see now yours is internally regulated other than our General Countries spec. You now need to ensure current can flow to the battery and also, that the sense system works to get current flowing. And always check grounding as stated above.
 

hobbes

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The voltage regulator was the second thing I thought of. My assumption was it is internally regulated and tested good. But, that is an assumption.
 

Steamer

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Not familiar with your particular wiring but, an alternator needs 12v going into it and usually is fed from the ignition switch and through a fuse. If fuses are good, I would next identify which wire on the (disconnected) terminal is the 12v feed, and check it for voltage with ignition switch “on” & not running.
 

hobbes

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On my 1974, there is a 10 amp fuse the goes from the ignition switch to the regulator. Like Steamer said, check that if your year has that too.
 
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Thanks for all the replies...digging into all of them. I checked engine fuse (10A)...good. That's the only fuse I know about. It has blown before and you get zero power. You guys think it might be possible that during the process of removing alternator (little tapping with hammer, etc.) I jiggled it enough to work on bench but not on truck? Just to check...I put it back in but haven't had a chance to check it with multimeter. Also, neglected to mention battery is almost 10 years old...it checked OK after charging, but think that could have anything to do with it not reading above battery voltage while running?

Thanks,
Rob
 

Steamer

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battery is almost 10 years old...it checked OK after charging, but think that could have anything to do with it not reading above battery voltage while running?
I would say no.
I put it back in but haven't had a chance to check it with multimeter
That would be the next step.

And if it still isn't charging,
I would next identify which wire on the (disconnected) multi- terminal is the 12v feed, and check it for voltage with ignition switch “on” & not running.
 
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OK...put old alternator back in, checked grounds (good), fired it up and charging to 14.2. I'm thinking just process of banging alternator around a little woke it up. Maybe some carbon on brushes, etc. At any rate working fine now. Can't believe 10 year old battery is still kicking but held full charge for 3 days. Appreciate all the suggestions. BTW, don't ever forget to disconnect battery when working on electrical stuff. Got in hurry and forgot to do it, and sparked + connection on my igniter. Fortunately didn't kill it. Fired right up. Thinking 41 year old alternator not going to last much longer...got another on standby. Thanks again.
 
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I have a battery in one of my cars that is also 10 years old. Nothing fancy, just a big box store battery. It stays on a tender, and the car has almost zero draw while sitting. Sometimes you get lucky. Running your old battery down, may have put too much stress on it, so I would plan for its funeral by having a jump box, or parking on hills! Glad you got it working, I have not had good luck with aftermarket alternators. If yours is OEM, look at getting it rebuilt if it gives trouble again. :)
 
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Thanks...yep alternator is OEM built like a tank....like I said hasn't been changed since new (81.) The new one I have in box is from Autozone...not so sure on that one, but it has lifetime warranty. Not a good idea for me...I'm on my third NAPA starter with lifetime warranty. When this one quits I'll get another! Yea that old battery is from Wally World...holding charge like a champ.
 
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On my FJ62 I was having charging issues, and I think that is one reason the guy sold it. He thought the radio was draining the battery. I tried two aftermarket alternators and both were acting weird. I splurged and bought a factory reconditioned unit and lights went off, and electrical system was back working properly.
 
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An old simple test to see if the alternator is putting out is to disconnect the battery negative cable while the engine is running. If the alternator is charging, the engine will remain running, if the alternator is not putting out then the engine will die.
 

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