Battery keeps randomly losing charge

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Hey everyone, recently my 1991 fj80 has been acting really weird—

The other day, when I went to start the car it cranked slower then usual. Stupidly, I continued to drive and the battery didn’t charge, so the car eventually died. Due to this, I thought it was the alternator.

I got the battery charged and tested at Napa, put it back in the car, and the car started and drove just fine for about and hour. On the way home, however, my battery lost charge again. Luckily I was able to make it home. Once home, I let it idle for a second to see what would happen, and the engine sounded really weird. It was like it was turning on and off again. I also saw that the oil pressure gauge was all the way down, but I wasn’t sure if that was due to low battery.

Are these all symptoms of a low bad and/or bad alternator, or is something else going on?

I have a video, but I still need to figure out how to post it.
 
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Voltage does not drop much when headlights are on and the engine is running.

Battery voltage is 12.3 volts if I did it correctly.

The battery gauge did read low when the car died, it was probably around 6 volts.

I found the fusible link, but how do I check it for continuity

Video of trying to start the car + picture of the dash

31D31939-107F-4C31-818D-630F8E31C74C.jpeg
 
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Check continuity with an ohm meter. They should read close to 0 ohms. Alternator output may be checked with an ammeter while running.

What do the battery terminals look like? You can try cleaning them all up and recharging to see if that makes a difference. All signs so far are a bad alternator or bad connections or wiring or a combo of either.
 
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The battery is clean and was just recharged and tested, so I don’t think that’s the issue.
I’ll go try to test continuity, but it might be a while before I test the alternator because the car currently isn’t starting
 
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Just looked at the fusible link more— I can’t figure out how to test for continuity, but all that wiring is bad so I want to replace it anyway.

Where is the connection that I circled in red?

CA11EC30-DCFB-43EA-89C6-FBF4014C20FB.jpeg


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I second that it's the alternator. Replace the brushes and replace the fusible link and the little black box it connects into, under $40 for parts, done.
You can replace the brushes without taking the alternator out by removing the cover only on the back of the alternator. Clean up the PO's wiring mess and you should be good.
 

Pierce

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I was having a similar issue. The battery would discharge over time. New fusible link, new battery. Replaced the battery cables with those from @Fourrunner and have had no issues since. The originals were showing signs of oxidation.
 
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Does anyone know what that connector is called so I can add the other side of it back into my wiring

(edit: I found it)

51A314A5-834E-4FB5-A04C-C6B139B6082F.jpeg
 
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Update:
I fixed the jank and installed a proper fusible link. It didn’t solve anything— the battery still isn’t charging. Based on this I am going to replace the alternator, which should be here Monday.

I took the old one out today and took some hoses out with it. Everything went well, but now I’m having trouble finding replacement hoses. If anyone wants to help with that, check out here
 

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