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Battery light is on!!


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The red warning bulb is connected to battery on one side and alternator charge on the other. As long as you have battery and charge there is no (or extremely little) current flow through the lamp to light it. If the alt stops charging current flows from the battery to the alt. and the bulb lights.


Check the fusible links and see they are in good shape and not obviously burnt.



Can you simply borrow a good battery and swap it and see if the light goes out? You did say new battery and new things are unproven in use.

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The voltage reads 14 or higher

The light is on all the time as long as the truck is running

I'll have to check what the voltage gauge reads when off, I haven't checked that yet.


Yes, Battery is new and the truck starts right up with no struggles, even when cold so I don't think it is the alternator.

I am going to check the fusible link, I think that I will find the problem there because I just swapped out the ignition and the horn fuse blew out and the batery fuse probably blew too ...huh?

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An alternator puts out more electricity the faster it spins. 

This is basically how the "Old School" external alternator and regulator worked. 

When you turned the key on, before starting the engine, electricity went from the battery, through the ignition key switch, to the voltage regulator, and then into the field coil of the alternator, and then to ground.

when the engine starts, and is turning slowly, and the system voltage is low, the voltage regulator keeps a full 12 volts on the field coil of the alternator.  As the engine speeds up, the alternator puts out more voltage, and current, and the regulator then disconnects full voltage to the field coil, and put a resistor in series with it.  This cuts down the output of the alternator.  If the battery gets fully charged, and the engine speed continues to go up, the regulator will disconnect the field coil completely.  Then the system voltage will go down, and the regulator will reconnect the resistor in series with the field coil.   The switching of the direct battery voltage, or a resistor, or disconnecting the field coil altogether happens fast, in some cases, you could probably hear the voltage regulator buzzing, or vibrating as it is doing the switching.


An electronic or relay less voltage regulator does the switching with transistors, instead of a relay.  An internal regulated alternator just moves the transistor switching "black box" inside the alternator case.


Hopefully, you understand my explanation on how the regulator works to control the alternator output, or voltage.


Now to your question, "I got around to driving the truck today and noticed that the light tends to turn off while I am driving at high speeds but turns on when the truck sits idling."

The field coil in a alternator is always spinning.  It is connected electrically to the internal alternator wiring by brushes, that ride on two copper rings on the rotor of the alternator.  If the brushes wear out, they do not make a good contact, and full battery voltage cannot get applied to the field coil, and the alternator does not charge at low RPMS, and the ALT light comes on.  When you speed the engine up, the alternator no longer needs full voltage to put out some voltage, enough to barely charge,, and cause the charge light to go off, but the alternator will not typically have full output then.

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Shortdog: if you can check the voltage (across the terminals of the battery is fine) with the engine running and with the engine off, that should give a clear signal of whether the alternator is working at all (whether it's working _well_ may be a different story).  You should see ~14.5V while running, ~11-12V while engine off.

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  • 1 month later...

Just outta curiosity.....what does a VERY dim batt light indicate(while running). Bulb is good....bright b-4 engine starts. Never have had an issue w/ charging or dim lighting @ night. I can leave the truck with the lights on for 45 min or more and it starts easily......diminished batt voltage quickly recovers. I have GOOD connections/ground straps in place. It doesn't bother me but I'm sure either Mike or wayno have the answer. I suspect the fuseable link(s) is the culprit....

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On at all times... brushes may be worn or close to worn out. You used to be able to get them at NAPA or elsewhere, for under $10.


On at all times... one of the diodes may be going, or bad, with lowered output.


If it goes out when revved... your idle speed may be too low, with less output.




Just make a jumper wire across the two fusible links. I doubt this is it

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After reading your last post mike I'm leaning towards the brushes. I remember having similar issue in an old vw bug with a generator(no diodes). In fact in the 'VEE DUB IDIOTS MANUAL' I think there is a page about gen/alt light issues. Gotta love John Muir's doodles/diagrams and explainations. Wish he had wrote a Datsun book.

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I have no idea why you would think I know about electrical stuff, but here would be my guess.


The alt. might be going out or having a connection issue, does it get brighter when you turn on the headlights, or is that when you see it glowing dimly, when the headlights are on?

When I turn my headlights on, my ign. light just barely glows, at least enough to see it during the day time hours, but I didn't notice it till driving home one night, I suspect that my alt. is going out, but it could be a loose connection also, so before I pull my alt out, I am going to clean and tighten all connections, if that doesn't change anything, I will wait for it to go out, as I don't travel in my work truck very far from home that often anymore.

In the past I have had the main wire connector on the alt go bad, that is the wire going from the alt to the starter post(battery), it went from dim/half on to very bright when I started fiddling with it, so I pulled the wire loose from the connection because I was afraid it would come loose while I was driving and ground out and cause a fire, taped the end with duct tape and I wrapped it around something till I got home, I fixed it by putting a new connector on the end of it and put it back on the alternator, no more ign. light glowing anymore till just recently.

I myself am not to worried about mine yet, as I have a volt meter, and the voltage has not changed.

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