mysterious electrical problem

1996 NISSAN TRUCK
135,000 MILES • 2.4L • 4 CYL • 2WD • MANUAL
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DWKLOOSTER
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The battery is not charging even though the alternator tests good. I have replaced the battery (though it is starting to fail again from occasional driving without an alternator) and checked for continuity between the positive terminal of the alternator and battery. I have also checked the relevant fuses (9 and 11) and tried switching what I believe is the bulb check relay (closest to the front on the passenger side of the engine compartment. Tell me if I'm wrong here.) with another nearby relay with the same number on it. I have also checked for corrosion on the sockets for the relays and fuses.

The charge light bulb tests good, but never lights up, except very dimly in the following scenario:

Sometimes, maybe 1 in four times, when i turn the engine from the on position back to accessory, the indicator lights will not turn off, except for the airbag light, which then turns back on after a few seconds along with a slight dim glow which slowly creeps into the battery light.

I have also noticed that when I disconnect the alternator, the yellow/black wire shows a voltage to the + terminal of the battery when the car is off, and a voltage to the - terminal when the key is turned on. This led me to check for continuity to the opposite battery pole in each case, and I found 13 ohms to - in the off position but no continuity to either pole in the on position.

I feel like I might have hooked jumper cables up backwards in the dark like a silly billy but am not 100% sure about that memory as I was very sleep deprived at the time. Is there a fried diode somewhere? I'm baffled.
Oct 6, 2020 at 2:34 PM
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KASEKENNY
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Couple things. First we need to check resistance through the fusible link. A lot of times when they have an issue they start with higher resistance.

Then we need to check voltage at each terminal of the alternator.

Let's start with this and go from there as that will show what direction we need to go.
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Oct 6, 2020 at 3:56 PM
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DWKLOOSTER
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when I disconnect the positive battery terminal and the main lug of the alternator and test resistance between these points I find .3 ohms, so I would think the fusible link is good. when those are still connected and I test the voltage at the alternator lug it is always the same as the positive battery terminal no matter whether the car is running or not. When I disconnect the wire harness and measure voltage the white wire is always the same as the positive battery terminal(I also found .3 ohms along this wire as well.) The yellow/black wire (should be yellow red according to your diagram, but it is continuous with the yellow/black wire to the instrument cluster.) it shows negative voltage(12 or so volts to the positive terminal) when the ignition is off(with 13 ohm resistance to the negative battery terminal) and positive voltage when the ignition is on, maybe sometimes a tenth of a volt lower than battery voltage. the ground wire shows continuity (.2 ohms) with the negative battery terminal wire as well.

Someone from justanswer.com told me to take the alternator to NAPA for a third test in case the folks at advanced auto parts were jokers. I will also update you if this turns a different result, the the other weird electrical behavior in the lights leads me to think there is something else wrong.
Oct 7, 2020 at 8:05 AM
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DWKLOOSTER
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Apparently Napa doesn't test alternators. I will follow a few more of your steps assuming the alternator is good before I do something crazy like buy a new one even though it might not fix the problem or spend almost as much money so a mechanic can possibly tell me what the free test already told me.
Oct 7, 2020 at 10:29 AM
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KASEKENNY
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Seems like all the wiring is okay, but I assume you did all these voltage tests with the engine off correct? Obviously the the resistance was taken with them unhooked but I just want to confirm that.

However, I would agree that we need to get the alternator tested even though it is unlikely that the other parts store was wrong we need to double check.
Oct 7, 2020 at 7:23 PM
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DWKLOOSTER
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I did voltage tests on the main alternator lug with the engine on and off and it is always the same as the battery. The other tests were done with the engine running but I tested the yellow black wire with the key in both the on and off positions. I have already taken the alternator to multiple stores and seen it test good.
Oct 8, 2020 at 9:09 AM
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KASEKENNY
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Okay. Let's step back. You may have done some of this but let's just go step by step so if we can redo it just to check it that would be great.

1.turn the ignition on does the charge light come on? If not, disconnect the S and L wires from the alternator and jump L terminal to ground. Does the lamp come on?

2. If the lamp did come on with the key in the on position, then start the engine. Is the charge lamp on? I assume it is not.

3. If it is not on when idling, run the engine to 1500 RPM and turn the head lights on. Is the lamp on at all? Even dimly? If so, let it idle and measure voltage across terminal B and L. It should be less then 0.5 volts. If it is more the alternator is defective. Diagram below shows this.

4. Bring the alternator back to 1500 RPM and measure voltage at terminal B and make sure S is properly connected. Voltage should be between 13 and 15.

Let me know with these things and we can go from there. Thanks
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Oct 8, 2020 at 6:58 PM