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Volt gauge drops to 0, but engine cranks and starts


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1977 280Z with upgraded alternator and voltage regulator jumped.

 

 

My gauge in the car will read 14.5v and car starts just fine.

 

If I turn on my running lights, all electronics die out in the car and gauge drops to 0.

Turn everything off and wait a while - power comes back on.

 

If I use my turn signals, nothing happens, but if I use my hazards - same thing. All electrics die and voltage reads 0. Same goes for fan.

 

My headlights work, but nothing else does. Even during the "all electrics dead" period, I am able to crank the engine and start the car.

 

Using a voltmeter, my battery gets 13.5v with engine running. With all electronics dead, it reads 12.5v at both battery and alternator, even though gauge in car reads 0.

 

Any ideas why?

 

Thanks!

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 You've clearly got a bad connection somewhere. Turn your lights on to create the problem, and then while it's screwed up, wiggle your largest fusible link. Should be the black one. Closest to the passenger strut tower.

If that doesn't do anything, try wiggling the following next:
Remainder of the fusible links.
The wires that go to the starter.
Any wire you can reach.

 

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I didn't even think about why the voltmeter wire would run through the flasher.  Not very rigorous.  But that's what the wiring diagram showed.  I thought.  Might have messed it up.  I think that there's a fuse for the running lights.  Might be overheating and opening.  Cools down and close.

The voltmeter doesn't really draw much current at all.  Almost none.  But I'd still trace the path of the wire that moves the needle.

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I'm not sure what I was looking at re my first comment.  Have to dig later.  This diagram shows a fusible link in line with the meter, as CO suggested, and a fuse.  Might be the same fuse that controls the lights.  The diagram below it on BE-33 gives some options although I can't follow the maze myself.

V Meter.PNG

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Nissan made some big changes from 76 to 77 if the wiring diagrams are correct.  I get the impression though that where the meter is tied in doesn't matter much, which makes sense.  It just offers a clue, a better one in your case.  In 76 there were all kinds of things branched off the VM source wire.  In 77, not much.  

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Thanks a lot for the help, gentlemen!

I've checked the fusible links and they are all ok. None of my fuses inside the fuse box look popped, but I'll replace them with new ones just in case.

Only things that work are headlights, electric fuel pump and cranking to start motor. I noticed that when everything shorts out, that annoying "beep" sound that indicates key in the ignition with the door open goes away, but if I disconnect my battery and reconnect it, I hear the annoying beep 4 times before it goes away again. This is is all whole all other electronics are out and volt gauge reads 0.

 

If I turn everything off and wait a few minutes, power comes back.

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The fuse wouldn't be blown, just barely connected.  Same with the fusible link.  The current flow causes heat which causes expansion which causes movement which breaks the electrical connection.  Could just be the ends not making good contact.  You could look at the diagram on BE-33 and pull the three possible fuses (all 10 A) that are shown, with the key On.  One of them might cause what you see.  Do the same with the fusible links.  

Edited by Zed Head
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Will do!

 

I was having an issue with my alternator pumping out too much juice and burning up my fusible link. I replaced the link....and now I'm having this issue. Could be either link isn't doing its job or maybe wiring inside harness got burnt?

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I'm just looking for common denominators and the black fusible link fits that category. Strongly.

The reason the headlights still work throughout this ordeal is that they have their own independent fusible link separate from everything else on the car. Rear outboard red.

The reason the engine still cranks and runs is because those functions are supplied by the other two red fusible links.

The black one supplies power to all of the "hot at all times" fuses, as well as everything powered by one side of your ignition relay (like the flashers and the gauges). So, I'm sure you've already checked the fusible links, but you have to start somewhere.

Turn your lights on to create the problem, and then while it's screwed up, wiggle the black largest fusible link. The one closest to the passenger strut tower. And also try wiggling the harness below the fusible link blocks, as well as anything area you worked on when you did the internally regulated alternator mod.

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13 hours ago, TheCrazySwede said:

I was having an issue with my alternator pumping out too much juice and burning up my fusible link. I replaced the link....and now I'm having this issue. Could be either link isn't doing its job or maybe wiring inside harness got burnt?

You have a before and an after.  This is where the old KIS principle fits perfectly.  Check that link.  Except - I think that you might actually have a different problem too, or only.  Alternators pump out juice according to demand, not all of the time.  Short circuits are high demand.  You have a short somewhere, most likely.  Find that short.

The reason for the link wiggling is to check the connection of the link to its terminal.  That's also the connection that can open up when it gets hot.  Short circuits cause a lot of heat. That's how the fusible links work, they melt to save their cousins, the harness wires.

 

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Aside from the fusible link CO mentioned, there are other common components in the circuit you are having trouble with. Check the condition of the terminals in C-9 and the point where the large white/red wire attaches to the fuse block. Look for signs of heat and corrosion. Take the connector apart and look at both parts to be sure it is clean and tight. Any sign of heat, such as a discoloration in the plastic housing or corrosion on the terminals, indicates a loose or dirty connection.

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Thanks a lot guys! You are all a great! Really appreciate the help :)

I'll feed some more background into the situation.

I recently went from an EFI L28 to a carbed L28

Only thing new, electronically, is an electronic fuel pump.

Since engine swap, I noticed my volt gauge would read past 16v. I thought maybe my regulator was bad or maybe alt. I replaced alternator with new internal regulated version. I jumped the cable that connected to the external regulator, as instructed on Atlantic Z. (Boy, that website is amazing!)

Since, I've noticed my alternator has still had issues where it outputs a lot of juice and burns my fusible link. The link that keeps burning is the one that goes from the alt and to the battery. Looking at the wiring diagram, it also connects into the voltage regulator (the old external one that now has its connections jumped.)

Another thing to note is that this issue I'm experiencing now is not new. I was having similar issues with the old alternator, too. And with the new one - but the issues sort of went away on their own. 

I'm thinking maybe there's a ground wire somewhere that isn't hooked up properly. When I did the motor swap. Or maybe my fuel pump hookup is jacked - who knows.

I'm going to do some back-tracking to try and find the sucker causing this issue, since I had no electronic issues prior to swapping my motor. Funny how I went from an electronically motored engine to a fully mechanical one and ended up with electronic issues, haha

 

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11 hours ago, TheCrazySwede said:

 I noticed my volt gauge would read past 16v.

Since, I've noticed my alternator has still had issues where it outputs a lot of juice and burns my fusible link. The link that keeps burning is the one that goes from the alt and to the battery. Looking at the wiring diagram, it also connects into the voltage regulator (the old external one that now has its connections jumped.)

Another thing to note is that this issue I'm experiencing now is not new. I was having similar issues with the old alternator, too. And with the new one - but the issues sort of went away on their own. 

Does the meter still show 16+ volts?  You might not have the S wire connected properly.  Maybe that is damaging the alternator's regulator, or passing too much current, although that should damage more than the just the fusible link,  The link is designated for quite a bit of current.

You can check both S and L at the alternator with the the T plug disconnected.  They should both show battery voltage with the key On.  I would do that first.

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It sounds like you might have two (or more) problems. Assuming that it all started when you did the engine/carb swap, start by looking for cut or pinched wires, unhooked ground between the body and the engine, etc. The first issue, voltmeter reading falling when turning on the electrical accessories, is likely a poor connection somewhere. Just good enough to power the volt meter but has a large resistance and therefore a large voltage drop with any real load. The second issue, blowing the fusible link at the alternator output, could be due to a bad alternator/regulator or a missing voltage sense at the S-terminal (as Zed Head already mentioned). Try letting the car run but don't race the engine. When the engine is at idle the alternator output won't be enough to blow the link and you will have time to perform tests.

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