Electrical diagnosis - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 25 (permalink) Old 04-26-2017, 04:51 PM Thread Starter
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Electrical diagnosis

Hi All,

1978 280z L28e 5spd.

All electrical power drops as soon as there is any charge on the system. If I turn any light on, ignition, cabin light, etc the voltage drops to zero. However, when I pull the park tail 15A fuse (top right), I can turn the head lights on and the voltage doesnt drop, but ignition and cabin lights make the voltage drops to zero. I have cleaned all the bulb sockets, every ground I can find (except the ones beneath the dash), and I have even made a ground direct to the chassis from the negative battery terminal plus the original ground from negative terminal going to the trans. All new battery cables. Alternator and starter terminals cleaned very well. I have replaced all the fusible links and checked all the fuses in the fuse box. I have cleaned many connectors as well, including the ones beneath the fusible links. I have disconnected the fusible links one by one and none made a difference. No difference when the rear lights or real side panel lights were disconnected. No blown fuses so far...

It seems like a grounding issue, but maybe Im wrong. On the fun side, while taking off the back left side light I saw a large D cell flash light wedge between the fender and cabin panel. Looks like it has been in there for 20 years lol.

Any suggestions? Thanks for your time. Electrical issues can be a pain.
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post #2 of 25 (permalink) Old 04-26-2017, 05:54 PM
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Are you watching a needle on a gauge or does every electrical device in the car die immediately, including the engine?

Put a meter on the battery and see what the battery itself is doing. Not the cable ends, the battery posts.
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post #3 of 25 (permalink) Old 04-26-2017, 10:30 PM Thread Starter
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Are you watching a needle on a gauge or does every electrical device in the car die immediately, including the engine?

Put a meter on the battery and see what the battery itself is doing. Not the cable ends, the battery posts.
I have my girlfriend in the car watching the voltage. I would turn the center dash cabin light on, so the voltage drops to zero. Then I would start pulling fuses to see if it ever goes up, which it never did. Then by myself I disconnected all the lights and it would still drop to zero whenever any electrical load is applied.

I hooked a jumper pack up to the battery and the problem was still there, so that would cancel out the battery, right?

The engine doesnt start because when I turn the key to the on position voltage drops to zero. Im sure I could jump start the starter by touching the positive and negative ends with a screwdriver at the same time, but starting the car isn't the problem right now as its in the garage.
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post #4 of 25 (permalink) Old 04-26-2017, 11:24 PM
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Sounds like dirty and/or loose connections. When current flows the corrosion heats up and expands, opening the loose connection. Yours would be pretty bad if just a light kicks things off. Check your fusible ink connections and battery cable connections. Twisting the battery cables on by hand often isn't good enough. Clean them up and clamp them down. And don't trust appearance, and don't think that WD40 is a good cleaner.
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post #5 of 25 (permalink) Old 04-27-2017, 01:46 AM
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Have you checked the fuse block connector and/or looked deep into or the back of the fuse block to see if there's a corroded or broken copper bar/wiring. Odd that the headlights would not shutdown the voltage with the upper right fuse out, but everything else from the fuse block kills it.

zippityzda
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post #6 of 25 (permalink) Old 04-27-2017, 12:01 PM Thread Starter
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Have you checked the fuse block connector and/or looked deep into or the back of the fuse block to see if there's a corroded or broken copper bar/wiring. Odd that the headlights would not shutdown the voltage with the upper right fuse out, but everything else from the fuse block kills it.
I cleaned up everything behind the fuse box. Also all the radio and hazard/defroster connectors beneath the center shifter area. I also cleaned up everything underneath the steering wheel with exception to the gauges which I couldn't reach.

These electrical issues suck. I don't know what else to do...
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post #7 of 25 (permalink) Old 04-27-2017, 12:02 PM Thread Starter
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It's an Arizona/Nevada car with zero rust. Every connection within the interior looks like new. The grounds in the engine bay needed cleaning up though.

Maybe I could buy the LED conversion kit for the tail lights. Think this would help reduce load...? Thoughts on options?

Last edited by jm280z; 04-27-2017 at 12:04 PM.
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post #8 of 25 (permalink) Old 04-27-2017, 12:28 PM
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It's an Arizona/Nevada car with zero rust. Every connection within the interior looks like new. The grounds in the engine bay needed cleaning up though.

Maybe I could buy the LED conversion kit for the tail lights. Think this would help reduce load...? Thoughts on options?
Don't spend money on LED lights unless you just want them anyway. Your problem, when you find it, is likely to be an inexpensive fix.

You didn't really answer the question regarding the voltage at the battery when things drop to zero. Your girlfriend sitting in the car isn't watching a voltmeter connected at the battery posts. Make that measurement. If the battery voltage measured at the posts is still +12V when the voltmeter in the dash goes to zero, you now know you've got a good battery. If it goes to zero, you've got a bad battery (yes I know you said it was new but ...). I suspect your battery is OK though based on being able to operate some lights in a certain configuration.

So, with a known good battery, start going down the line checking voltages with a load on the system so your dash voltmeter shows zero. Move the voltmeter to the cable connector at the battery, if you've got +12V, move to the other end of the cable and measure. At some point as you travel down the circuit you'll find zero volts. When you do, somewhere between the point with zero volts and the last +12V point is your problem. Yes, there will be branches of the circuit and it can get tedious, but that's a pretty sure fire way to find and fix your problem.

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post #9 of 25 (permalink) Old 04-27-2017, 01:40 PM Thread Starter
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Don't spend money on LED lights unless you just want them anyway. Your problem, when you find it, is likely to be an inexpensive fix.

You didn't really answer the question regarding the voltage at the battery when things drop to zero. Your girlfriend sitting in the car isn't watching a voltmeter connected at the battery posts. Make that measurement. If the battery voltage measured at the posts is still +12V when the voltmeter in the dash goes to zero, you now know you've got a good battery. If it goes to zero, you've got a bad battery (yes I know you said it was new but ...). I suspect your battery is OK though based on being able to operate some lights in a certain configuration.

So, with a known good battery, start going down the line checking voltages with a load on the system so your dash voltmeter shows zero. Move the voltmeter to the cable connector at the battery, if you've got +12V, move to the other end of the cable and measure. At some point as you travel down the circuit you'll find zero volts. When you do, somewhere between the point with zero volts and the last +12V point is your problem. Yes, there will be branches of the circuit and it can get tedious, but that's a pretty sure fire way to find and fix your problem.
Sounds good. Ill start tracking it all with a volt meter. Ill start at the battery.

The car doesnt start with a jumper pack on either, so thats why I don't think its the battery as well. Maybe I'm wrong. We'll see
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post #10 of 25 (permalink) Old 04-27-2017, 07:49 PM
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If I was you, I'd grab a multimeter and start testing continuity to ground on the positive circuit. Find where sections begin to branch from the main post, and test continuity to ground. Most will be fine, but you should notice once branch with a problem. Continue going down the branch, and you should be able to narrow it down rather quickly. When you test each branch of wiring, you can knock that off your list as it not being the problem.

I did this sort of thing when diagnosing my fuel injection issues, I would go to the ecu and test, when it failed, I'd run down the wiring with my meter until I found the spot where it was grounded, a quick patch work and I was back together.

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