Electrical short trouble shooting

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ed B
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Electrical short trouble shooting

Post by ed B »

This all started because my Ammeter did not work. My car still seemed to run fine and most everything else worked as well. Most folks on line said that the line to the Ammeter was probably by-passed and that would explain why everything still works but the Ammeter.

I wanted to have a working ammeter, so I took a chance on a $15 (plus $15 shipping) combo meter on line. I hooked it up to test (but did not start the car as the oil pressure line was not connected yet) and pulled the switch for the headlight and was pleasantly surprised to see the needle move to the left. I also checked all my wiring and it appears that the original wire from the alternator to the ammeter and then to starter solenoid were all still in place and not by-passed. I figured that if I put an ohm meter on the two line ends (both being removed from the ammeter and not touching each other) that it would still show them to be continuous if the original wires were by-passed. I am by no means an electrician nor do I understand it very well, but that part seemed to make sense to me.

Before permanently reinstalling the combo meter, I installed an inline fuse in the white/red wire form the ammeter and then decided this was a good time to make sure I had no shorts. I knowledgeable person I work with told me an easy way was to disconnect the negative wire of the battery and then check the voltage between negative battery terminal and the unconnected negative wire and that it should read nearly zero volts and if it did not, then pull the fuses one at a time to see where the short was. That is where my main question lies. Does anyone know if this is a valid procedure to check for shorts, because I did not get very good results. The volts across the posts of the battery was 12.40 volts and the voltage for the negative post to the unconnected negative wire was 2.00 volts. I proceeded to pull each fuse until all the fuses were not connected and still no change. It gets worse.

When I reinstalled all the fuses and then connected the white/red wire from the alternator to the ammeter and then the white wire to the other post of the ammeter, to starter solenoid, the voltage from the neg battery terminal to the unconnected neg cable jumped to 12.40 volts. I checked both the white/red wire and the white wire and found that neither wired was grounded. So is it true that the reading should be nearly zero volts and if so, does anyone have any idea why my readings are so far off??
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steve_car
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Re: Electrical short trouble shooting

Post by steve_car »

Hi Ed,
I am sure your friend meant to connect up an ammeter, not a voltmeter between the negative battery terminal and battery cable. The ammeter would then normally read a little bit because of your clock and maybe other things. If it reads a lot, the method of disconnecting fuses to figure out what circuit is drawing excess current is a good way to start.
Your reading of about 12V between the battery neg post and the cable is normal also.
Steve
Last edited by steve_car on Fri Jul 15, 2016 1:14 am, edited 1 time in total.
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dbrick
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Re: Electrical short trouble shooting

Post by dbrick »

First thing, if you don't have it is to download and print the schematic from the WIKI. YOu could have a single or double row fuse box, different diagrams.

I do this all day long, tracing dc circuits, so if I'm not detailed enough on something, please ask. The method is correct. The basics of a volt meter is it tells you the difference in voltage between the two leads. On a simple circuit,(+battery to switch to light bulb to -battery) with the switch on and bulb on you will only read voltage across the battery or light bulb, where there would be a difference. If you turn off the switch, you will see 12v across the open switch, but if you remove the load, (the lightbulb), or cut a wire, the reading will be zero By disconnecting the battery negative, you are basically opening the circuit, like turning off a switch. If something is trying to draw power, it will show voltage, as the meter is jumping the open circuit you created.
Image
You need to start with everything normal and connected except the battery -terminal. Read battery voltage for reference then put one lead of the meter on the disconnected neg cable, other on the battery neg terminal. It should read zero volts or close to it. If it reads battery voltage, then something is drawing power, like the light bulb in the simple circuit above or a dead short to ground like a wire pinched. If it's something less than battery voltage is tougher, like a corroded switch or water damage.
Next step is with the meter still connected, remove any fuses that are always hot, horn, headlights etc until the meter changes or drops to zero. If no change, unplug the voltage regulator. Still no change, carefully disconnect the big wire from the alternator. Gets tougher from there, If this doesn't find it, call me tomorrow night or on the weekend and we'll work it on the phone. 90eight 591 989two

not 100% related, but excellent article from Fluke relating voltage to waterflow.
http://www.fluke.com/fluke/uses/comunid ... sevoltdrop

Dave Brisco

Take my advice, I'm not using it"

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ed B
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Re: Electrical short trouble shooting

Post by ed B »

dbrick wrote:First thing, if you don't have it is to download and print the schematic from the WIKI. YOu could have a single or double row fuse box, different diagrams.

I do this all day long, tracing dc circuits, so if I'm not detailed enough on something, please ask. The method is correct. The basics of a volt meter is it tells you the difference in voltage between the two leads. On a simple circuit,(+battery to switch to light bulb to -battery) with the switch on and bulb on you will only read voltage across the battery or light bulb, where there would be a difference. If you turn off the switch, you will see 12v across the open switch, but if you remove the load, (the lightbulb), or cut a wire, the reading will be zero By disconnecting the battery negative, you are basically opening the circuit, like turning off a switch. If something is trying to draw power, it will show voltage, as the meter is jumping the open circuit you created.
Image
You need to start with everything normal and connected except the battery -terminal. Read battery voltage for reference then put one lead of the meter on the disconnected neg cable, other on the battery neg terminal. It should read zero volts or close to it. If it reads battery voltage, then something is drawing power, like the light bulb in the simple circuit above or a dead short to ground like a wire pinched. If it's something less than battery voltage is tougher, like a corroded switch or water damage.
Next step is with the meter still connected, remove any fuses that are always hot, horn, headlights etc until the meter changes or drops to zero. If no change, unplug the voltage regulator. Still no change, carefully disconnect the big wire from the alternator. Gets tougher from there, If this doesn't find it, call me tomorrow night or on the weekend and we'll work it on the phone. 90eight 591 989two

not 100% related, but excellent article from Fluke relating voltage to waterflow.
http://www.fluke.com/fluke/uses/comunid ... sevoltdrop
Thanks Dave, I may take you up on that this weekend (after I read the article and check my results again). Glad to hear his method was correct, but not happy hear since it shows a problem.

Your diagram shows the way I had done the test. Before I disconnected anything and before I replaced the ammeter, I was reading the full battery voltage as per your diagram. My brother told me I had a dead short. It went down to about 5.5 volts when I disconnected the fuse for the common (lower right on my fuse box -with 8 fuses). This is supplied by the white wire from the starter solenoid which is connected direct to the positive of the battery. I tested the white wire to a ground with my ohm meter and it did not show any continuity, so I figured that meant it was not shorted - so a mystery to me. When I removed the fuse for the horn the voltage went down to 2 (with the common already disconnected). The horn relay was pretty corroded so I cleaned up the contacts and the voltage went down to 2. Could not get it less than 2. I later found a wire that was broken at the base of one of the connections on the voltage regulator and one that was broken within the insulated casing. I ordered a new one the same time as the horn relay (that is another story - not thrilled with the communications and response from the vendor - they still have not sent anything out yet after I ordered it monday). So the results in my first post above are with the voltage regulator disconnected. I did not think it would make a difference, but I could be wrong on that.
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ed B
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Re: Electrical short trouble shooting

Post by ed B »

ed B wrote:
dbrick wrote:First thing, if you don't have it is to download and print the schematic from the WIKI. YOu could have a single or double row fuse box, different diagrams.

I do this all day long, tracing dc circuits, so if I'm not detailed enough on something, please ask. The method is correct. The basics of a volt meter is it tells you the difference in voltage between the two leads. On a simple circuit,(+battery to switch to light bulb to -battery) with the switch on and bulb on you will only read voltage across the battery or light bulb, where there would be a difference. If you turn off the switch, you will see 12v across the open switch, but if you remove the load, (the lightbulb), or cut a wire, the reading will be zero By disconnecting the battery negative, you are basically opening the circuit, like turning off a switch. If something is trying to draw power, it will show voltage, as the meter is jumping the open circuit you created.
Image
You need to start with everything normal and connected except the battery -terminal. Read battery voltage for reference then put one lead of the meter on the disconnected neg cable, other on the battery neg terminal. It should read zero volts or close to it. If it reads battery voltage, then something is drawing power, like the light bulb in the simple circuit above or a dead short to ground like a wire pinched. If it's something less than battery voltage is tougher, like a corroded switch or water damage.
Next step is with the meter still connected, remove any fuses that are always hot, horn, headlights etc until the meter changes or drops to zero. If no change, unplug the voltage regulator. Still no change, carefully disconnect the big wire from the alternator. Gets tougher from there, If this doesn't find it, call me tomorrow night or on the weekend and we'll work it on the phone. 90eight 591 989two

not 100% related, but excellent article from Fluke relating voltage to waterflow.
http://www.fluke.com/fluke/uses/comunid ... sevoltdrop
Thanks Dave, I may take you up on that this weekend (after I read the article and check my results again). Glad to hear his method was correct, but not happy hear since it shows a problem.

Your diagram shows the way I had done the test (BTW I had the printed wiring diagram from wiki with me for the whole test). Before I disconnected anything and before I replaced the ammeter, I was reading the full battery voltage as per your diagram. My brother told me I had a dead short. It went down to about 5.5 volts when I disconnected the fuse for the common (lower right on my fuse box -with 8 fuses). This is supplied by the white wire from the starter solenoid which is connected direct to the positive of the battery. I tested the white wire to a ground with my ohm meter and it did not show any continuity, so I figured that meant it was not shorted - so a mystery to me. When I removed the fuse for the horn the voltage went down to 2 (with the common already disconnected). The horn relay was pretty corroded so I cleaned up the contacts and the voltage went down to 2. Could not get it less than 2. I later found a wire that was broken at the base of one of the connections on the voltage regulator and one that was broken within the insulated casing. I ordered a new one the same time as the horn relay (that is another story - not thrilled with the communications and response from the vendor - they still have not sent anything out yet after I ordered it monday). So the results in my first post above are with the voltage regulator disconnected. I did not think it would make a difference, but I could be wrong on that.
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dbrick
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Re: Electrical short trouble shooting

Post by dbrick »

Take a look at the small harness from the main body harness down to the starter under the master cylinder. There is a plug that goes bad and the wire can get pretty ratty from heat. Call anytime, got to get it fixed so you can get to Watkins Glen!

Dave Brisco

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ed B
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Re: Electrical short trouble shooting

Post by ed B »

I'll look at it tomorrow. I did look at that area earlier and found that the PO had added a twist nut connector to the green and yellow horn fuse and relay wire, that is supposed to go to the solenoid, but they re-routed it directly to the positive side of the battery. I checked that wire and did not find any shorts. It a very frustrating process when you don't get any success or answers.
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Re: Electrical short trouble shooting

Post by ed B »

Some trouble shooting pictures.
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Re: Electrical short trouble shooting

Post by krispoulin »

dbrick wrote:First thing, if you don't have it is to download and print the schematic from the WIKI. YOu could have a single or double row fuse box, different diagrams.

I do this all day long, tracing dc circuits, so if I'm not detailed enough on something, please ask. The method is correct. The basics of a volt meter is it tells you the difference in voltage between the two leads. On a simple circuit,(+battery to switch to light bulb to -battery) with the switch on and bulb on you will only read voltage across the battery or light bulb, where there would be a difference. If you turn off the switch, you will see 12v across the open switch, but if you remove the load, (the lightbulb), or cut a wire, the reading will be zero By disconnecting the battery negative, you are basically opening the circuit, like turning off a switch. If something is trying to draw power, it will show voltage, as the meter is jumping the open circuit you created.
Image
You need to start with everything normal and connected except the battery -terminal. Read battery voltage for reference then put one lead of the meter on the disconnected neg cable, other on the battery neg terminal. It should read zero volts or close to it. If it reads battery voltage, then something is drawing power, like the light bulb in the simple circuit above or a dead short to ground like a wire pinched. If it's something less than battery voltage is tougher, like a corroded switch or water damage.
Next step is with the meter still connected, remove any fuses that are always hot, horn, headlights etc until the meter changes or drops to zero. If no change, unplug the voltage regulator. Still no change, carefully disconnect the big wire from the alternator. Gets tougher from there, If this doesn't find it, call me tomorrow night or on the weekend and we'll work it on the phone. 90eight 591 989two

not 100% related, but excellent article from Fluke relating voltage to waterflow.
http://www.fluke.com/fluke/uses/comunid ... sevoltdrop

Apologies for highjacking this, but I am also trouble shooting some electrical issues. I found this thread while searching the forum and performed the above methods. There were no changes in the voltage reading (neg. terminal / disconnected neg. cable) when I removed any/all of the fuses, but when I disconnected the voltage regulator, the reading went down to practically zero (a few mV).

So, does this mean my issue could be as simple as my VR has gone bad or is this just narrowing down the search? If only narrowing down the search, what do I do next?

(Recently smoked my ammeter. It's currently "replaced" with a 30A fuse. I think I read that blowing the ammeter could likely have ruined my VR, too.)

Thank you!

Edit: I wanted to mention that my VR clicks when I connect the battery. I don't remember that happening before very recently.
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ed B
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Re: Electrical short trouble shooting

Post by ed B »

You are welcome to the thread if it helps you, no sense starting from scratch. Most of this stuff is over my head, but Dave was able to walk me through a lot of it and help me out. Maybe he or someone else can help answer your questions as well.
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Re: Electrical short trouble shooting

Post by Curtis »

This will show you how your charging circuit works.

http://www.kendo-usa.org/datsun/datsun_ ... _69_70.pdf
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Re: Electrical short trouble shooting

Post by krispoulin »

dbrick wrote:You need to start with everything normal and connected except the battery -terminal. Read battery voltage for reference then put one lead of the meter on the disconnected neg cable, other on the battery neg terminal. It should read zero volts or close to it. If it reads battery voltage, then something is drawing power, like the light bulb in the simple circuit above or a dead short to ground like a wire pinched. If it's something less than battery voltage is tougher, like a corroded switch or water damage.
Next step is with the meter still connected, remove any fuses that are always hot, horn, headlights etc until the meter changes or drops to zero. If no change, unplug the voltage regulator. Still no change, carefully disconnect the big wire from the alternator. Gets tougher from there, If this doesn't find it, call me tomorrow night or on the weekend and we'll work it on the phone. 90eight 591 989two

not 100% related, but excellent article from Fluke relating voltage to waterflow.
http://www.fluke.com/fluke/uses/comunid ... sevoltdrop

Today I performed the above troubleshooting method again, just to get my head back into this after a few days away from the car.

This time, with the 30A fuse (labeled Lights / Ammeter) out and the voltage regulator disconnected, the voltage slowly drops down to just over 1 V. Is the fact that the voltage doesn't drop instantly, but decreases slowly, a clue as to where the short might be?
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Re: Electrical short trouble shooting

Post by krispoulin »

Disconnecting the power to the radio eliminated that slow decrease in voltage. (I know just enough about electricity to get in trouble, so I'm guessing there are capacitors in there that slowly dissipate after power source is removed.)

It didn't prove to be the source of the short, though, so I'll get back to hunting that down soon.
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Re: Electrical short trouble shooting

Post by krispoulin »

Sorry to keep asking essentially then same question, but after almost 6 hours at this today, I'm getting a little confused.

One quick scenario:
12V to ground except when I pull 30A fuse AND disconnect voltage regulator. If I put either one of them back in circuit, the 12V returns.
Does this narrow it down for anyone?

I'm getting back to this right now and will keep hunting, but any words from those with more experience at this would be greatly appreciated.

Thanks!
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Re: Electrical short trouble shooting

Post by Linda »

While we are waiting for someone with real knowledge I will just mention the 2 things i think of with electrical circuits: clean connectors and clean/secure all grounds. The VR grounds via the screw holes into the body, and it is a good idea to add a ground wire as well. Maybe try cleaning that. Then clean all other grounds too, at least with a shot of Electronic spray cleaner, like CRC, after removing neg battery cable.
Maybe the cavalry will show up :?

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