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tracing a possible electrical short


RJK

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Hi all, I suspect I have an electrical short somewhere, and could use some help going about how to find it. I recently was replacing my alternator, and initially inverted the + and ground wiring to the alternator(combination of not labeling the stock wires, swapped location of posts on new alternator, and my being a bonehead). Upon connecting the battery, the fusible link did not blow, but cooked the insulation off the wire. The wiring lived for maybe 6-10 seconds before I realized what was happening and disconnected the battery; I cannot be sure I didnt cook some insulation off a wire elsewhere in the car.

 

 I have replaced the fusible link, and pulled the wrapping off the section of wiring from the starter to the alternator-I saw no shorts or burned insulation. With the alternator wires completely disconnected, when I attempt to reconnect the battery terminals, I get sparking that definitely does not look normal. I need to go about testing the system in a manner that is A-safe, B-logical, and C-not so wasteful. In particular, I don't want to keep blowing fusible links, if possible. I have a DVM, sufficient spare wire, fittings, and tools to repair and rebuild wires. Thanks so much for any possible culprits, starting points, etc.

Edited by RJK
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If your not so comfortable in looking for short circuits you could use some automatic fuses on places where you have the fusible links.. or normal fuses in a fuseholder.. it's much cheaper to blow some fuses then blow these fusible links!

NEVER use bigger fuses always the right amperages or less!

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The first and most important thing to do in order to get help tracking down a possible short is to tell us the year of your car! With that, we can tell you specific things to try. 

Did you get a new fusible link to replace the damaged one?

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Thanks folks! Pics and specs here:

 

-1972 240z, MSD ignition, otherwise stock.

-I have replace the fusible link. I also bought one spare. I bought the nylon connectors, so I have a T connector for the new alternator. I still need to wire the "cross" pin of the alternators' T connector to the alternator harness.

-when I was removing the wiring from the starter, the threads on the +12V post seized, so I replaced the starter with a new Delco unit.

-I unbundled the wrap of the wiring from the starter to the alternator, so I could see any possible short there. All the wires look undisturbed. My next guess for a short location would be at the ignition switch, but that is just a guess, and why I'm here! 

See attached pics; thanks for the help!

IMG_1822.JPG

IMG_1823.JPG

IMG_1824.JPG

IMG_1825.JPG

IMG_1826.JPG

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4 hours ago, RJK said:

Thanks folks! Pics and specs here:

 

-1972 240z, MSD ignition, otherwise stock.

-I have replace the fusible link. I also bought one spare. I bought the nylon connectors, so I have a T connector for the new alternator. I still need to wire the "cross" pin of the alternators' T connector to the alternator harness.

-when I was removing the wiring from the starter, the threads on the +12V post seized, so I replaced the starter with a new Delco unit.

-I unbundled the wrap of the wiring from the starter to the alternator, so I could see any possible short there. All the wires look undisturbed. My next guess for a short location would be at the ignition switch, but that is just a guess, and why I'm here! 

See attached pics; thanks for the help!

Now THIS is the way to frame a request for help. 

If you are worried about having a short that will fry the fusible link, there are several ways to assess the risk.

  1. Remove the fusible link and use a voltmeter and measure from the wire coming off the solenoid (for the fusible link) to the positive battery cable. If there is not a short, that should read 0V
  2. Measure resistance between the wire coming off the solenoid (for the fusible link) to ground. If it reads less than 10 ohms, you have a significant load or short. If you have less than 1000 ohms, you will have a pretty good battery drain.
  3. Use a 12VDC test light between the wire coming off the solenoid (for the fusible link) to the positive battery cable. If it lights up, you have a short. (Be sure to test the test light across the battery terminals to make sure you have a good bulb.)

I would post photos with examples, but I have a gas tank sitting in the way on the garage floor right now.

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Steve has this well in hand, but let me just add a couple thoughts...

On 10/16/2021 at 9:00 PM, RJK said:

Upon connecting the battery, the fusible link did not blow, but cooked the insulation off the wire. The wiring lived for maybe 6-10 seconds before I realized what was happening and disconnected the battery; I cannot be sure I didnt cook some insulation off a wire elsewhere in the car.

In theory, that's exactly what the fusible link is supposed to do. It's supposed to cook and burn out before the wiring inside the harness cooks and burns out. The copper wire inside the fusible link is smaller than the wiring in the harness and it's intended to fry open before the wires inside the harnesses do.

Point is... In theory, if the system was designed properly and fusible link did what it was supposed to do, then, in theory, rest of the wiring "should" be ok. In theory.

Second thing is you said "when I attempt to reconnect the battery terminals, I get sparking that definitely does not look normal." - If you have an Ammeter with a 10A range, you could quantify the amount of that current before you get all worked up about the spark. By that, I mean... If you haven't measured the current, you don't really know if it's a problem or not. You could be chasing a non-problem.

Maybe (just maybe) the spark WAS just a normal in-rush thing charging all the systems of the car and once that initial hit was gone, it may drop down to the milli-amp or micro-amp range? Connecting an Ammeter in series with the fusible link would give you an idea of how much current you're talking about. Half second ought to be enough for your meter to get a reading?

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On 10/16/2021 at 6:00 PM, RJK said:

when I attempt to reconnect the battery terminals, I get sparking that definitely does not look normal.

You're implying big sparks, with snapping noise.  The clock is the only thing that is on when the key is off, I believe.

A simple "amp meter" is a 5 amp fuse in an inline fuse holder.  Connect it to the negative terminal and hold the other end on the negative post.  If it blows you have over 5 amps flowing.  Or use a circuit breaker "fuse", mentioned in a different thread.

You could also remove the clock fuse and use a test light between negative terminal and post.  It should not light.

Anyway, only the clock should be pulling amps.

Electrical can be frustrating.  You can't see it.

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Thanks much, folks. I appreciate this! As soon as I get a chance, I'll pop a 5A fuse in line with the ground terminal, and assuming it doesnt blow, measure my current on a DVM. That's exactly the kind of "first step" I was looking for!

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And I would recommend that (since this whole thing started with an alternator replacement) you do your first fuse test with the alternator disconnected. Maybe the voltage regulator unplugged as well?

If the low amperage fuse (or meter) survives that first step, then plug the voltage regulator in and try it again. And keep working in steps like that?

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I have a 1973 so I am not sure that your set up is exactly the same as mine. 

Having said that, I believe up under the passenger side you will see a bank of connector blocks.  Your wire harness is separated into a few pieces.  If memory serves me there are 4 larger connectors with 10 pins in each.  There is a also a few individual connectors.  The 4 larger connector blocks (black white green and blue) connect the dashboard  or instrument harness, hatch area harness, to the engine harness.  The relays for things like the horns, and wipers are located on the passenger kick panel and are connected with individual connectors.  There is also an “L” shaped connector that connects the hazard relay. Another for the main power from the battery to the fuse block.  Can’t remember other detail at this moment.

 Anyway, I would start by taking copious notes and labeling the various individual connectors, and I would disconnect the battery and leave it disconnected until the issue is resolved.  Write down the order of the 4 larger connectors, they are colored and I believe keyed.  I think there is a good chance that the problem is one of the relays, but that is just a guess from some of the things that you posted.  Don't conclude anything based on guesses.  You can troubleshoot this to the point where you can eliminate the actual issue, and not waste time or money replacing items that are not damaged.

After labeling all of the connectors I would disconnect them from the engine harness and see if the short still exists.  If it is gone than reconnect one connector at a time to determine which harness has the issue.  If the short is not removed then I would disconnect the individual connectors.  If everything is disconnected and you still have an issue then the problem is likely to be in the engine compartment.  I say likely because there is always the possibility that a previous owner has added or modified something.

Once you figure out which area has the issue then I would get a schematic and track it down.  I have a laminated schematic in the garage and will post a picture when I can, a bit later today.  If you do not have a meter or short detector you need to buy one,  you can get something fairly inexpensive.

You may be overwhelmed, frustrated and confused right now.  Electrical systems are similar to plumbing, without some of the sloppy mess.  Go slow, take your time do one thing at a time and draw a conclusion.  If your not sure post the question.  Write down what you are doing and what the results are so the forum can help you.  It may take a while but you will get through it.

 

 

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So, I took a few snap shots from the service manual:

 fsm engine harness.jpg

top right corner depicts the 4 connector blocks that are tied together.  Pin out detail is on right.

 

fsm instrument Harness.jpg

 

fsm body harnessjpg.jpg

 

Below is a lamenated schematic that I think I got from Bonzai Auto works

schematic.jpg

 

Hope it helps.  Let me know if I can help.

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On 10/17/2021 at 2:24 PM, SteveJ said:

Now THIS is the way to frame a request for help. 

If you are worried about having a short that will fry the fusible link, there are several ways to assess the risk.

  1. Remove the fusible link and use a voltmeter and measure from the wire coming off the solenoid (for the fusible link) to the positive battery cable. If there is not a short, that should read 0V
  2. Measure resistance between the wire coming off the solenoid (for the fusible link) to ground. If it reads less than 10 ohms, you have a significant load or short. If you have less than 1000 ohms, you will have a pretty good battery drain.
  3. Use a 12VDC test light between the wire coming off the solenoid (for the fusible link) to the positive battery cable. If it lights up, you have a short. (Be sure to test the test light across the battery terminals to make sure you have a good bulb.)

I would post photos with examples, but I have a gas tank sitting in the way on the garage floor right now.

Ok, so I just tested: 

  1. Remove the fusible link and use a voltmeter and measure from the wire coming off the solenoid (for the fusible link) to the positive battery cable. If there is not a short, that should read 0V--- I DO NOT HAVE THE BATTERY LIVE, SO I DIDNT READ THIS IN VOLTAGE. IN OHMS, IT'S OVER 1K.
  2. Measure resistance between the wire coming off the solenoid (for the fusible link) to ground. If it reads less than 10 ohms, you have a significant load or short. If you have less than 1000 ohms, you will have a pretty good battery drain. -THIS READS AT 2.6OHMS TO 3.1 OHMS, DEPENDING ON WHICH CHASSIS LOCATION I USE. BUT CONSISTENTLY IS AROUND 3 OHMS.
  3. Use a 12VDC test light between the wire coming off the solenoid (for the fusible link) to the positive battery cable. If it lights up, you have a short. (Be sure to test the test light across the battery terminals to make sure you have a good bulb.)--DONT HAVE ONE OF THESE YET.

So I think it's safe to say I have a short somewhere, or a load? Should my next area of focus be at or near the ignition switch?

 

(PS-forgot to mention that for now, as a baseline, I am working with battery disconnected, alternator disconnected, and fusible link also disconnected. 6 pin VR harness adapter is seated)

 

Edited by RJK
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I may have not given you a clear methodology for testing. I went out to my car to verify.

For the resistance test, measure from the fusible link connector in the engine bay wiring harness to ground. See the photo.

20211018_182105.jpg

I tested my car, placing the other test lead on ground. It was about 880 Ohm.

20211018_165730.jpg

That would mean a potential draw of around 13mA.

Can you measure the resistance at the same points?

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Thanks Steve-yes, I was measuring the same test points it looks as though you were measuring. To clarify, at the fusible link that connects the starter to the car wiring(single pin molex style connector), I disconnected the fusible link, and measured with positive probe at the CAR'S wiring harness side of that connection(in your image, it would be the old and faded side of the connection), and the ground probe at numerous chassis points. With the battery disconnected, I am averaging around 3 ohms.

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Is the alternator connected or not?

If it was connected, try the test with the alternator disconnected. 

If the readings are still low, post the year of your car so I can tell you what to disconnect at the body/engine harness junction to try to isolate the short better.

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11 hours ago, SteveJ said:

Is the alternator connected or not?

If it was connected, try the test with the alternator disconnected. 

If the readings are still low, post the year of your car so I can tell you what to disconnect at the body/engine harness junction to try to isolate the short better.

alternator is disconnected until i know i'm not sucking current or creating a fire somewhere. so not connected on reading.

 

1972 240z, only upgrade is MSD ignition, wiring appears to be stock.

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So locate the two connectors circled. They will be around the dash on the passenger side. Disconnect them and repeat the resistance measurements in the engine bay. Disconnecting those wires should isolate the wires from the alternator and battery in the engine harness from the dash harness. 

Also check resistance from the white/red wire at the alternator to ground.

image.png

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Ok, more diagnostics, and pics.

 

I found the molex connector in the PS footwell that links to the fusible link wire. In the pics, see a 4 pin connector; when I pull it, my impedance goes OL. Obviously, I need to figure out where this traces to, but this is progress.

IMG_1861.JPG

IMG_1862.JPG

IMG_1863.JPG

IMG_1864.JPG

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And as Captain Obvious, I'm compelled to point out that resistance readings like what you're taking are dependent on switch positions. By that, I mean...

If something like your headlight switch is on, you'll read low resistance from the +12 volt side of the harness to ground, and it's perfectly normal. Same thing goes for the ignition switch. And the inspection lamp, etc.

In other words... when you're taking resistance readings between hot and ground, you need to be double dog sure that all the accessories are off and the key is in the OFF position.

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R - Right Headlight

R/Y - Left Headlight

R/L - Inspection Light

G/L - Front/Side Marker Lights

Of those 4 wires, only the R/L wire doesn't go through a switch.

Pull the 4th fuse down on the right side of the fuse block. Reconnect the 4 pin connector and test again.

image.png

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On 10/19/2021 at 8:00 AM, SteveJ said:

So locate the two connectors circled. They will be around the dash on the passenger side. Disconnect them and repeat the resistance measurements in the engine bay. Disconnecting those wires should isolate the wires from the alternator and battery in the engine harness from the dash harness. 

Also check resistance from the white/red wire at the alternator to ground.

image.png

More info:

1-disconnecting this single pin, white>white red connector at the PS footwell ALSO drops impedance to OL. 

2-impedance from red/white alternator wire to ground is 2.6 ohms.

3-removing 4th fuse down on right side has no effect on the ~3ohm reading I get from fusible link wire to ground.

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36 minutes ago, RJK said:

More info:

1-disconnecting this single pin, white>white red connector at the PS footwell ALSO drops impedance to OL. 

2-impedance from red/white alternator wire to ground is 2.6 ohms.

3-removing 4th fuse down on right side has no effect on the ~3ohm reading I get from fusible link wire to ground.

Photos would help to show what was disconnected and where you were measuring. 

Basically, I'm trying to get you to isolate the circuit. Here is a simplified view using the wiring diagram on page BE-5

image.png

The red line represents the white/red wire. The blue line represents the white wire.

The wires do not intersect directly. The white wire goes from the fusible link through its connector at the junction of the engine harness and dash harness. From there it has two branches. One goes to the fuse box, and the other goes to the ammeter.

The white/red wire goes from the alternator through its connector at the junction of the engine harness and dash harness. From there it goes to the fuse box and comes back out where it branches. One branch goes to the ignition switch, and the other goes to the ammeter. 

The ammeter is the spot where the white and white/red wires intersect. 

Again, make sure you have BOTH connectors disconnected. Make sure the white/red wire is disconnected from the alternator. Also make sure the sense wire is disconnected from the alternator. If you are going to find the short, isolation is critical. At that point, test for the white wire to ground and white/red to ground.

If you get OL readings on the white/red, start reconnecting one at a time and re-test.

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SteveJ-AMAZING info here. very clear. thank you SO much. it's complicated stuff; I had to read your entire paragraph 2-3 times before I could visualize what you were describing, but now I think I can. In essence, it seems as though the white wire and white/red wire run thru a network, that includes the fuse box, ignition switch, and ammeter when they travel into the cabin. by disconnecting the molex connectors, and the connections in the engine bay, I am breaking apart that network. now, by testing at the connecting points, i can see where the network is grounding out(or close to it). very clearly put. I'll implement this procedure today, and report back.

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23 minutes ago, RJK said:

SteveJ-AMAZING info here. very clear. thank you SO much. it's complicated stuff; I had to read your entire paragraph 2-3 times before I could visualize what you were describing, but now I think I can. In essence, it seems as though the white wire and white/red wire run thru a network, that includes the fuse box, ignition switch, and ammeter when they travel into the cabin. by disconnecting the molex connectors, and the connections in the engine bay, I am breaking apart that network. now, by testing at the connecting points, i can see where the network is grounding out(or close to it). very clearly put. I'll implement this procedure today, and report back.

I think you are understanding the testing concept now.

Now imagine a 3 inch bundle of wires, all of them are grey. One of those wires was not run correctly, and you have to find out which one it is. That really helps improve diagnostic skills. I find wiring diagrams and wiring harnesses with color codes to be SO much nicer. LOL 

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