tracing a possible electrical short

[RJK]

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 October 17, 2021 by RJK

[heyitsrama]

I would identify the circuit that had the cooked insulation and start there.  Your DVM might have a beep mode where it detects continuity within a wire, you can use this to attempt to see if a particular wire has a short.

 

Pictures? 

 

[dutchzcarguy]

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!

[SteveJ]

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?

[RJK]

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!

[SteveJ]

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.

[Captain Obvious]

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?

[Zed Head]

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.

[RJK]

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!

[Captain Obvious]

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?

[qz16]

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.

 

 

[qz16]

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

 

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

 

 

 

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

 

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