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, 20213 yr by RJK

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.

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

That would mean a potential draw of around 13mA.

Can you measure the resistance at the same points?

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.