SteveJ

If I was doing the wiring, provided the following conditions are true. There is no need to make the wiring easily reversible. There is no need for the wiring on the old ignition circuit. You are just needing the main 12VDC switched source for the Haltech. Here is my thinking. Buy two latching 1 pin connectors from Vintage Connections. Buy some extra male and female pins for this type connector. Buy an open barrel crimping tool if you don't already have one. (Amazon link for a crimping tool) Buy an inline fuse holder. Identify which BW wire at the engine harness/dash harness is the one on the ignition circuit, noting the connector and position. (I'll detail how to identify the wire later.) Back out the pin for that BW wire from the connector on the engine harness side. (Vintage Connections tool) Cut a short length of wire, maybe 3 to 4 inches. On one end, crimp a female pin onto the wire and insert into the female 1 pin connector. On the other end of the wire, crimp a pin that matches the type you backed out in step 5, and put it into the engine harness connector to replace the wire you backed out. Strip, crimp the pins, and put the male and female connectors onto the wires of the inline fuse holder. On pink Haltech wire, crimp on a pin and attach the remaining connector to plug into the one on the end of the inline fuse holder. Use no larger than a 10A fuse in the inline fuse holder.

Now THIS is something I can work with. The good news is that this wire is not shorted to ground. While 15 ohms is low resistance, you could be reading through other branches on the circuit. That resistance would equate to less than 1A of current when energized. You just don't want the end of the wire floating around in the engine bay where it could short while you're driving.

The connector changed to a round connector.

It's possible. The problem with most relay installations is that the owner hacks into the wiring. Now, @Captain Obvious provided me key information that would allow a relay kit to be created for the 78 that does not involve cutting any wires and could essentially be tucked away nicely. Unfortunately I don't have an unmolested 78 at my disposal to see that scheme to fruition.

So tell us the OD and height. My mind is formulating evil plans as we type!

@gotham22Do you have a micrometer that you could measure the dimensions of the tab in better condition?

Yep, that's a problem. It melts down/wears down because of the heat from all of the current flowing through the switch. Don't try to get a 3D printed version. It will only melt down faster than the original piece. @Captain Obvious You're better with plastics than I am. Can you think of a hard plastic that could be drilled/milled/lathed into a replacement?

So that would beg the question about where he tapped into the ignition circuit to begin with since he said he didn't know how the circuit was routed. More questions than answers.

That's a good piece of the puzzle. Did you have the points ignition or a first gen Pertronix in the car when he did that? The worst case scenario is that I can probably walk you through making a replacement for that wire that you can "graft" into the dash harness. You know you wanted to have a reason to pull the dash. 😉

That's why I'm trying to get more information on the testing methods. There could be a hot spot from corrosion at the black/white, possibly short duration short circuit in the past, or someone tapping into that circuit downstream, increasing the load on the circuit.

What meter are you using? Does it only have one lead? Take photos of your testing setup and don't skimp on details.

I don't know if I've ever heard anyone say they weren't happy with Ron's work. @zclocks

Get resistance readings, not tone. I think I have mentioned in one of my YouTube videos that you can get a "false positive" with a tone. Also please be more descriptive with your measurement techniques and testing procedures. Where you put your probes is important. Here's an example, for the B/Y wire, it goes through the solenoid and on to ground. If the wire is connected at the solenoid, you could read the resistance of the wire in the solenoid when doing a measurement. Depending upon how your meter is set up, that may set off the tone. If you're not too experienced with multimeters, these videos may help.

Those wires feed fuses, but technically, there is no fuse from the alternator to the ignition coil positive. The wiring diagram shows a fusible link at the alternator, but I don't recall ever seeing that in the wild on a 240Z. I could have used one there 27 years ago.

You might want to read through this thread:

I hate dealing with those.

Have you looked in the passenger footwell area? If you have a big aftermarket evaporator, it blocks you from easy access.

Several good excuses to come down to the Atlanta area in your Z: The Mitty - Vintage racing weekend at Road Atlanta on the last weekend in April https://hsrrace.com/themitty2022/ Fall Historics - More HSR vintage racing at Road Atlanta https://hsrrace.com/fallhistorics2022/ Z Nationals - I usually just go to the track day on Friday when the show is at Z1 Motorsports. With their expansion, they don't have space on their grounds for more cars. This year track day is at Atlanta Motorsports Park. https://www.znationals.com/

I told you to disconnect the engine harness from the dash harness and re-test. If you test the engine harness and find no short, you can suspect the short is in the dash harness. That black/white certainly got overheated. You are quite possibly correct that it is on the ignition circuit branch out to the ballast resistor. You can verify that, but it takes using a multimeter with long leads. By the way, this can be handy for that purpose: https://www.harborfreight.com/electrical/electrician-s-tools/30-ft-retractable-test-leads-58024.html (Note, they are only designed for low current applications.)

Thank you. I'm fortunate to live in a Z oasis in the Southeast. Yes, the 260Z was at Birmingham. I drove it out to Barber for Parade Laps & to watch autocross, and I took it to Little Talladega to shoot photos and video for the second track day. I had it at the People's Choice show, too. It was parked next to the blue 280Z with the RB swap. My car is on the bottom row in the middle, just above the ZCCA logo..

I gave you instructions on how to figure out if the short was in the dash harness or engine harness. After that, you look for damage to the harness or evidence of tampering.

The closest I could find with a quick search was here: https://zcarsource.com/seat-upholstery-set-240z-70-73-butterscotch-vinyl-new/

For the IAC, if you need a switched source, the black/white at the voltage regulator is a better choice. That wire is electrically the same as the black/white that goes to the ballast resistor. The only thing is that the wire at the VR goes through the fuse box before going out to the VR. If the insulation ever gets damaged, the fuse should give some protection to the rest of the wiring.

Who asked about the IAC current draw? The fuse is to protect the wiring. If you have a short in an unfused circuit, you could have an electrical fire unless you are lucky and take out the fusible link first. I gave you a safer alternative because the wiring is fused on that branch.