Need assistance identifying OE wirinng

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Dec 5, 2022
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Indianapolis, IN
My Car
1973 Mustang Sportback
Hi everyone,

First time posting here and would appreciate any advice or diagrams anyone might have. I recently purchased my '73 as a 90% complete vehicle built specifically for road racing but capable of street driving. The original owner had a custom wiring harness built for the car, and stated it was a Painless setup but matches nothing on their website. It started having an issue with the turn signals and I've been trying to diagnose it. At the moment I have replaced the turn signal switch, and flasher relays. So I'm trying to trace the issue through the wiring. I need help identifying some of the wires coming from the original turn signal switch, also I found the 73 was different from the 71-72 and this is a non-tilt column.

The issue I am having now with this setup is the taillights and headlights turn on, the brake lights work, and when I click the turn signal the taillights get brighter but do not blink. The front turnlamps also do not turn on at all. I'm thinking either the new flashers are shot, or something is disconnected somewhere, perhaps between the switch and flasher relay.

Before removing any thing the passenger side front turnlamp and tailight turn signal would work, but the driverside didn't. The switch itself was super sloppy and still from 1973.

There is a yellow wire and a blue wire near the hazard tab. I have no idea what these go to, and they were capped on the previous switch that I am replacing. I just need help identifying those and don't understand why the wiring diagrams aren't easier to find.

Thanks all,
It might help if I included a photo of the part, red arrow pointing to specific wires.


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Maybe a photo of the fuse box so we might be able to I’d what harness you have? It might be a long shot, but we’re kinda working blind. Im assuming you have a VOM to check the wiring with. To figure out the problem, you have to trace the path of the circuit to see what is failing. If the lights in the front are not working, I would start there. The flasher needs the 4 bulbs to light to create enough current to enable the flasher. If two don’t light, it won’t work. So check and see if the bulbs are good, then check the voltage to the plugs, etc, going backwards toward what’s providing the power. Let us know what you find. Steve
Perhaps it’s an Amercian Autowire setup?
I'm not asking for any assistance with the fuse box. I just need the two wires from the turn signal switch identified. The rest of my post was just to give context to the work.

It's also not just the front. Both front and back aren't working for the turn signal, but again that's another issue for another time.

Most people here know OEM wiring, not custom stuff. So if they ask for additional information, they are looking for more context to understand your problem. There are downloadable versions of the wiring diagrams available on the site. The '73 diagram is here...
Yea, the reason I thought of the pic of the fuse box was that someone might recognize it and help I’d what brand it is, in case it’s not a Painless. Someone might have an old wiring schematic of it and be able to provide it to you.

The reason no one has identified the two wires is that we don’t have enough information to help. You can test the new switch for continuity to verify if it’s working first; most of us have found even new parts sometimes are junk.

And what did the voltage testing of the front lights provide?
My Go To place for wiring schematics for First Generation Mustangs/Cougars is Forel Publications. If you are using a turn signal for 73 Mustang that has the factory colored wiring their 73 Mustang colorized schematics is really pretty decent. It shows not just the wiring schematics, but connector and ground locations as well. Here is a link to their site, below. Very reasonably priced PDF file format.

Attached are some schematic snippets you may find helpful.


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  • MatthewBarela_C702-C702_Circuit_1973Mustang_20200929.pdf
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Thanks mrgmhale! That is exactly what I was looking for and found the old school diagram on here. I think I've identified the blue wire as going to the flasher relay and the yellow as being for the horn.

Steve McMahan, knowing what the wires are on the switch has nothing to do with the fuse box since one is factory and the other is custom. I just needed to know what the OE purpose was for those wires which was found using the wiring diagrams.

Thanks everyone
You'll also want to check your grounds, make sure the chassis is properly grounded and the ground signal is actually getting through all of the body panels and spot welds to the lights. Make sure the ground wires from the lights are well grounded. Lack of grounds can cause many electrical gremlins, including what you're experiencing.
I'll definitely look into the grounding issue. I was wrong on the blue wire coming from the turn signal switch. That is also for the horn, along with the yellow. It was the light blue that was for the flasher. After comparing the wiring I have in the car to a few different companies I think it is an American Autowire setup, but one of the smaller ones as there are only a handful circuits. I wondering if an issue could be the use of rear LED taillights and standard bulb front turnsignals. I know LEDs require a specific flasher. Mine seem to be a little loose and wobbly for my taste even after replacing them.

I'm wondering if the flasher being slightly loose could cause a grounding issue. I'll have more time tomorrow to test out all the grounds since I'm going out to the garage between meetings to mess with this.


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