Under dash intermediate power loss

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73 Grande will undergo three phase build process. Phase 1 is to make roadworthy. Phase 2 is interior/exterior restoration. Phase 3 is ++ performance.
I have a question, hoping this is a typical problem with these vehicles. I’ve got some issue with the under dash wiring. If I reach under the dash, to the right of the steering shaft, and grab those wires that run along from that point to the center, I can cause the power to be interrupted to the ignition switch, just by moving, slightly, the harness. Is there one connector or plug I should focus on that is the likely cause? All help is appreciated.
 
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73 Grande will undergo three phase build process. Phase 1 is to make roadworthy. Phase 2 is interior/exterior restoration. Phase 3 is ++ performance.
Thanks for the help. I checked that first and it’s not that. That’s what I first thought, but when I check the yellow wire in that flat harness, when I have the issue, there is no power there. I’ve already checked everything under the hood, where I did my changes to incorporate the 87 TBird SD EFI harness. All of that checks out fine. It’s past the firewall for sure.

Does anyone know where connector c211 or c211a is located? If I’m reading the wiring diagram correctly, that is the last connector from the battery + before it arrives at the ignition switch. It seems like a logical place to have a loose connection. Thanks.
 
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73 Grande will undergo three phase build process. Phase 1 is to make roadworthy. Phase 2 is interior/exterior restoration. Phase 3 is ++ performance.
I have power on the interior side of the bulkhead connector, so I don’t think that’s it. Here is a pic of the area that when I move the large yellow wire, you can hear the relays receive power, then lose power:

A4235AB4-10A7-42B2-9D4D-C2451F5B233F.jpeg
That black connector, above the tan column connector, seems to be the issue. I was trying to see if I got voltage on the large yellow wire, when I moved that wire, but I can’t get my electric meter to cooperate. Where in the harness does the large yellow wire branch to the smaller yellow that goes to the ignition switch? That also could be the issue. In the diagram I have, it appears to be a splice that creates that. But I haven’t located it yet.
 
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You might try grabbing the wires at the back of the connectors and wiggle them around a bit with some needlenose pliers. You can often find loose wires and pins without risking the connector. That's a bit harder on the main connector, but if you can get the pliers on the wire, it's an alternative.
 

Sheriff41

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That black connector doesn't look completely together. Whenever I open one of the old electrical connectors I spray it with CRC contact cleaner followed with CRC 2-26 lubricant. It makes fully reconnecting them easier.

Don't mean to sound like a CRC rep but theirs are the products I've been using.
 
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73 Grande will undergo three phase build process. Phase 1 is to make roadworthy. Phase 2 is interior/exterior restoration. Phase 3 is ++ performance.
I did do the needle nose pliers thing and that’s how I confirmed the intermediate connection. That’s a good idea. I think I can use my brake cleaner to clean it before I try to separate it. But, where does #37 Yellow merge with the smaller gauge yellow that feeds the ignition switch? Is it at the top of the fuse box? The reason I ask is because I cannot verify if moving the wire is causing a failure at the connector or at the splice, since I can’t find it.
 
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73 Grande will undergo three phase build process. Phase 1 is to make roadworthy. Phase 2 is interior/exterior restoration. Phase 3 is ++ performance.
Ok, after pulling things apart, I found the problem. Plug C-309 wasn’t fully engaged. It looks like someone had pulled it apart before and I was able to get it fully engaged after cleaning and applying dielectric grease, but it will not snap closed, if that’s the right terminology. I think I’ll put a very small plastic wire tie between the two half’s to keep it from pulling apart again. That was the issue.
 
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