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1966 Mustang GT Fastback 289 4BBL
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Discussion Starter · #1 · (Edited)
Working on my 1966, I did some overhaul in the engine compartment, and also replaced the reverse light switch. I put everything back together, and now my tail lights will not shut off. I assume I have a short or something to that effect and will probably eventually figure it out, but has anyone else had this problem and might know where to start looking that might save me some time? Thanks.
 

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1966 Mustang GT Fastback 289 4BBL
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Discussion Starter · #5 ·
I wasn't sure they were related. The backup lights still do not work either after replacing the switch. I think there may be grounding issues throughout the car. I am cleaning up a previous 'restoration' that was more concerned about look than correctly doing things. They do not appear to share a ground, but I will check.
Strange. The parking lights should feed right off of the light switch with nothing to do with the backup lights. Do they share a ground?
 

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Do you have the ground wire from the engine to the firewall? Without it something might be trying to ground through the parking lights. Take some battery jumper cables and clamp one of them from the negative battery post to a good body ground and see if that cures it. If it does your missing the ground from the engine to the body
 

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1966 Mustang GT Fastback 289 4BBL
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Discussion Starter · #7 ·
Thanks for the suggestion, I just tried that with the jumper cable and there was no change. I also, to eliminate one more thing, unplugged the reverse switch as well. I think my solution is going to be starting at the taillight and working backwards.
 

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Thanks for the suggestion, I just tried that with the jumper cable and there was no change. I also, to eliminate one more thing, unplugged the reverse switch as well. I think my solution is going to be starting at the taillight and working backwards.
If you have a voltmeter you do what is called a "voltage drop test". Its fast and it will find the spot of resistance quickly. I've on a few Ford forums and I explain how to do this at least once a week. I need to save it so I can cut and paste it. This time I will just give you this guy's video because I'm going to go work on my own cars. You master this and you can solve 90% of your electrical problems.

 

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1966 Mustang GT Fastback 289 4BBL
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Discussion Starter · #10 ·
Thanks for the info on this. I will study this out...I'm gonna have time because I just about took my finger off with my chainsaw today doing yard work. No car work for me for a few weeks!
If you have a voltmeter you do what is called a "voltage drop test". Its fast and it will find the spot of resistance quickly. I've on a few Ford forums and I explain how to do this at least once a week. I need to save it so I can cut and paste it. This time I will just give you this guy's video because I'm going to go work on my own cars. You master this and you can solve 90% of your electrical problems.

 

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1966 Mustang GT Fastback 289 4BBL
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76 Posts
Discussion Starter · #12 ·
Well, after much frustration and head scratching, I have found my issue. I finally stopped chasing wires and tore into the wire harness as a last effort. Looks like I need a new wire harness and I am surprised that the lights shorting and coming on was my only problem. Looks like one of the wire got HOT at one point and melted itself inside the harness. I think replacing it should do the trick! Thanks everyone for your help.

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You can fix that harness. . Cutout the burn sections. Slide some heat shrink tubing on there. Then solder in the new wires and then shrink the tubing. That would be a good repair. What I would do first is find out why that wire melted so you don't do the same thing with your new harness. Something is drawing more current than the wire can handle or the insulation wore away and the wire shorted against another wire. Its not a ground problem because that reduces current. I would separate them all out and see if one of those wires is getting hot.
 

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1966 Mustang GT Fastback 289 4BBL
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Discussion Starter · #14 ·
You can fix that harness. . Cutout the burn sections. Slide some heat shrink tubing on there. Then solder in the new wires and then shrink the tubing. That would be a good repair. What I would do first is find out why that wire melted so you don't do the same thing with your new harness. Something is drawing more current than the wire can handle or the insulation wore away and the wire shorted against another wire. Its not a ground problem because that reduces current. I would separate them all out and see if one of those wires is getting hot.
After seeing the cost of the new harness, I am thinking repair would be a viable option. I will tear in to it soon and also will try to nail down why it happened. The green on the wire indicates that this was an issue from years ago. Thanks for the advice!
 
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