12v aux wire origination - Diesel Forum - TheDieselStop.com
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post #1 of 3 (permalink) Old 09-12-2018, 09:13 PM Thread Starter
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12v aux wire origination

Hello, my name is Doug and I'm new here. I have an '89 F250 7.3idi. I know where the 12 volt pin is on my trailer connector. The problem is I cannot find the lead coming from the hood. And yes I have checked the wires with key on. The connector has been spliced into the harness by a previos owner, minus that wire. All other functions on my trailer are working. I have a small 5th wheel that has a 12v battery, thus the need for the charge lead to be connected.
Can anyone tell me where that wire comes from at the alternator end so I can trace it out or run a new one?
Thank you very much,
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post #2 of 3 (permalink) Old 09-13-2018, 01:12 PM
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This link requires an e-mail, but it doesn't have to be your real one:
BBB Industries- Premium Alternators, Starters, Power Steering Products | TSB's & Wiring Diagrams
When you see the "WIRING DIAGRAMS" button, click it before selecting the system. If you can't find the diagram for your year, try a later similar truck.

It doesn't connect to the alternator - it probably originates near the L hood hinge, where the other relays are (originally under a black plastic cover). This is what the trailer relay harness looks like on '87-91 F150s & Broncos (& possibly F250/350s):

(phone app link)

(phone app link)

But the charge wire might come straight from the starter relay as a simple fusible link.

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Last edited by klhansen; 09-14-2018 at 12:08 AM. Reason: forum rule violation
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post #3 of 3 (permalink) Old 09-16-2018, 07:43 PM
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Your's may be the same as my '91 and if I remember right, all my factory trailer wiring was at a plug one side or the other at the very rear of a rear fender. Probably the left because that's where my in-bed trailer plug is mounted. And mine did have an "always hot" 12V wire. Of course if a previous owner made changes in your case, all bets are off.

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'91 F-250 SC 4X4, 5spd. ATS Turbo, 3:55 diff.
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