2004 trailer running light issue

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Oct 3, 2023 | 11:19 AM

rlward

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2004 trailer running light issue

I have a 2004 F250Super Duty with the 6.0 diesel, 4x4 with the extended cab. My trailer running light circuit doesn’t provide power at the trailer plugs. The fuse is good. Ive read about a relay located somewhere in the circuit but cannot find it. One mechanic said it was integral with the fuse box located under the dashboard on the driver side and that the entire fuse box had to be replaced. Others have said it’s under the hood on the driver side.
All the other trailer lights and the brake circuit work, so the issue is confined to the running lights.
Where is the relay located and do I have to get a new fuse box. Thanks.

Oct 3, 2023 | 11:40 AM

redford

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From: Stephensville WI

The trailer taillights and running lights are the same circuit at the plug. If your taillights work, but the trailer running lights do not, it is a trailer issue.

If you also do not have taillights, get a volt meter and check the truck side for voltage. This website has a useful diagram.

https://www.ajtnt.com/Info/7-Way-Diagram

Do not spend money or replace anything until you verify voltage (or lack of voltage) and trace the wiring path.

Oct 3, 2023 | 01:30 PM

rlward

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The trailer taillights and running lights are the same circuit at the plug

According to the wiring diagram from Ford they are both controlled by the light switch but the trailer lights are controlled by a fuse and relay.
I want to know exactly where that relay is located and is it replaceable.

Oct 3, 2023 | 01:43 PM

redford

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From: Stephensville WI

That question has already been answered on the duplicate question you posted in the trailer towing forum.

Oct 3, 2023 | 02:41 PM

rlward

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Actually it has not. The answer provided is not correct for the 2004 super duty

Oct 3, 2023 | 04:27 PM

redford

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From: Stephensville WI

You can download a copy of your owners manual at https://www.ford.com/support/vehicle...owner-manuals/

Per the manual, the only relay used in towing is relay 305, trailer battery charge relay. That isn’t applicable. Fuse 38 is for the running lights. That might be your issue.

Oct 3, 2023 | 05:22 PM

rlward

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In my question, I stated that the fuse is good.
Again, where is the relay in the running light circuit located in a 2004 F250 Super Duty with the 6.0 diesel?

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Oct 3, 2023 | 06:59 PM

projectSHO89

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From: St Louis

Quote:

Ive read about a relay located somewhere in the circuit but cannot find it. One mechanic said it was integral with the fuse box located under the dashboard on the driver side and that the entire fuse box had to be replaced.

The "one mechanic" was correct as to the location of the trailer running lamp relay, anyone who states otherwise did not do their homework for the vehicle under discussion.

As far as repairing the issue, assuming the relay is the issue (and it probably is if fuse F38 is good), the easiest repair would be to replace the whole box as that mechanic suggested. However, either the box can be opened by a competent individual and the circuit board-mounted relay can be replaced or an external relay can be scabbed around the box to fix the problem. Both procedures have been documented in this forum.