Blowing 20A Fuse to my Running Lights

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FJRdoc

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I've been working on final bugs in my Duster restoration when a new one just popped up.All wiring harnesses were replace with M&H harnesses during the build. All electrical components were working fine. Recently, my turn signal flasher stopped working. I purchased a new one from Auto Zone and fixed that issue.
The other week I had the car running in the garage at night when I turned on the lights and did a walk around. I noticed that none of my running lights were working. I looked at the fuse block and saw that the tail lamp fuse was blown. I replaced the 20A fuse and checked again. Blown again! I removed the drive kick panel and pulled the plug to the rear harness. Replaced the fuse, and tested again. The fuse is blown again.
Headlights, turn signals and hazard flashers are working. All grounds had been gone over multiple times during the rebuild. Just wondering if anybody has any ideas or insight as to what else I should check. I have my FSM out looking at the wiring diagram. I hate electic Gremlins!!
 
Unplug the bulk head for the headlight harness. If it still blows the fuse check the Headlight switch or dash lights.
 
I have a spare headlight switch that I may just exchange the current one with.
 
Replaced the headlight switch but the fuse still blew when I turned on the lights.
 
Better to trace the problem if you can.

Yes - start with your FSM.
See what else that fuse shares and where the hot from any of the wires after the fuse could be shorting. Also check the bulbs and sockets. With battery disconnected you can 'ohm' out shorts, especially those to ground.

To figure out the details, check the wiring diagrams in the back of the chapter.
Just highlight the ones of interest. If you're using a printed book, you can use tracing paper.
Ths is from a '69 DodgeFSM. '70 is probably a bit different with side markers and the column key.
Many colors will be the same, and probably the basic circuit layout will just be an evolution of '69.
upload_2019-10-23_14-9-41.png


Since L6 is the front markers, the tail lights are also L.
Lets jump right to the inside and follow L6 to U, and from the tail L7.

Looks like the colors stay the same.
upload_2019-10-23_14-26-23.png

But we do find that at least some cars got a headlight warning buzzer - so one more possible circuit to check for shorts.

I'd start by clipping a continuity or ohmmeter onto the circuit - maybe at the fuse box and see if there's a short to ground.
If there isn't any continuity, then turn on the headlight switch (the battery is disconnected, yes!) and see if a ground short shows up.

If so, the short must be in or on the other side of the headlight switch. Start disconnecting sockets until it goes away.

If still no short to ground, then suspect cross wiring.
 
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I had one of those side marker lights short intermittently, drove me crazy trying to find it. The center pin was shorting to the case.
 
OK. Thanks for the ideas. I don't know when I'll get a chance to try and sort things out.
 
First thing to do is determine what all is on that fuse circuit. If the wiring is as factory, that is detailed in section 8 of the service manual. There is usually a list right before you get to the diagrams section.
 
My guess is a grounded out feed wire to the front parking lights or front side lights. But I'd check the wire to the cigarette lighter first.
 
Diagram shows the fuse marked "instrument feed" at one end of the box. That is all the little illumination bulbs on orange wires, inst' panel, shift indicator, ash tray, radio, etc... Remove that fuse temporarily will further isolate the park lamp circuit.
We know a short to ground beyond this fuse should blow this fuse. We don't know what a back feed into a orange wire from another circuit might do.
If/when you determine the fault is at a exterior lamp, the license plate lamp is one of them. Good hunting
 
Thanks for your help guys. I might try to work on the car this weekend. I've been sick the last couple of days and haven't had the desire or energy to sort out the problem. I'll keep you all posted.
 
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