Brake, turn and hazard lights don't work - 1985 F150

[LastDeadLast]

I hope someone can help with this one.

I'm trying to get my F150 back on the road after some general maintenance from sitting in a garage for so long and I've run into an interesting problem:

Every time I step on the brakes or turn the hazards on I blow the number 1 (15 amp fuse) that is labeled:  "Stop/Hazard Lamps; Speed Control".

This is a recent development because they originally worked ok.  This started when I replaced the backup and tail lights with LED lights.  I changed out the blinker modules for electronic units and everything worked great.  I tested the hazard lights, tail lights, left/right signals and stop lights.  All worked.

The next day, I was showing a friend the difference in brightness and nothing worked. Nothing looked off, but every time I would press the brake or turn the hazards on, the number 1 fuse would blow.

To debug this, I have done the following:
1.  Put back the old incandescent lights (1157's) tail/brake lights and replaced the blinker modules with the original units.  Nothing changed.
2.  Replaced the turn signal / hazard switch.  No change

Does anyone have any ideas?  I'm struggling with where to go next.

Thanks for any help!

-Shannon

[Jake Papageorge]

Shannon, I suggest you do a voltage drop test on this circuit!  Both the load and ground side!  You made a pretty dramatic change in the amount of current going through these circuits with your LED experimentation and if you had a weak link somewhere (corrosion in a wire, questionable contact on connector, cruddy ground) the changes in the amount of current these circuits carry can push a questionable connection "over the edge."  

Also, consider look at ANY places on the harness where you have moved or jostled the wiring.  The tail light harness is one place to look.  Look for any damage or corrosion, black spots on insulators or connectors, etc.  Always go back to anywhere you touched and look for the obvious!  This is likely going to reveal your problem.

Always go for the "simple" and move to more complicated stuff.  

If you have never done a voltage drop test, check this out. It will help you out:

https://www.fluke.com/en-us/learn/blog/automotive/electrical-automotive-troubleshooting

[LastDeadLast]

ok.. continuing my investigation.  I am at the back tail lights.  I am getting continuity between the brake light lead, the turn signal lead, and ground.

Something strange is up.

-Shannon

[Jake Papageorge]

Shannon, don't rely on a continuity test.  I do not even trust resistance tests.  Circuit resistance can reap havoc on a circuit that, essentially, uses a circuit breaker to operate the turns and hazards.  Do a voltage drop test and don't forget to test grounds as well as load side.  I have had many incidents where a circuit checked good with continuity but had a ton of voltage drop.  You only need one strand of copper in those wires to satisfy a continuity test.  

[LastDeadLast]

Fixed it.  Wow.  So I started tracing wires as you suggested and apparently the PO had removed the fuel tanks and had them cleaned (I was told this when I bought the truck).  Well, when they reinstalled the fuel tank retainers for the front tank they caught the rear wiring harness and mushed it against the frame rail.  

I must've aggravated the situation when I was messing with the brake lines around a few days ago.

Anyway, luckily none of the wire strands were broken, just the insulation.  It was a quick fix once I discovered it.

Everything works now!!!

Thanks for the guidance, I doubt I would have gone that route if you hadn't suggested it.

-Shannon

[Jake Papageorge]

That's awesome!  I am so glad you figured it out!  Nothing more satisfying than nailing down a goof problem like that!