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Frustrating day chasin electrical gremlins .
I thought I would post this hoping this would help someone else.
Trailer running lights would not work. Stop and blinkers would.
Checked fuse: it was Ok
Can't be ground because the other lights work. So I checked pin voltage on truck: 12.2V
No voltage on trailer though. Must be trailer harness, right?
Replaced one suspect connector on the trailer. Still no go.
Patch in a cable and another plug. Still no go.
Checked truck. Still 12.2V.
Double checked a new brake controller I installed recently and MORE. Lots more.
Finally got smart and hooked up another trailer. No running lights there either. Dang. Must be truck.
But How? I got 12.2v at the plug?
Looked at fuse again. But this time, I happened to hold it up to the light to look for evidence of burning by holding the outside edges of the fuse legs between my fingers and one the legs just collapsed and broke.
Put in a new fuse and everything was wonderful. This whole ordeal took 4 hours of work.
Moral? A fuse does not have to be burnt to be bad. The crack was solid enough to carry the full 12V potential but not enough to carry current to even cross the resistance of the trailer plug or carry a load.
I hope this helps someone!
2006 F350 Flatbed "plain and ugly"('cept got a chrome grille now!)