Brake lights blowing fuse - Update

[blown motor]

When I take it out for a run the fuse for the brake lights blows. I have followed the wires looking for a bare spot but can't find one. I put a board between the brake pedal and the seat and 10 minutes later the brake lights are still on but I'll bet a dollar to a donut that after the next run the fuse will be blown again. It happens every time. What's happening here?

[jimynick]

The reason a fuse blows is that it's asked to flow more current than it's rated filament can handle. The most common cause is, as you said, a short, but if you have an intermittent short that comes and goes it can be tough to see/find. You have standard bulbs in the t/l's Murray? You have the specified rating fuse in it? What if you were to attach another temporary ground to the bulb holder to make sure that it's not hunting for a more current intensive way home? Something in the brake light switch that's current hungry but which once connected doesn't suck more current- (your keeping the lights on for 10 minutes and no issue) ? Just some thoughts. Good luck hunting. 

[Rich G.]

Unplug the harness at the kick panel going to the back lights. Go for a spin like you would when it would blow the  fuse. If it still blows you can eliminate that whole section. Nothing worse then an intermittent problem.

[anlauto]

Take tin foil from your cigarette pack and wrap the fuse, problem solved 

[blown motor]

Take tin foil from your cigarette pack and wrap the fuse, problem solved 

But I don't smoke. Knew I shouldn't have quit. 

[RJChallenger]

 :thinking:I wish I knew you had this problem the other day, when I was there. I would have really enjoyed helping you. Pull each light bulb from the back end. Look inside the glass bulb, to see if  the element has broken and  is bouncing around shorting tail light to brake. Pull the bulb from each socket making sure the 2 contacts aren't mutilated on the bulb. check the 2 contacts inside each socket and make sure the plastic insulation on the wires isn't broken where they go into the metal socket .

[blown motor]

20 amp fuse as called for. 1157 base LED bulbs.

[Chryco Psycho]

I agree it could be a faulty bulb

[anlauto]

I've had no luck with those LED bulbs...

[blown motor]

I've had no luck with those LED bulbs...

Interesting!

[jimynick]

Just swap 'em out for the test and remove one more variable. If you haven't got any regular 1157's let me know and I'll gear you up matey. 

[blown motor]

I have 1157  :dunno:bulbs but if it's the LED bulbs that's the problem then why didn't the fuse blow when I had the brake held on fit 10 minutes? 

[Bullitt-]

  The only thing I've experience similar to this was an 80's D100 that someone had spliced a trailer connector to the rear harness that had chaffed and would occasionally blow the fuse. So anywhere you may have worked on that could have changed a harness that may have created a situation for a short would be suspect.

  Looking at my '73 Service Manual I see there are a couple of other fixtures that are serviced by the same fuse.... trunk lamp, dome lamp and hazard flasher that includes the stop lamps, front turn signal lamps and turn signal indicators. 

[blown motor]

@Bullitt-  Interesting that you mention an 80 D100. This is on a 79 D100 cab put onto an 83 W150 chassis. The brake lights are on their own circuit. There is no trailer hitch but there are a couple of snipped wires back there and who knows what the PO has done. Maybe I need to unwrap all the wiring behind the rear axle just to make sure there's no gremlin hiding within.

[blown motor]

Well this morning I swapped out the LEDs for regular 1157 bulbs and blew the fuse so that's not it. I pulled the wire off the outward side of the brake pedal switch and went for drive, all good. So the trouble is beyond the switch which means I don't have to contort myself to try and follow wires under the dash. I had put a board from the brake pedal to the seat with the key off and it was all good, today I tried it with the engine running and, again, all good. So the fuse doesn't blow unless the truck is moving. I drove across the road to jaw with the neighbour, three times on the pedal at low speed and blew the fuse.
This is on my 79 Power Wagon with an automatic transmission and the fuse box is under the glove box lid. So I'm in the shop, put a new fuse in and leave the glove box door open so I can see the fuse. Back out, brake and put it in park, all good. Foot on the brake and pull it into reverse, it's good. As soon as I slide it into neutral the fuse blows!
Where do I go from here? Is the problem tied into the NSS somehow?