Emerg & Brake Light Fuse Blowing

[FloridaFordGuy]

I'll preface this by saying electrical stuff isn't my strong suit. When I was putting everything back together at the end of my restoration about six months ago, the brake lights weren't working. I checked the fuse box and, sure enough, the Emerg & Brake Light fuse was blown. I got another 20A fuse, plugged it in and that fixed it. Easy-peasy, brake lights worked fine.

I don't know exactly when it happened, but I noticed last weekend that they weren't working again. Sure enough, fuse was blown. This time, when I pushed the fuse in and it connected both sides, it immediately blew. I noticed that the brake pedal wasn't coming all the way back because the stoplamp switch was sticking. I got that all lubed up good so that the switch is fully depressed when the pedal comes back. I plugged another fuse in and it blew right away. I even tried disconnecting the hot battery cable, plugging in a new fuse and reconnecting the cable. It sparked immediately and blew the fuse.

So I'm perplexed. If the stoplamp switch is fully depressed, I don't know how there can be continuity. Even if there is, I wouldn't think the fuse (Bussman AGX-20) would blow anyway. Any thoughts on where I should start would be greatly appreciated.

Thanks!

[sparky72]

There is probably a short to ground somewhere in the wiring in that circuit.

[cep62]

To save money on fuses , get a sealed beam headlight and rig up jumper wires with alligator clips.
clip to each side of the fuse terminals. now the headlight is the fuse.
instead of melting wires the headlamp will glow.
the worse the short , the brighter the light gets.
now you can search for the problem , when the light goes out you found it
it could be a wire got pinched during assembly , or some hacked trailer wires in back.
I new a guy who also wired an old key buzzer too so he could hear from under the truck.

[basketcase0302]

Taking one hour to learn to read a digital voltage/amperage meter here will benefit you hundreds of times in the future.
A simple test lamp might help but if it's a direct short to ground like it sounds it might even blow the lamp in a test lamp.
Like CEP said go back and verify there aren't a lot of hacking into the wiring harness, (and the brake light wiring has a lot to trace down) but don't let it kick your butt. Rule out one part of one circuit at a time and slowly but surely you'll narrow it down.
I replaced the brake light switch with a dent style on mine and never looked back, (as it was always sliding in the sheet metal bracket and next thing I knew I'd have a dead battery cause the brake lights had been stuck on).

What year is the truck?

[jimmy828]

Taking one hour to learn to read a digital voltage/amperage meter here will benefit you hundreds of times in the future.
A simple test lamp might help but if it's a direct short to ground like it sounds it might even blow the lamp in a test lamp.
Like CEP said go back and verify there aren't a lot of hacking into the wiring harness, (and the brake light wiring has a lot to trace down) but don't let it kick your butt. Rule out one part of one circuit at a time and slowly but surely you'll narrow it down.
I replaced the brake light switch with a dent style on mine and never looked back, (as it was always sliding in the sheet metal bracket and next thing I knew I'd have a dead battery cause the brake lights had been stuck on).

What year is the truck?

I agree with the multimeter. It can be a life saver. I'm always using mine to use on 4 different vehicles that i own. Plenty of knowledge on You Tube to get familiar with the multimeter.

[FloridaFordGuy]

This is a 1970.

Thanks for all the help. I guess I should have mentioned that I do have a multimeter, and I've used it to confirm the stoplamp switch appears to be in good shape, and I believe the emergency warning flasher is functioning as well. Beyond that, I'm not certain how to set up the multimeter to test the 20A fuse terminals without blowing the thing up.

basketcase0302, what is this 'dent style' brake light switch of which you speak?

Thanks again for your help. I have a long weekend coming up, so I plan to spend a good part of it crawling around looking for bad wires. I'll update with what I find (or don't).

[basketcase0302]

Here is the 1970 light schematic, (to be able to trace wire to wire on the light wiring):




And here's a really good easy to read laminated color coded one, (I always carried in the glove box so when stuck on the side of the road to aid in troubleshooting):

https://www.ebay.com/itm/324241873151?_ ... %3A2334524

You know the fuse is blowing so an amp reading is a waste of time. And like suggested above start at the bulb sockets, (pull them out/test to ground/test continuity of that wire as it might be grounded/shorted in the harness in between the cab and the socket). Finding the "dead short" by tracing the wires using the "ohm" setting on the multimeter is how you'll find the issue. Using the ohm/resistance scale on your meter it will confirm which wire has the "short to ground". Get familiar with reading the meter's continuity/ohm scale to ground first-then start from the fuse and "go backwards" testing each wire, (after the bulbs and switches have been tested for continuity/grounding). Here's a good tutorial video on using the meter for continuity:
https://www.youtube.com/watch?v=x3nqlXM6nOM

The "dent" (1973-1979 Ford truck) style brake light switch uses a different wiring harness to hook up and is a lot more dependable, (source the wiring harness from a boneyard/junkyard from any 73-79'. Pretty sure someone has it aftermarket but a quick search I couldn't pin one down):
https://www.ebay.com/itm/293777530864?f ... SwtThe868R

[FloridaFordGuy]

That's awesome! Ok, I have a project for the weekend.

Thanks so much for the help! I'll repost when I have it fixed

Bryce