The Stovebolt Site Forums The Shop Area The Electrical Bay 56 GMC headlight switch fuse blowing

Hey guys... So, I'm a complete novice with electrical. I am open to all suggestions, as I can't speak with any authority at all...

I just put an NOS 56 GMC headlight switch in my truck. Everything seemed to be working fine, and then the dash lights went out. So, I pulled the fuse and it was blown. Put another fuse in, and it blew again instantly. I am using 4 AMP SFE fuses. I'm going to try eliminating accessory lights, and see if I'm overloading the circuit or something. I have an NOS accessory junction block that I am running for the accessories.

ok... well, just as an expirament, I disconnected the accessory panel lamp wire from the junction block, and retried, an no fuse blown. So... I did something wrong as far as the accessories go. This is progress, huh?

Yep, you've isolated the circuit that's overloading. Now you need to determine if you've got a dead short, or just too much current draw for the fuse. Try this- - - -make up a jumper lead with an inline fuse in it. You can get an accessory fuse holder at any auto parts store, and attach an alligator clip to both wires. Splice extender wires to the fuse holder leads if necessaary. Put a big fuse, maybe a 20 or 30 amp, into the holder. Attach one end of the jumper to a known source of voltage like the battery terminal on the ignition switch, for instance, and touch the other clip to the wire you just disconnected. A big flash and a blown fuse confirms that you've got a dead short, a hot lead attached directly to ground. If the lights burn normally when you connect the jumper, and the wires don't get hot, your circuit is simply drawing more current than the fuse you selected for the light switch can handle. Step up in fuse size until you've got a little bit of overload capacity- - - -if a 4 amp fuse blows, try a 6 amp, or maybe a 10 amp. Just don't get ridiculous and go with a 20 or something similar. The fuse protects the wire, not the end item, so if the circuit you're powering has a #16 wire, for instance, use a 10 amp fuse or smaller. #18 wires need about a 7 amp, and really small wires like a #22 or so should be fused to 5 amps or less.

If your dash lights are drawing more than 6 or 7 amps, you've probably got a short somewhere.
Jerry

Well, so far so good. I don't know exactly what I was doing wrong, but I did manage to get everything working again. I was running an NOS Chevy truck junction block, then switched to a correct NOS GMC junction block I found in my stash of stuff. They actually are different, and even install in different places, strangely enough. Hooked that up, and and just double checked all my connections against installation instructions, cleaned and tightened everything, and it all seems to be working great... Thanks for the help!!