what would cause 80A fuse to blow?
 

the other day i was driving and between shifts (3rd to 4th maybe?) the 80 Amp fuse popped. i was able to pull over and replace the fuse and get her home. what would cause the fuse to pop like this? i was driving it real easy since its a rebuild with only ~350 miles on it. car is 87 NA with s5 rebuilt shortblock and all s4 longblock accesories.

Intermittent short to ground, or just old age. If it does it again, start chasing the wiring harness back and see if anything is chaffing.

overfilled the alternator fluid

Check the wires around the ACV to make sure they aren't burned.

ACV? swapped it out for emissions test but i wouldn't think i would have anything to do with the main fuse...

Most items are protected by other fuses other that the Main 80a fuse.

Seeing as how the wire going to the alternator is only protected by the 80a fuse............look to see if that large output wire from the alt is close to touching bare metal. Like a intercooler on a turbo car.

That same wire also goes directly to he ignition switch and feeds parts of the interior fuse box by passing pwr thru that igniton switches contacts.

So look for any mickey mouse aftermarket installation that picks power off the ignition switch OR any of the wires that go from the ignition switch to the interior fuse box. NOT the wires leaving the interior fuse box, but any of the wires that leave the ignition sw and feed the interior fuse box. They's be colored black/white, black/red or black/green, pure blue . In other words the color of the wires leaving the ignition switch pigtail connectors. Not likely those unless a aftermarket bs has been installed.

Shifting from one gear to another might involve the neutral switch and the clutch switches. Neutral sw only puts a gnd on a pin of the ECU and that would not do it. Clutch switch would apply power to a pin in the Cruise contrlo unit and also a pin on the ECU..............but that power is protected by a fuse in the interior fuse box. So it's not either of those items.

I'd be looking at the large output wire of the alt touching something and or some aftermarket item attached to the pure black wire on one of the ignition switch connectors.

Or that just mentioned black wire has a connector about six or so inches below and inboard of the engine bay fuse box. A single wire connector..........black in color. Comes directly from the engine bay fuse boxes middle bolt that is attached to the 80a fuse in that box. That connector gets mishandles and abused. Look for it and make sure it's connected together good and not frayed.

That same black cable that feeds the alt and igniton switch also feed the two circuit breakers on the very top row of the interior fuse box. ...but those breakers would have tripped prior to the 80 main fuse blowing. Those two feed the rear defroster relay in the rear of the car and ??? the heater I think.

checked acv, alternator wiring, ignition wiring, radio wiring, nothing shows signs of shorting.

however, when hooked the battery back up, it arc'd a bit more than usual, and i heard a pop from the alternator. aslo, while at a stoplight at one point i was watching the volt gauged and watched it fluctuate and drop down to ~12v and the engine began to struggle. the light turned green and i was able to continue driving, but i was then greeted with the smell of electircal burning... alternator dead?

The only REASONABLE thing that can cause the MAIN FUSE to blow is the cable b/t the alt and engine bay fuse box grounds out.......the black wire that feeds the ignition switch grounds out...........and I guess if the guts of the alt short out it'll blow the main fuse. We can't tell from here. You are our eyeballs.

i have a s5 alt im going to swap in since i have it, and the current one was a shiny junkyard find that has given me issues since i installed it.

as for wiring i cant find any visible shorts, but i dont feel like tearing the harness apart to find it...

i think as a precaution im going to create a new alternator charge wire, but i just wanted to make sure that im reading the wiring diagram correctly: the wire goes from the B post on the alt directly to the 80A fuse? no other splices/devices?

bump

Usually the alternator wire or starter wire (main power wires) are touching something and grounding out. Could also be if the battery is laying loose in the car and is rocking around and hitting the PS pulley or some other metal part with the positive terminal somehow.

Does anyone here know how to use a multimeter? Use it to check the circuit for continuity to ground. Shake the wiring while you're doing it. Something that only breaks the 80a fuse is probably a large gauge wire that is part of your charging or starting system. Maybe your starter solenoid is intermittantly shorting out internally while you drive.