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74 620 + battery terminal sparking ? !

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Background- 74 PL 620 stock wiring harness, upgraded to internally regulated alternator & aftermarket fuse block installed after stock unit melted. Aftermarket ignition switch installed.

Todays issue- connected + battery cable & fusible link wire to positive terminal on battery & i got some intense sparking. Enough to semi melt the thick fusible link wire.

I cut the fusible link wire away from the other 3 wires it was connected to so now its hanging loose along with the other 3 wires. When i touch the fusible link wire to the black wire [comes out of the harness leading into the cab] & the other 2 wires i get sparking. That black wire connects to the fuse block on one end which i believe is tied into the ignition.

What ive done so far to diagnose- disconnect alternator wires & connect fusible link wire...sparks..

disconnected power to starter & connect fusible link wire...sparks..

disconnected all wires @ ignition switch & connected fusible link wire...sparks..

installed new fuse where black wire goes to on fuse block & connected fusible link wire...sparks..

Im at a loss here guys. Ive been starting the truck several times a week for a month in preperation of a long trip with no issues. No wires have been altered or moved around at all. Whats causing the sparking when i connect the fusible link wire to the solid black wire ???

Heres a pic of the 4 wires. The black one @ the top is the one thats sparking..the red wire is the fusible link & connects to the + battery terminal...


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Solid black wires are ground wires. They will spark if connected to the battery POS.

Ok understood. But...ive had these 4 wires hooked together for years with no issues until today.

I need to look at the 74 wiring diagram & double check my connections.

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Anybody else have any ideas ??? Ive looked at the 74 wiring diagram & now im really confused. Some of the wires on the diagram color wise dont match whats on the trucks wiring.

Im to the point of ripping EVERY stock wire off the truck & starting over !!!!!!!!!!!!!!!!

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Test the wires.


For example, that black wire. If it has continuity with the engine block, it is a ground wire. Don't attach it to the '+' battery terminal.


For the other, do they have 12v or 0v? What about when the key is ON?


I don't see a Fusible Link in your photo.

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Ok. The thick black wire goes to one side of the fuse block...the blk/red wire [on bottom] goes to the alternator & connects to the power terminal...the wire in the 3 o clock position goes back into the harness & im assuming to the headlights...the BIG red wire goes to the + side of the battery. The BIG red wire "was" hooked up to the power wire coming off the alt & i believe the 2 black wires were hooked together.

Ive not tested the wires yet to see what they read &.....I was just searching "fusible links" to see how it gets wired in. Whats strange is i wired it like it was & it worked fine for years..then all of a sudded i connect the + battery cable & i get sparks !

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