Keep Blowing 80A main fuse

[gijoe985]

I'm trying to help my brother with his 85 pickup. Sadly, I didn't help take everything apart, but he had to swap motors and now that everything is getting back together he is blowing his 80A main fuse every time he hooks up his battery. I think he said he also blew the fuse (maybe relay) to the ignition switch as well.

Any thoughts? I've done a bunch of electrical troubleshooting in the past, but for some reason I am drawing a blank on this. What could I do with an DVOM to help pinpoint where it is shorting?

 

[bcstoy]

It seems as if you having a short on the hot wire to the ignition switch. Take some continuity test of the wire against itself and against the ground. Wihtout seen it, it is hard to tell. Short are the most annoying thing to deal with.

 

[Bogo]

I'd use a continuity tester* and start testing wires, and I'd start with the ignition one gioven what you said. If it proves shorted, then I'd start tracing it, and wiggling it to find the spot where it is shorting.

* Some DVMs have a continuity test function where they will beep if there in continuity.

 

[toy4xfun]

i would start with the wire going to the alternator, giving you said he just pulled the engine, also test the alternator if the wire is good.

 

[cruisernerd]

could pull all your fuses and see which has continuity to ground. there should be a ohm value on one side of the fuse. proly wanna remove your battery cables first.

 

[bcstoy]

The difficult thing about electrical circuits is that they are a combination of series and parallel circuits. So the first step would be to tract the series part to make sure the wire is healthy one section a time. Then if things check OK move on to the parallel feeds to make sure that all of them are healthy. Changes are that somewhere a bare hot wire is touching metal. BWT do not forget that after feeding the load eletrons need to go back to the battery and they do so through the ground side. I left my main ground on my LC disconnected when I did the HG and after running it quit but that was easy to spot since the main ground has a big connector.

 

[gijoe985]

Put the meter on Ohms and measure from the + battery terminal (not hooked up to the battery) to ground (black wire from battery screwed to inner fender right at the battery).
It will read as a short = very low Ohms, like 1 or 0.something. Start removing things from the circuit. Disconnect the starter first at the starter. Keep checking for the Ohms to jump up. Yank fuses until it does. When it does rise, you will at least have a place to start tracing the short from.

This method worked great. An alternator wire was grounded. Thanks!