'71 340 Duster has a short

[MrDuster]

Looking for some help to get Mr Duster up and running again. Mallory dual point distributer and stock coil(I think--round black). Couple years ago I took out the rally dash and had it sent off to a FABO member who did a great job renewing it. I opted to have the amp gauge display like a volt gauge and wired it back in per instructions. Worked great for a year. Then amp(volt) gauge started wavering at idle...went away at higher rpm. One day while out with my 15 year old daughter I did a tight left turn in a cup-d-sac and it died...DARN!! I was just about to let her drive it for the first time. I found a 30A fuse in the main power line in the engine compartment before it goes through the firewall blown. Replaced it and drove it home. Month later took my daughter and 6 year old son out for a weekend drive and during another tight turn it died. I put another fuse in and it immediately blew. I towed it home and pushed it in the garage. I plan to work on it over the next couple of days as we are finally getting rain in San Antonio. I had a new wiring harness put in back in the mid-90's, but I've never done the MAD mod drilling through the firewall plug. I plan to start at the firewall connection and move into the cabin and look for something grounding it...I checked quickly once but found nothing. I had continuity from the firewall side of the fuse holder to ground and no continuity from the engine side of the holder to ground. The wire from the fuse to the firewall connector sure looks small. I'm not running anything that would pull much power...radio is out, heater not connected...just dash, lights, turn signal, etc. I'm up for any suggestions/instructions on electrical troubleshooting to find a short to include how to disconnect the firewall plug without messing it up...I've never taken it apart. Thanks in advance for any help!

 

[Dubob]

If the fire wall side of the fuse holder is going straight to ground, personally I’d set it up where I could watch the meter (assuming that’s what your using) and start pulling fuses from fuse box to see if ground goes away. I’d try not to disturb anything else until I determined what electrical branch off fuse box made the ground go away. Not saying it couldn’t be prior to fuse box. But that’s where I’d start.

 

[MrDuster]

If the fire wall side of the fuse holder is going straight to ground, personally I’d set it up where I could watch the meter (assuming that’s what your using) and start pulling fuses from fuse box to see if ground goes away. I’d try not to disturb anything else until I determined what electrical branch off fuse box made the ground go away. Not saying it couldn’t be prior to fuse box. But that’s where I’d start.

 

[MrDuster]

Thanks Dubob, waiting on my buddy to come who's a lot smarter on this stuff then I am. That's a great idea...I'll start there with him holding the meter in the engine compartment and me pulling fuses...will post back what we find. Crazy day here with the drywall guy showing up to fix our ceiling from an air conditioner water leak from an improper install...now AC guy showed up too...will be good to get the Duster running again as we've had to push it in and out of our garage the past month to have access to the attic.

 

[MrDuster]

Fuse box looked ok...found the firewall connector to be melted where the main power wire goes through. Pulled out the bad wires and plan to push through new wires tomorrow. Also plan to rewire the alternator over to the starter relay with a fusable link. Black wire coming into the cabin was melted...certainly got very hot at some point. The amp gauge has been converted to a voltmeter and the red and black wires are crimped together behind the dash.

Will there be continuity on the cabin side due to the voltmeter and/or dome light bulb? We still have continuity to ground when checking the larger red wire in the cabin before it passes through the firewall ...not sure if thats normal or indicates a problem??

I can look up and see the connections and everything looks ok externally. Our plan is to temp wire everything and see if it still blows the fusible link...if so, we'll keep digging looking at the voltmeter and dome light...if not, we'll fire it up and then solder /shrink wrap it up. Thanks a bunch for the advice...I feel like I'm getting closer!

 

[MrDuster]

Thanks Dubob, waiting on my buddy to come who's a lot smarter on this stuff then I am. That's a great idea...I'll start there with him holding the meter in the engine compartment and me pulling fuses...will post back what we find. Crazy day here with the drywall guy showing up to fix our ceiling from an air conditioner water leak from an improper install...now AC guy showed up too...will be good to get the Duster running again as we've had to push it in and out of our garage the past month to have access to the attic.

We did pull each fuse one by one and had continuity through it all...my buddy thinks the voltmeter and/or light bulb might be causing the continuity...we did close the doors so the door switch to the dome light wouldn't be a factor.

 

[MrDuster]

We did pull each fuse one by one and had continuity through it all...my buddy thinks the voltmeter and/or light bulb might be causing the continuity...we did close the doors so the door switch to the dome light wouldn't be a factor.

Ran new wire through the firewall plug, rewired alternator over to starter relay with #10 wire and fusible link, cleaned up firewall connections and DC-4'd them, cleaned up fuse box, and some wiring left over from a failed attempt at electronic ignition years ago (think I had a lemon control box). Once wired up there was no continuity to ground on either side of the fusible link going to the firewall...so I guess the idea of the voltmeter or light bulb providing continuity didn't pan out in this case. Pulled 5 amps through the firewall with the ignition switch on. Mr Duster started right up and the voltage is good. Voltmeter rock steady now too which is nice. My buddy did find the dome light inop so we'll trace that down another day. Happy to be able to drive it again! Guess I was bit by the dreaded corroded power plug terminals through the firewall. Next time I see the voltmeter not rock steady I'll investigate instead of waiting till I have a problem. Thanks for the advice Dubob! Happy Moparing....