Blowing fuse issues with 30 amp fuse

O

old31

Well-known member
Any ideas where to start looking or what to do. While driving the car blew a fuse and died. Every time I try to put a new fuse in the fuse instantly burns.

I am running 6 V with a perTronics electronic ignition.
 
If you are running an alternator, a 30A fuse is too low. An alternator will output about 60 amps for about a minute immediately after starting to charge the battery.
 
Start disconnecting one load at a time. Begin by disconnecting the horn (horns cause lots of shorts), then the headlights (they are next most common place for shorts), then remove the dash panel and check the wiring connections at the ammeter for frays and exposed wires. Check all the rest of the wires under the dash too. Then remove the cover of the junction box and check for frayed wires there.
 
Bob, I’ve been running a 30 amp fuses in all my cars with alternators and I’ve never run into a problem.

But the problem now is not running the car. The problem is just trying to install a new fuse because once it’s installed, it blows.

I came across this article I had saved and that’s what I’ve been using is the light. What I have found so far as when I hook up the wire that goes from the starter to the right side of the terminal box that’s when it blows. I tried a different ammeter, but that did not work.

I have disconnected the wires to the alternator and checked each wire in the terminal box and I see nothing.
 

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A wire of 16 gauge wouldn't even carry 30-amps very long. Fuses protect the wire. Not the components. If it's blowing fuses then it has a short to ground somewhere. Start by disconnecting the light and horn power from the Gen or Alt. Look at the wire connections inside the terminal box and see if there is any signs of overheating in there such as melted or burned insulation on wires. If you have a dead short in the charging line, the ammeter will show it as a major discharge.

The safe way to check it out is to start isolating individual circuits (disconnect them from power in) and run an ohms check from one terminal on each wire to ground and make sure there is no continuity to ground in any circuit wire you check. Related switches should remain in the off position. The ignition breaker circuit would have to have points open to check it to ground. Lighting circuits will have a bulb to ground on the end of the circuits so they have to be checked with bulbs removed for a short to ground check. Motor circuits such as horn and electric wiper circuits are the same way, as anything connected to ground on the other end will have some continuity reading.

There is also the possibility that two wires are shorted together in a pair or bundle of wires. A continuity reading between two or more unrelated circuits will indicate a short between the two or more wires. Check everything before powering the system back up. This may also require removal of the dash since wires can short in there too and ammeters can self destruct. The wire inside the pop out type armor cable can go bad too but it generally just kills the coil function if open. If it shorts to ground then the coil is hot all the time which can cause it to self destruct. That all depends on what type of switch set up is in the vehicle.
 
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old31 said:
Bob, I’ve been running a 30 amp fuses in all my cars with alternators and I’ve never run into a problem.

But the problem now is not running the car. The problem is just trying to install a new fuse because once it’s installed, it blows.

I came across this article I had saved and that’s what I’ve been using is the light. What I have found so far as when I hook up the wire that goes from the starter to the right side of the terminal box that’s when it blows. I tried a different ammeter, but that did not work.

I have disconnected the wires to the alternator and checked each wire in the terminal box and I see nothing
Click to expand...

I was just looking for that article in the Tiny Tips book for you. It is in volume 2, page 55. That is the method I use to find shorts.

BTW, both Volumes of Tiny Tips is great to have. All the many tips from readers of the Restorer Magazine in one (actually two) place. Something to make life a little easier, as you don't have to search thru your old magazines..
 
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Try disconnecting your instrument panel light. The repro's switch was shorting out. This happened to me. I don't have much of any repro parts on this old truck. Either the stoplight switch or the map light, after that short was found in the light, I replaced with an original that was worthy of restoration. No problems now.
 
This is one of those questions that can't be solved online. Your short can be anywhere.
Get out the wiring diagram and start checking to find which component/wire is shorted to ground.
 
Follow up:

It is fixed.

And it was my fault from the start. I found a wire inside of the gas tank tunnel that was labeled "do not use". I checked the other end of the wire and I had it attached to the + side of the ammeter. About 4 years ago, I changed to electronic ignition, and this wire was not needed. It is amazing that it took this long for it to show its ugly head and short out.
 
When I have a short like when you first described the problem, I attach a voltage appropriate bulb (6V or 12V) across where the fuse goes. Made the bulb attachment with wires (about 6" long) attached to alligator clips that will connect to the terminals of the fuse and to the bulb base and center. Put heat shrink tubing around the bulb. With a short, the bulb will be on so disconnecting the various circuits will tell you which one is bad when the light goes out. With the bulb there is chance of burning up a wire.
 
Bob Bidonde said:
If you are running an alternator, a 30A fuse is too low. An alternator will output about 60 amps for about a minute immediately after starting to charge the battery.
Click to expand...
The NuRex is 30 amps. Just looked at their page. I looks like they don't offer the high output one anymore. https://www.nurex.com/alternator-6-volt
My car has the high output and 30 amp fuse - works fine.
 

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Again, I have never, ever blown 30 amp fuse with the Nurex high output alternator, or the Powermaster 82006, 60 amp alternator.
 
FWIW even the higher output alternators on a weak battery will charge at 10-15 amps initial and no more, even with accessories on. The only time you'd blow a fuse is if your system is drawing or supplying more than 30 amps to the battery.
 
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