New Harness and Burnt Fuse Holders

S

Snortin Vache

Joined
Mar 25, 2023
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Hiya,

I’ve been slowly but surely making a new wiring harness for my 73 roadster. I have been using Greg Marsh’s write up for a 74 Commando and have gotten my supplies from British Wiring.

Last weekend I had the harness finished and had it on the bike for testing before I wrapped it. I had everything finished except I had not yet spliced my battery sub harness (14 gauge/ 28 strand wires that connect to the positive terminal of the battery, the capicitor, and my Sparx voltage regulator) to the 14 gauge red wire that runs the length of the bike to the headlamp shell where all the connections are made. As such, I jumpered the positive terminal of my battery to the 14 gauge ground wire in the headlight shell (making all my red wires common) to test things. When I turned the key to ignition plus lights (a blue yellow wire running to the two position switch in the shell, through the high/dip handlebar switch to the headlight), my jumper wire started to get hot. The alligator clip from the positive terminal of my battery melted the insulation on my 4 way snap connector (connecting all my grounds in the headlight shell), the blue wire from my high/dip switch started to melt, and the fuse holder with a 35 amp glass from British wiring melted through the plastic finally breaking the circuit and stopping a fire. The fuse never blew; it just melted through the side of the fuse holder that comes right off the negative terminal.

Just today, I unplugged everything except my battery connections, the capacitor, the voltage regulator and alternator pick ups, and lastly, the bullets in my high/dip handlebar switch. The horn was left disconnected in the switch. I fitted a new a new fuse holder and a 25 amp glass fuse. I then jumpered the positive terminal to my ground strap wire, thus making the reds in my main harness and elsewhere all common. I activated the high beam by itself, then the dip by itself, and then the flasher button to activate the high beams without going through the two position switch. Each of these three tests independently smoked the fuse holder, but never burnt up the fuse. The bulb is the same one I got with the bike and never had issues with before. It is a halogen and it still works.

I don’t know what to do next and why my fuse never blows but the fuse holder melts.

Any advice would be appreciated. Thank you.
 
Did you fit a new fuse holder every time with these three tests? The last bike i rewired (a mk2a 850) i used one of the flat blade fuse holders, the fuses are far cheaper and easier to get hold of. A 25A fuse is too high, a 17.5 amp flat blade fuse is plenty.
 
There are two fuse ratings, blow and continuous. If you mix them up then you get issues such as a short not blowing the fuse and the heat continues. So the old Lucas glass fuses were marked 17A continuous and 35A blow. As Dobba says a modern fuse of 17.5A is correct and I actually use a 15A and it never blows. So if your 25A fuse is the continuous rating it will not blow until 50A but will take 24A continuous without blowing.

To cut down on the number of fuses you blow then instead use a bulb, if you have a short then it will glow.
 
Very short on details here.

This fuse holder?
New Harness and Burnt Fuse Holders
 
My guess is you have something backwards OR a hot wire is pinched to ground somewhere. The handlebar clusters can pinch a wire to the handlebars and cause a dead short. The headlight shell is also a common spot where a bare wire can contact ground either on the shell itself or to another wire when you insert the headlight into the shell. It pays to shrink tube as much as you can inside the head light shell.

The other thing is a reverse polarity on a component, where you are connecting the "hot" to a side of a component circuit which is a grounded terminal. Sorry if my comment seems obnoxiously obvious...

I haven't had any electrical issues in a long time.... (hard to believe actually) but my favorite method to sort things is the replace the battery in the circuit with your continuity tester. (one that makes a tone when the circuit is complete) Then go around disconnecting one component at a time until the tone goes off.... Hopefully that finds the offending circuit without needing to burn up 100 fuses...
 
Snortin Vache said:
Hiya,

I’ve been slowly but surely making a new wiring harness for my 73 roadster. I have been using Greg Marsh’s write up for a 74 Commando and have gotten my supplies from British Wiring.

Last weekend I had the harness finished and had it on the bike for testing before I wrapped it. I had everything finished except I had not yet spliced my battery sub harness (14 gauge/ 28 strand wires that connect to the positive terminal of the battery, the capicitor, and my Sparx voltage regulator) to the 14 gauge red wire that runs the length of the bike to the headlamp shell where all the connections are made. As such, I jumpered the positive terminal of my battery to the 14 gauge ground wire in the headlight shell (making all my red wires common) to test things. When I turned the key to ignition plus lights (a blue yellow wire running to the two position switch in the shell, through the high/dip handlebar switch to the headlight), my jumper wire started to get hot. The alligator clip from the positive terminal of my battery melted the insulation on my 4 way snap connector (connecting all my grounds in the headlight shell), the blue wire from my high/dip switch started to melt, and the fuse holder with a 35 amp glass from British wiring melted through the plastic finally breaking the circuit and stopping a fire. The fuse never blew; it just melted through the side of the fuse holder that comes right off the negative terminal.

Just today, I unplugged everything except my battery connections, the capacitor, the voltage regulator and alternator pick ups, and lastly, the bullets in my high/dip handlebar switch. The horn was left disconnected in the switch. I fitted a new a new fuse holder and a 25 amp glass fuse. I then jumpered the positive terminal to my ground strap wire, thus making the reds in my main harness and elsewhere all common. I activated the high beam by itself, then the dip by itself, and then the flasher button to activate the high beams without going through the two position switch. Each of these three tests independently smoked the fuse holder, but never burnt up the fuse. The bulb is the same one I got with the bike and never had issues with before. It is a halogen and it still works.

I don’t know what to do next and why my fuse never blows but the fuse holder melts.

Any advice would be appreciated. Thank you.
Click to expand...
You are using too high an amperage fuse or you have the fuse in the wrong part of the circuit. The Brown/Blue 14 ga wire is the only connection to the battery negative and the other end of that wire goes to one end of the fuse holder. From the other end of the fuse holder, it's again Brown/Blue to the master switch with splices to the regulator and if installed, the capacitor.

When testing I use a AGC 5 or AGC 10 fuse. The 14-strand wires are capable of continuously carrying 7 or 8 amps (depending on where you look). Other than testing with low amperage AGC fuses, you should not use AGC fuses. The original fuse the British bike manufacturers and Lucas specified was usually 35 amp but that is NOT an AGC 30 or higher fuse!!! The closest thing I can find today is a SFE 20. A SFE 15 would be better but the length of SFE fuses changes with amperage and SFE 20 is the only one the right length.

I also test each circuit independently - if I've made a mistake, I know it right away and basically where to look. My phone number is on every page you look at - call me.
 
It was the fuse holder pictured above from British Wiring. I had the spring in the fuse holder in series after the contact rather than the spring around the wire with the contact completing the circuit. The spring heated up like a toaster wire until it melted the plastic of the holder.

Obvious mistake when you see it, hopefully this helps someone else who’s equally clueless.

Thanks everybody.
 
marshg246 said:
You are using too high an amperage fuse or you have the fuse in the wrong part of the circuit. The Brown/Blue 14 ga wire is the only connection to the battery negative and the other end of that wire goes to one end of the fuse holder. From the other end of the fuse holder, it's again Brown/Blue to the master switch with splices to the regulator and if installed, the capacitor.

When testing I use a AGC 5 or AGC 10 fuse. The 14-strand wires are capable of continuously carrying 7 or 8 amps (depending on where you look). Other than testing with low amperage AGC fuses, you should not use AGC fuses. The original fuse the British bike manufacturers and Lucas specified was usually 35 amp but that is NOT an AGC 30 or higher fuse!!! The closest thing I can find today is a SFE 20. A SFE 15 would be better but the length of SFE fuses changes with amperage and SFE 20 is the only one the right length.

I also test each circuit independently - if I've made a mistake, I know it right away and basically where to look. My phone number is on every page you look at - call me.
Click to expand...
Agreed, 15 amps is about the max for 14 gauge stranded anyway. Those plastic Buss fuse holders melt pretty easliy. I have taken to using the blade fuse holders and fuses. For the main fuse on my commando I am using a mini circuit breaker that fits the blade fuse holder. It seems pretty reliable and is very convenient. See e.g. https://www.amazon.com/Automotive-R...69&sprefix=blade+fuse+cir,aps,112&sr=8-3&th=1
 
kommando said:
There are two fuse ratings, blow and continuous. If you mix them up then you get issues such as a short not blowing the fuse and the heat continues. So the old Lucas glass fuses were marked 17A continuous and 35A blow. As Dobba says a modern fuse of 17.5A is correct and I actually use a 15A and it never blows. So if your 25A fuse is the continuous rating it will not blow until 50A but will take 24A continuous without blowing.

To cut down on the number of fuses you blow then instead use a bulb, if you have a short then it will glow.
Click to expand...
While over the Scottish Border in 1967 I had a random fuse blow and used up the spare fuses trying to get home. Stuck in the middle of nowhere I noticed an old pencil in the gutter. I cut it down to suit the fuse holder and bingo the bike started and off I went. I soon found out the cause when smoke billowed out of the side panel when I applied the rear brake.
 
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