Main fuse size - 30amp blowing

[tmb99]

Just recently my bike started blowing the 30amp main fuse. It does it intermittently so it's a little difficult to diagnose. I have done wiggle tests on all wires without luck. It's all new and looks good.

I had friend who knows a bit about this stuff have a look and we couldn't find anything obvious. Checked coils and charging system and couldn't find anything going to ground. However do still suspect it is something in the charging system.

He suggested putting in a larger fuse. So I put in a 40amp and it has stopped blowing... which is great, however it's making me very nervous that its going to catch on fire!

End question here is 40amp too large for the main fuse?

 

[chopit25]

I am no expert but I have read a ton here when I wired my bike and I would say that is too much. Plus going to bigger amp fuses will only mask a problem until it causes a much bigger problem.

 

[ANLAF]

On my bike - custom wiring job for my chopper - I have two basic circuits, one for the charging system and the other for everything else (lights, etc).

I was blowing my main fuse (a 20amp is big enough for my system). I disconnected the lighting (at the fuses) and the 20 amp still blew. I got fed up with fuses blowing so I put in a trip switch, very inexpensive, and just press to reset). So then I reconnected the lights and disconnected the charging system, pressed reset on the 20 amp trip switch and the lights came on. Bingo! the fault therefore was on the charging side. I was on points and the old rotor/stator then, and I found it was earthing at the bushes until I put in nylon screws. Fault found and remedied.

Anlaf

 

[ANLAF]

By the way, tmb99, wiggling wires might find a fault if you are lucky, but a multimeter is one of the best tools I have ever had - well worth, and inexpensive.

Anlaf

 

[ANLAF]

One more thing, tmb99, these are old bikes - connections will be dirty and some corroded. The majority of electrical faults are simple dirty connections. Take apart and clean what you can, and refasten. Check as you go with your new multimeter (go on! you know it makes sense). Disconnect the lighting at the fuses and start cleaning/measuring with multimeter at the battery connections.

Be patient and work through. Doing a visual is not enough.

Anlaf

 

[tmb99]

I have now done some searching and the stock fuse was 20amp? So doubling the size is not a good idea...

I'll clarify a little; the fuse blew, so I replaced it and went for test ride, few minutes of riding and it blew again, replaced it road side and it only lasted a few seconds this time. Replaced it again and it lasted until I got home - so that's why I'm saying it's "intermittent".

I have a multimeter however this isn't going to show an intermittent issue?
I've worked my way around the loom wiggling, as that could show where it's going to ground.

How would I test the charging side? Stock charging system with points.

 

[ANLAF]

tmb99, it sounds a connection problem. start at the battery, disconnect, clean connections till shiny, replace. Eliminate that most obvious connection then move on.

Look at your wiring diagram, and disconnect the appropriate connector blocks or remove fuses to eliminate.

If it is intermittent is is highly likely to be a dirty connection or a failed cable - hence, clean and multimeter.

Anlaf

 

[Brian902]

TBM99,

You have a short somewhere. Broken or bare wire touching ground or another wire most likely.

Did you do anything lately that might have pinched a wire?

Remove all the other fuses but the ignition and the 20 amp and test ride will help indicate if it's in the light or signal circuits.

Good test for charging system is multi-meter set for 20 volts DC on positive and negative post of battery. Should show at least 12+ volts at idle and increase to around 14+ volts at or about 3,000rpm. If it's less than 12volts before starting, battery could be shot, if it doesn't climb to 14+ volts, brushes could be bad, or regulator/rectifier is bad.

Never use a higher fuse than a 20 amp in the main and 10 amp in the rest. Any higher cause a circuit overload which results in many burnt wires. EXPENSIVE.

Trace the wire harness looking for wires with a melted covering or ends that look unusual. Somethings it necessary to peel back the main harness to find wires in the main loop that have shorted to each other.

Good Luck.

 

[ANLAF]

Just be patient and thorough, tmb99 - you will find it.

Anlaf

 

[ANLAF]

By the way, did you make any changes to the wiring before you started shorting out?

Check out the safety relay if you have the electric start wired-up. If that is faulty or a dirty connection, you might be shorting on the left-hand casing.

That's multimeter territory.

Anlaf

 

[XSLeo]

tmb99, you don't say what year your bike is or what mods have been done. A lot of people put this in their signature. Thus way it's always there and we don't have to ask.
If your bike is an older on and has just one fuse it is much harder to trace a short. The later bikes with 4 fuses it was easier.
Often the tail light hit wire gets chafed enough so it shorts onto the side of the socket. The tail light is fed right off the main fuse. A short there will blow the main fuse.
Try unplugging the tail light and see if it still blows fuses.
Leo

 

[vincemoccio]

I had a very similar issue with my bike. The person a bought it from had an led strip as a tail light with tiny 20g wires. I found a shorted wire and fixed that. I slapped a 1977 Honda tail light on the bike. Seems to work fine. Then I bought a new led tail light and for some reason the bike just started and died. Seem to have blown a starter relay of some sort. Any suggestions?

 

[XSLeo]

vincemoccia,The starter relay has no connection to the lights. It sounds like you may have hooked the new tail light up wrong and are again shorting out.
Possibly hooked the ground up to a hot.
A bit more info on your bike may help. The year, model, any mods.
A lot of us put that info in our signature so it's right there where we can see it.
Leo