blown fuse

[mikeyboy1]

any ideas on why My 78K blows the 15 amp fuse? I replaced it w/a 30 amp last year & forgot about . have had no problems .just put the 15 back in & it blew.was HOT when I took it out.put the 30 back in-no problem.not hot. so where do I start looking?
thanx  Mike

[TOOLS1]

First thing I would check is the soldered connections on the back of the fuse block. Those go bad and cause the fuse to heat up and blow.
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[mikeyboy1]

did that,still blew after 2 days. any other ideas ?

[Re-run]

Well, in essence, a fuse is just a simple piece of low temp metal. It runs a certain amount of current and then it get s to hot to continue and the metal melts.

Now, you are making it 2 days before it goes, and a 30 amp does not blow. We can likely rule out any sort of short. So what we are looking at is too much amp draw.

This can be caused by a couple things, additional or different equipment such as hotter coils. It can also be caused a poor connection that is causing a higher amount of draw.

First thing to always do, go through the connectors. Even if they look good on the outside, they might be worn/ dirty on the inside. I have seen pristine looking connectors on the outside, but the inside connections are junk.

So, inspect each connection and make sure they are tight. Shoot each one with contact cleaner that is safe for plastic.
This includes the connections for the coils. If you have dirty coil connections, it is going to pull harder to try and overcome the poor connection and the coils are on your main fuse.

This is likely to solve it. If not, then it is time to start running tests on the components on your main circuit.

[mikeyboy1]

thanx rerun will do

[mikeyboy1]

well I cleaned ALL the connections-still gets hot -even hotter when he hi beam is on.
last nite the fuse blew at 50mph on a DARK road ! no power -NO LITES !! was like having my eyes closed.managed to keep it strait & stop .Christ was that scary ! so needless to say-this is more than just an annoyance.what to check out next ?

[Re-run]

What is the wattage of your headlight? Did you put in a silverstar or something similar with a higher wattage?

[mikeyboy1]

will have to check.thanx Re

[Re-run]

If you have a meter capable of it, you should hook it up to your main fuse location and measure the amp draw. You'll take the fuse out of course as you want everything to go through the meter.

Another question, what coils are you running? Don't remember if I asked that before. If you have hotter coils, electronic ignition, or a combo of both, that could increase amp draw. Heck, it may be a bunch of things increasing draw. I think we need to see what the bike is pulling through the main though.

[mikeyboy1]

the headlite is a sealed beam. found a small wornout spot in wire insulation & fixed it. checked & cleaned all grounds. checked all connections. takes longer but the fuse still heats up.I did add horns which required a wire be run to the relay from the battery.also found a loose plug wire. will try to measure amp draw tomarrow

[Re-run]

Yeah, it has to be additional draw from somewhere. I had an h4 halogen on my bike and a dyna S ignition. Never blew a fuse and I ran 1 slightly under, 14 amp since I couldn't find a 15.

[mikeyboy1]

I did replace the coils.got the from cb750 supply.nothing fancy,just stock I assume

[Re-run]

Well, I wouldn't think that would do it. People have switched to 3 ohm coils(stocks are 5) and afaik, no one every went to a higher amp fuse.

[mikeyboy1]

so this morning there's a dead mouse under the bike ! and it wont start.figure she got in the airbox.took it off-it's clean.dont find any chewed wires.getting amp readings all over the map on the fusebox.testing the coils for resistanc -same results only I feel one of them is hot.so I assume that's been the culprit rite along & ordered new ones.even though they were new last May.as for the mouse--who knows ??
stay tuned !!

[mikeyboy1]

well I was wrong about the coils-they're fine. what I finally discovered was the battery was almost dry.I filled it when I put it in back in april.dont know where the water went. doesnt appear to be cracked. filled i& charged it-end of problem.fuse stays cool.put in a new fuse panel anyway.

[Truck]

Keep an eye on that battery, a lot of times they will go bad if they get that low for awhile.

[Re-run]

Check your charge system. If you put water in this april, they usually don't lose water that fast. If your system is charging to high though, it could be cooking the water out of the battery. Charge it and start the bike. rev to 2500 rpm and take voltage reading. 14.5 should be about max.

[Rockwork]

Is it safe to put a 30 amp fuse in the main (15 amp) slot?

I've not blown a fuse on my 78 CB750F in the 3 years I've had it until today. Riding streets of NY it just died. Replaced it with a 15 amp, and 15 mins down the highway it blew again.

I will replace the tube fuse holder with a blade holder as recommended here, but that's moving up to a 20 - 30 amp breaker. So is it okay to use a 30 amp tube fuse now and move up to the 30 amp blade if the system was designed for 15 amp fuse?

I just want to make sure I'm not overlooking a problem elsewhere.

[TOOLS1]

A short will blow any amp fuse, but an overloaded system will heat up to the point of blowing a fuse. You might have some dirty/loose connections causing the fuse to blow. I would replace the fuse with a blade type 20 amp circuit breaker. I would only use the 30 amp fuse in an emergency to get home.
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