Electrical Fuse question

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vuuduu21

vuuduu21

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I'm nearing completion of a restoration of a 72 combat and i installed a gel battery i got from commando specialties. to my amazement when i hooked it up everything seems to work as it should, although i havent tried the ignition circut as i'm not ready to fire it up yet. (it has an older Boyer installed by the PO)

last week i went to hook up the tail light and the main fuse was blown. It was a Buss 20A. i checked the battery voltage and i still have 12.8 vdc on it. i replaced the fuse w/ a Buss 20A and everything seemed fine. today whe i was messing around with it cheching the brake light switches it blew again. i didnt have the headlight on just had the key in the first position. replaced the fuse and everything works. is there some type of capacitor that is drawing power, or the ignition coils? or do you think its just a dead short? from everything i read so far the 20A Buss fuse should be the right size. should i look into a bigger fuse?

Thanks, Rich
 
vuuduu21 said:
I'm nearing completion of a restoration of a 72 combat and i installed a gel battery i got from commando specialties. to my amazement when i hooked it up everything seems to work as it should, although i havent tried the ignition circut as i'm not ready to fire it up yet. (it has an older Boyer installed by the PO)

last week i went to hook up the tail light and the main fuse was blown. It was a Buss 20A. i checked the battery voltage and i still have 12.8 vdc on it. i replaced the fuse w/ a Buss 20A and everything seemed fine. today whe i was messing around with it cheching the brake light switches it blew again. i didnt have the headlight on just had the key in the first position. replaced the fuse and everything works. is there some type of capacitor that is drawing power, or the ignition coils? or do you think its just a dead short? from everything i read so far the 20A Buss fuse should be the right size. should i look into a bigger fuse?

Thanks, Rich
Click to expand...
Double check your ground at the lamp and/or run a dedicated wire around the taillight bracket to the frame. There are many point to loose ground back there.
 
No. Don't use a bigger fuse. Actually put a 12V bulb (like the one in the tach) in place of the fuse and see if it light up, brightly, and when. When it does, that may be your clue to what is shorting. Or if you have a VOM, you could put it in line, but it wouldn't be like a fuse, but it will tell you how much current is being drawn with different things switched on.

Sounds to me like you have a short somewhere. You might be able to detect it with the ohmmeter part of a meter, without the battery of course.

Dave
69S
 
DogT said:
No. Don't use a bigger fuse. Actually put a 12V bulb (like the one in the tach) in place of the fuse and see if it light up, brightly, and when. When it does, that may be your clue to what is shorting. Or if you have a VOM, you could put it in line, but it wouldn't be like a fuse, but it will tell you how much current is being drawn with different things switched on.

Sounds to me like you have a short somewhere. You might be able to detect it with the ohmmeter part of a meter, without the battery of course.

Dave
69S
Click to expand...

+1
 
concours said:
DogT said:
No. Don't use a bigger fuse. Actually put a 12V bulb (like the one in the tach) in place of the fuse and see if it light up, brightly, and when. When it does, that may be your clue to what is shorting. Or if you have a VOM, you could put it in line, but it wouldn't be like a fuse, but it will tell you how much current is being drawn with different things switched on.

Sounds to me like you have a short somewhere. You might be able to detect it with the ohmmeter part of a meter, without the battery of course.

Dave
69S
Click to expand...

+1
Click to expand...
+2

Remember, on the Commando, the engine is isolated from the frame by rubber mounts - front and rear isolastics, head steady rubbers, even the muffler mounts. This means the frame is isolated from the alternator which is why there are so many ground wires in the wiring loom.

It sounds like there may be a short in the brake light circuit. Check the big connection block by the taillight assembly, or a skinned wire back there. also make sure the connections are right - its possible you may have plugged the brake light wire into a ground wire.
 
I'll have to keep working it. the brake light worked fine, then just snap the fuse blew when i wasn't doing anything.
 
Pull the red wire off the blue capacitor and see if there is a small spark when touching it against the terminal. This could short even without the ignition on.
 
DogT said:
No. Don't use a bigger fuse. Actually put a 12V bulb (like the one in the tach) in place of the fuse and see if it light up, brightly, and when. When it does, that may be your clue to what is shorting. Or if you have a VOM, you could put it in line, but it wouldn't be like a fuse, but it will tell you how much current is being drawn with different things switched on.

Sounds to me like you have a short somewhere. You might be able to detect it with the ohmmeter part of a meter, without the battery of course.

Dave
69S
Click to expand...

Replacing the main fuse with a bulb works great for troubleshooting a short and limits current (i.e. a 12W bulb would limit current to about 1 amp), but using the meter is not a good idea...VOM ammeters typically can only handle 10 amps or so and a dead short will pull much more than that and maybe blow the internal fuse in the meter or damage the meter.
 
I'm a lousy electrical trouble shooter, but I remember my Pops using a light bulb in this way too.
 
Sorry, my mistake at using the VOM in place of the fuse. Most are only fused at 10A if at all.

Dave
69S
 
OK, update. went to Autozone and bought a pack of 20 amp fuses. with the key off, I pulled the red wire off the 2mc, no noticble spark. i orderd a new one from Steadfast anyway since it looks original. turned the key all the way on, w/ lights, shook the bike, rocked the forks back and forth, wiggled the entire harness, no blown fuse. fixed the front brake light circuit, replaced the brown wire from the front junction behind the frame head tube, all the way to the connector at the tail light assembly. checked everything out everything works. i'm sure as soon as i get the tank and body panels back on i'll have a problem again. thanks again for all the great ideas. the only thing i didnt test yet is if there is a current draw with the key off. that't be another night
 
You don't have a volt/ohm/ammeter? You can get one for less than $20 and should have one. Make sure it reads down to at least 0.1 volt (i.e. 10.3VDC, better 10.31VDC).

The 2MC is not necessary if you have a battery and mostly is better left disconnected unless there is a reason for it (read no battery).

Blown fuses are an indication something is wrong, more fuses are not a solution.

Trouble shooting is a solution.

Dave
69S
 
Do some wiggle jiggles bouncing and tugging in the dark to see what lights up.
I have had a short in tail lamp where the uninsulated ground lead strap was touching a power terminal in bulb holder. Of course you can put in a bigger fuse then with fire extinguisher on hand watch and smell for bubbling insulation or smoke signal in a wire. A couple years ago had a ballast resister go bad and heat up so much it smoked paint near it but didn't blow fuse.
 
Do some wiggle jiggles bouncing and tugging in the dark to see what lights up.
I have had a short in tail lamp where the uninsulated ground lead strap was touching a power terminal in bulb holder. Of course you can put in a bigger fuse then with fire extinguisher on hand watch and smell for bubbling insulation or smoke signal in a wire. A couple years ago had a ballast resister go bad and heat up so much it smoked paint near it but didn't blow fuse.
 
I have a digital multi-meter, i just ran out of time to work on it. i got a supply of fuses to replace them in case they blow while i'm working the problem. possibly this weekend i'll have time to sort it out.
 
VOM testing completed as follows:

key off - 0.000A
park position - 1.45A
Ign. only - .195A
ign. / lights - 4.71A I saw the inrush current hit 43A.

i also checked my 2MC and it was holding 12.5VDC after several minutes.

turned everything on, wiggled the wiring harness, short not found. possibly the wire i replaced in the front brake light circuit was not just broken / disconnected but contating the frame? i'm not usually that lucky.
 
Did you see a bare spot on that front brake wire? If not, it'll be back. An intermittent short... I get them all the time on machinery. One method I've used successfully is ADDING fuse holders to break it apart , take and put a male bullet and a female on the fuse holder leads, snap these in place where the wires branch off for each device. Like, at the ignition switch, where power heads out to lights, another to the ignition (split the stop lamp switch on a seperate fuse). Under the seat, the horn circuit, inside the headlight. Best way I know to catch a rat. Cut the monster in half, then that half in half, etc.
 
No i took that wire out of the circuit, but thats a great idea on the fuse holder. I'm going to make one up.

Thanks!
 
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