KZ400 fuse destroyer

The bike is burning the main fuse (20A).

It all started a month ago, suddenly the main fuse blew. I thought, these are fuses from almost 40 years ago, it could be normal. I replaced and all was well (except the idle is unstable, looks like is going to stop, but I didn't think that was related). Yesterday the main fuse blew again, I put in a spare and after about half a minute it blew again. I had to push the bike home.
Each time it burned while the engine was running and warm.

- I have checked the coil (green Dyna), correct resistances and the wires do not touch the mounting post
- I have ordered a resettable fuse so as not to accumulate a pile of blown fuses, it will arrive tomorrow
- I still have to check the voltage with the engine running to see if there are problems with the regulator (it's electronic, it seems unlikely). But I'll wait for the resettable fuse so I don't burn anything

Anything else I can try?
I also have this little gadget that suddenly appeared but I have no idea if it will work for this hehe
 

Sounds like you have a live wire shorting out someplace. Normal suspects are where the loom enters the the head light bucket. As you turn the handlebars the loom can rub on the headlight bucket hole and if it doesn't have the rubber grommet fitted can wear through the insulation and short out. 
Look for broken/worn insulation on any main live which is normally brown. 

I would suggest you read through the "short circuit" fault diagnosis process in the attached guide. It will get you there without working through a bucket of fuses

www.kzrider.com/forum/forum-index/faq-wi...-fault-finding-guide

Wookie58 wrote:

I would suggest you read through the "short circuit" fault diagnosis process in the attached guide. It will get you there without working through a bucket of fuses

www.kzrider.com/forum/forum-index/faq-wi...-fault-finding-guide

Thanks mate. We all want a quick and easy solution, but sometimes things go by calm down, study and understand.

I had a very difficult problem with the carburetion a long time ago and that was the only way to solve it.

Did you leave the stock wiring when you hooked up your reg/rec?
The original rec/rec battery charging wire isn't fused(white/red stripe) on a 400H.
If you tied into a line that is fused, you may overload the fuse when the reg/rec starts sending more current to charge the battery.

It could really be anything.
What did you add/modify electrically to the bike?
This could also be an item that only fails when hot.

You could also check the coil's primary resistance immediately after the fuse blows.
Just disconnect one of the coil's primary wires and measure resistance.
Compare that with the cold coil primary resistance reading.
 

It could be a rubbed wire. Common places are under the tank, and wiring going into the headlight shell if the rubber grommet is all rotted and falling apart as most are by now. It could also be bad fuse clips causing the fuse to run red hot and melt. Usually you can see signs of plastic melted near the 20a fuse if this is the case, and rather than the fuse blowing, one end melts loose inside.
This was a huge problem and common. Touch the fuses and see if the 20a is hot. They run really warm even in normal operation, but it will be too hot to touch when the clips go bad. They can heat up from normal to red hot in a matter of seconds.
Really bad, and all the miles I rode at night I was just praying those clips would not leave me dead dark in the middle of the road.

I see in your signature that you replaced the handlebar controls. Once I replaced a right side control and a few months later I had the same problem. I had pinched a wire when I Installed the new control. An old guy told me a long time ago that the first place to look is the last places you worked.

F64 wrote:

Did you leave the stock wiring when you hooked up your reg/rec?
The original rec/rec battery charging wire isn't fused(white/red stripe) on a 400H.
If you tied into a line that is fused, you may overload the fuse when the reg/rec starts sending more current to charge the battery.

It could really be anything.
What did you add/modify electrically to the bike?
This could also be an item that only fails when hot.

You could also check the coil's primary resistance immediately after the fuse blows.
Just disconnect one of the coil's primary wires and measure resistance.
Compare that with the cold coil primary resistance reading.
 

I renewed portions of the wiring but I kept the colors, the diameter of the cables and even the type of terminals.

Question: Can a coil that shorts when heated really blow the fuse? The issue of the blown fuses has happened at the same time as the loss of idling stability. That makes me think a lot about the coil.

Anyway, I haven't tested it yet. I've been busy moping and moping haha.

RonKZ650 wrote:

It could be a rubbed wire. Common places are under the tank, and wiring going into the headlight shell if the rubber grommet is all rotted and falling apart as most are by now. It could also be bad fuse clips causing the fuse to run red hot and melt. Usually you can see signs of plastic melted near the 20a fuse if this is the case, and rather than the fuse blowing, one end melts loose inside.
This was a huge problem and common. Touch the fuses and see if the 20a is hot. They run really warm even in normal operation, but it will be too hot to touch when the clips go bad. They can heat up from normal to red hot in a matter of seconds.
Really bad, and all the miles I rode at night I was just praying those clips would not leave me dead dark in the middle of the road.

When the fuse blew the bike stopped and I went immediately to replace it. It wasn't hot, maybe it got cold fast but the amazing thing is that the new fuse lasted barely 30 seconds. That makes me rule out a temperature problem in the fuse box.

I have to do tests, there is already a long list.

Kelly E wrote:

I see in your signature that you replaced the handlebar controls. Once I replaced a right side control and a few months later I had the same problem. I had pinched a wire when I Installed the new control. An old guy told me a long time ago that the first place to look is the last places you worked.

Thanks, I'll check this out too.

Yeah, the coil could have internal wire insulation issues. As the coil gets hot the insulation breaks down which shorts internal windings.
If you have the old coil, you could swap it.
Or just check the resistance of the coil out of circuit when fuses start blowing.
 

F64 wrote:

Yeah, the coil could have internal wire insulation issues. As the coil gets hot the insulation breaks down which shorts internal windings.
If you have the old coil, you could swap it.
Or just check the resistance of the coil out of circuit when fuses start blowing.
 

I heated the coil with a heat gun (until it couldn't touch with my fingers) and this is what I got:

COLD: 14,50 kohm secondary, 3,2 ohm primary
HOT: 18,30 kohm secondary, 4,0 ohm primary

I think it's within reasonable limits.