Blown 35A fuse, dead electrics and a small fire

Hi members. I started a small fire on my UK 1981 Kawasaki KZ440A. I have two questions to help me get to the root cause behind the reason for the fire.

Symptom - All electrics died during a journey. I'd been on the bike and moving for less than 5 minutes. Prior to the event, no issues. The battery is less than two months old.

Issue - A blown fuse. The fuse was actually a 35A fuse in the place of the 20A fuse.

Attempted fix - Roadside assistance came to my aid after I had checked two things: 
1. The battery was ok. It had 13.3V ahead of connection to the charger, which then indicated a full charge within a few minutes.
2. The ignition switch wiring, using a ohmmeter it checked out ok in the Off position and the On position.

Roadside assistance checked the fuse and replaced it. The engine restarted and all appeared good without me riding it. It idled stationary for a few minutes then I switched it off and returned home to get my gear.

The next day I restarted it. Within minutes, the fuse blew again. Naively, I followed the "Get home" advice in the user manual: wrap the fuse in tinfoil and re-insert. A few minutes after restarting the bike, smoke and a small fire started just ahead of the petrol tank. Needless to say the tin-foil fuse was flicked out. Mea culpa. Every day is a school day I suppose, and I was lucky the fire did not lead to a burn-out - I know that.

The fire appeared to be the brown wire leading to the horn. It is damaged from the fire spot into the main wiring loom under the tank. No other wires appear to be affected, except the red/white wire attached to one end of the 35A fuse - The solder betwixt wire and fuse clip has melted, and some burn on the fuse holder at the other side of the 35A fuse. There is some melting of the black electrical tape along the main wiring loom.

I've attached three photos. One of the fuse, one of the front-end of the fire-damaged brown horn wire, and one of the main wiring loom's seared electrical tape.

My two questions are:
1. Is the root cause most likely the rectifier? If so, Would the following be a suitable replacement:  Regulator Rectifier for Kawasaki KH400 KZ440 Z400J Z400LTD Z440LTD (electrexworld.co.uk) ? I've been unable to remove my rectifier yet. The two bolts are very hard to access.
2. Why did the horn cable go on fire do you think? I assume that's 12V therefore not on the rectifier 20A circuit? Could the horn somehow be the root cause?

I noticed before this incident, the full beam is not working. I've checked the bulb and it's ok, but I know the full-beam flasher shares a rocker switch with the horn. Coincidence, I wonder?

If anyone in the forum can help me diagnose things, that would be great. Thank you in advance.

James

Open that loom up and inspect the wires.
Hard to guess what caused it without investigating it further.

35A fuse?  You are literally playing with fire!  Count yourself lucky that the entire bike didn't burn to the ground.  

wrote:

My two questions are:
1. Is the root cause most likely the rectifier? If so, Would the following be a suitable replacement:  Regulator Rectifier for Kawasaki KH400 KZ440 Z400J Z400LTD Z440LTD (electrexworld.co.uk) ? I've been unable to remove my rectifier yet. The two bolts are very hard to access.
2. Why did the horn cable go on fire do you think? I assume that's 12V therefore not on the rectifier 20A circuit? Could the horn somehow be the root cause?


 

1) Why do you think the rectifier is bad?  Is the rectifier visibly damaged?  Anything melted?  I doubt the rectifier is the problem.

2) When the fuse blew the first time, it demonstrated that you have a fault.  It showed there is a high current short circuit to ground.  Generally high current makes wires very hot.  If you find something is melted after a fuse has blown, it's a good bet that the thing that melted was in the path of the high current.  So yes, the melted horn wire was probably related to the fault.

Fuses are included in vehicle harness in order to prevent damage to the wiring in the case of a fault.  That is, to prevent wires and connectors from melting.
In general, fuses never blow unless you have a serious electrical issue.  Once a fuse does blow, you know know that you have an electrical fault in your system. 
If you replace the fuse with another fuse, it's likely to just blow again unless you find and fix the fault.  If you replace the fuse with a penny or a random piece of metal, you are pretty much guaranteed to melt your harness or catch something on fire.  As you are now well aware, I'm guessing.

I think you're going to have to look through the entire harness assembly and search for melted wires and connectors.

How long was the 35A fuse in the bike? Did roadside assistance replace the fuse with a 20A fuse or another 35A fuse? 

Before fitting a new fuse work through the "short circuit" element of the guide on the below link
www.kzrider.com/forum/forum-index/faq-wi...-fault-finding-guide  

Hi members. I started a small fire on my UK 1981 Kawasaki KZ440A.

Please put the fire out.

35A fuse?  You are literally playing with fire!  Count yourself lucky that the entire bike didn't burn to the ground. 

If the 20 amp fuse blows install a 30 amp fuse.
If the 30 amp fuse blows install a 40 amp fuse.
If the 40 amp fuse blows install a 50 amp fuse.
If the 50 amp fuse blows you have real problems.

1. Is the root cause most likely the rectifier? If so, Would the following be a suitable replacement:

Rectifier is not the problem.
Remove wire harness, remove tape, look for burnt wires, report back here.

 

Thank you F64. Please see the following and if you have any advice, it would be appreciated.

I have opened up the loom and found the following:

1. The brown horn wire is destroyed all the way to where it and another brown wire (as a pair) connect with a single brown wire. Please see the first picture. There is no further damage either to the source brown wire or the second brown wire in the pair

2. Four damaged/melted (but not destroyed) wires (Please see the second picture):
White/Red - Turn signal switch to Turn Signal Control Unit
White/Green -  Turn signal switch to Turn Signal Control Unit, or brake light failure indicator to brake light failure indicator switch
Blue/White - Turn signal switch to Turn Signal Control Unit, or headlight switch to 10A fuse.
Red - Distance Sensor to Turn Signal Control Unit, or ignition switch to tail/brake light.

My guess is the horn wire melted the other four wires due to direct contact, rather than any of the four wires being the over-heated source.

I can repair all these wires with replacement lengths and appropriate labelling. 

I'm still scratching my head as to why the 35A fuse blew (I acknowledge it should have been 20A. I wasn't aware one of the previous owners had chosen to use 35A until this incident)? That fuse connects to the rectifier/regulator, starter relay, the ignition switch, and the battery. Would a peripheral device, such as the horn or the turn signal controls, blow this fuse if faulty? My head is perhaps in the space of domestic wiring, where there are circuits, each with a fuse and I'm thinking, the horn isn't on the 20A fuse, However, my thinking may be faulty. 

Thanks DOHC. You're guess is right - I am now very well aware 
I guessed the rectifier only because the blown fuse is directly connected to it, the starter relay, the battery, and the ignition switch. The battery tested ok and showed no damage, the ignition switch tested ok, and so I figured perhaps the rectifier was generating more current than it should be (over-rectifying), and blew the fuse. A pure guess as I don't know if a rectifier can do such a thing. 

I don't know but certainly for as long as I've had it - 4 years. Having said that, very few miles done. Less than 100. Roadside assistance confessed to knowing little about motorbikes and yes, replaced the 35A with another 35A.

wrote:

Before fitting a new fuse work through the "short circuit" element of the guide on the below link
www.kzrider.com/forum/forum-index/faq-wi...-fault-finding-guide  

This is a great way to narrow the possibilities without having to gut your electrical system. Follow the steps to the letter and in the sequence given. Being patient and methodical will pay big dividends, especially with electrical problems.

Thank you Wookie58. That is really appreciated.