Headlight blowing fuses

[Joshwuh]

Since I cut out my horn my high beam on my headlight has been blowing the fuse. I then connected the two horn wires together to see if it would do anything and the high beam stopped blowing the fuse but it also wouldn't work. Now both my high and low beams blow the fuse. I have been using 20 amp fuses to get the light brighter but both 10 and 20 amps are getting blown. I know nothing about electrical stuff. Where should I start to look first?

I heard that faulty earth leads are often the causes of electrical problems. Should I unwrap all the wires to trace them to their connections?

Also, both the main and the headlamp fuses have been blown but at different times. Lately, it's been the main fuse. I'm assuming this is two separate issues?

Thanks in advance for any guidance. BTW '77 XS400

 

[leedutcher]

check inside the right hand control... remember these are old bikes and the wires inside get brittle and may be bare causing shorting....

 

[Joshwuh]

Thanks for the advice. I was going to check it today but the bottom screw is stripped so I'll wait til tm so I can get a new screw. I checked the left hand control for the high/low switch and everything seemed fine except for a small tear in the wire's coating which I covered with electrical tape as a temporary fix.

 

[Joshwuh]

The right hand control looked good. The wires weren't frayed and the soldering was still in good shape. It's blowing the main fuse and the bike runs with the light off so I'm assuming that the fuse holder is good (I already replaced it this summer). I'm assuming my next step should be to follow the cable from the fuse box to the headlight to look for tears and bad connections? I was holding off on doing this because all the wires are wrapped together in a rubberish housing and I'll have to cut it off and have all the wires hanging everywhere to trace it :-/

 

[Joshwuh]

There are some frayed and or torn wires under my rear fender leading to my tail light and 1 or 2 wires with the rubber coating torn going into my tail light. Is there a chance that these are the cause of my headlight not working and blowing fuses?

Details:

When I switch my lights on the Neutral light dims and no other lights come on (dials included)
- after a few seconds the fuse blows (sometimes the holder is smoking hot)
Rear light works when brakes are applied with or without the lights switched on
When key is turned to the far right position dial lights come on as well as tail light

 

[HoughMade]

Sounds like a possibility to me.

 

[CadXS]

Joshwuh, for the fuse to blow the headlight circuit has a short somewhere. The fuse is there so when the short happens it will deliberately blow as the weak part of the circuit. Don't keep putting larger fuses in (10a max) as this will draw more amps, which equals heat and possible fire, before it blows.

For the short to happen the feed from the battery is not reaching the headlight. Normally for this to happen the feed circuit must have a bare spot where the insulation is worn away. When the bare spot comes into contact with a bare metal part (the frame) that acts as the return path to the battery for the circuit (ground in USA?), the path to the headlight is short-circuited and because there is no headlight providing resistance the circuit overloads and the fuse blows.

As the tail light co-acts with the headlight it is part of the same circuit so any damaged or frayed wires as you mentioned in your post could definately cause a short and the headlight fuse to blow. The brake light circuit is seperate even thought the bulb is housed in the same unit as the tail light.

There is no reason yet to cut the wiring loom apart. The shorts tend to happen where the wires stick out of the loom and can be subjected to some form of mechanical abrasion or corrosion.

If you have been putting 20a fuses in the headlight circuit this could also be causing the main 20a fuse to blow as well. With no fuse discrimination it would be a race between the two fuses to which would blow first when a short in the headlight circuit occurs.

As you described in the post above check for any bare patches in the insulation of the headlight (+ tail light, its the same fuse) circuit where it could come into contact with the frame. Replace or insulate by wrapping in electricians tape any bare patches, clean any plug couplers in the circuit and check for damage and corrosion and then let us know what happens.

 

[Joshwuh]

Wow, Cad thank you for the explanation. I couldn't have asked for a better answer! Instead of giving me a fish and feeding me for a day you have explained fishing to me so that I can learn to feed myself :-D