Topic: CB400F main fuse keeps blowing! help

[mathbishop]

I have a 1975 CB400F that for the past 7 months or so has been riding flawlessly, had no issues whatsoever. Just took it out for a ride and after about 20 min of riding the bike loses all power due to the main fuse blowing. And it keeps blowing.

I havent really made any changes to the bike, so I dont know how or why a short could occur.

Does anybody have any ideas of where I should start looking for the issue?

Thanks guys

[jonda500]

A good wiring diagram would help you track down your short - starting at the battery, track down where the power goes to when you turn on the ignition and begin checking each wire for possible rubbing on the frame. Or starting at the other end, disconnect one by one every thing that turns on with the ignition checking inbetween whether you can put a fuse(or thin slither of alfoil) without it blowing. Or.. -and I can't recommend this although it has worked for me- turn the ignition on and short the terminals of the fuse that's blowing with careful brief contacts, whilst keeping your eyes peeled for smoke and noting the location of the source of the smoke! But this could seriously damage your wiring loom if not done extremely carefully.
   However if your handlebars have been converted to external wiring from the switches the first place I would look is where the switch blocks clamp around the handlebars to see if the wires are being crimped where they exit the switchblocks. Best of luck with it, John
   

[mathbishop]

A good wiring diagram would help you track down your short - starting at the battery, track down where the power goes to when you turn on the ignition and begin checking each wire for possible rubbing on the frame. Or starting at the other end, disconnect one by one every thing that turns on with the ignition checking inbetween whether you can put a fuse(or thin slither of alfoil) without it blowing. Or.. -and I can't recommend this although it has worked for me- turn the ignition on and short the terminals of the fuse that's blowing with careful brief contacts, whilst keeping your eyes peeled for smoke and noting the location of the source of the smoke! But this could seriously damage your wiring loom if not done extremely carefully.
   However if your handlebars have been converted to external wiring from the switches the first place I would look is where the switch blocks clamp around the handlebars to see if the wires are being crimped where they exit the switchblocks. Best of luck with it, John
   

I put on some clubmans a while ago, and in doing so destroyed my stock right hand controls (start, kill switch, etc.) I was able to salvage the left hand (turn signals, headlight, horn) but the wires are a rats nest and not in the best condition. After replacing the fuse I had the bike running and just wiggled the wires coming from the right hand controls and the headlight went out. Im doubting that it was just a coincidence. Im gonna order a new left hand and go from there.

*crossing fingers*

[Bodi]

Try wiring a tail light bulb in place of the main fuse. A short in the system will have this lamp at 100% bright. Normal load will be much dimmer. Just wiggle stuff until you find where the short is.
Most likely places are - inside the headlight shell, in the switchpods or the wiring to them, anything kludged together by a PO, and finally the harness section that flexes at the steering head.
For a wiring diagram: only use the real Honda one for your year and model!!! All the rest (clymer etc) are complete crap for finding problems: they are OK for a basic understanding of the wiring of typical bikes but they all leave out of vital circuit details specific to your bike.

[HondaMan]

I might suggest this, first of all: the contacts on the old glass fuses (and blocks) are now corroded. This makes them run quite hot, which melts the fuse. It might not actually be a short...

Remove the fuse and shine up the inside of the fuse clips, and maybe squeeze them a little closer together, if they don't hold nice and tight. If you happen to have a .22 caliber gun bore cleaning brush, it is perfect for the shining-up task. If not, try some fine emery paper, maybe on a small bolt, spin it in the clips. Clean well afterward.

If this fixes it up, maybe consider one of my ATC/ATO Fuseholders? I think I've sold more of them for the 400F bikes than all the others combined, this last year. It uses modern automotive fuses, holds a whole bunch of spares inside, too.