Topic: Bike only lives for 6.3 miles.

[schmidbc]

Well my triumphant return to the mean streets of Cincinnati has taken a bad turn.  While on my inagural test ride after rebuild, my electrics went dead.

I was on the return leg of my 7 mile ride when I switched off the high beam upon approaching an intersection, when I looked down after leaving the intersection my bike had lost all power.  No headlight, no dash lights, no nothing.  The best I could do was to pull over while I was still coasting.  I've inspected the fuses and then I hung it up.

I have no idea of where to go from here and I would be glad to take suggestions from anyone.  The bike is a 1976 cb750f.  It was functioning properly until the power down and I know the battery to be in good condition and still have a full charge courtesy of my Yuasa battery monitor.

[SteveD CB500F]

If you have no lights or ignition it must be the main fuse or the ignition switch.
Remove the fuse and test it.  Then replace it and test continuity between the fuse plate terminals - you'd be amazed at how these 30 year old electrics can hide the problems!
If OK, trace the +12V from the battery terminal to the ignition switch. I had a fractured solder joint on the back of my switch which refused all attempts to resolder.  I bought a new switch in the end.

[schmidbc]

I agree that the ignition switch may be the problem.  There is evidence that the ignition and dash assy have recived some sort of shock, the cover on the dash assy is broken as has been rigged to stay on until I pony up the $$$ for a "new" one.  Still though, I'm not sure about these types of fuses.  I'm going to start with that easy fix and buy new fuses to replace them.  Hopefully I just can't see that they're blown because I don't know what I"m looking for.

Do you think is my be the switch assy where you put the key in, or the switch assy under the side cover mounted to the battery box?

[Bob Wessner]

Hope you resolve your current issue. Given the age and current symptoms, it might not be a bad idea to, in the near future, go through the entire harness and clean all the connectors and grounds. Even though they may be "working," perhaps not at their best. Time and crud begins to create resistance.

[alan calvert]

The ignition switch is certainly a possibility. I have had 2 switches in 2 years where a wire has come loose from the back and resists any attempt to solder on again . The switches now are pattern an of poor quality

[schmidbc]

Bob, great idea.   However someone beat you to it; I cleaned the connections whilst I had the bike-in-a-box stage of development going.

Well in the light of day the problem was clear, my 15amp fuse in the fuse box had blown.  The filament of the fuse looked corroded and though it was not blown, it didn't pass a continiuty test.  So new fuses mounted, I'm off and riding again.  I should've replaced all of the fuses when I did the rebuld, but I guess I was too anxious to get rolling when I got to that stage.  Just goes to prove that patience is a virtue.

There is one thing left that bothers me, the fuse box lid melted where that fuse had been.  I'm wondering if the corrosion in the fuse caused the meltdown becuase it kept the fuse from blowing quickly enough or if my problem is more deeply rooted.  There's a sting going that is title "wtf melted fuse box" I believe, maybe I'll check there.  Any repsonses here would be appreciated too.  Maybe a result of the new, upgraded dual output 3 ohm coils?

Steve D, thanks again for your help.

      -Brian.

[TwoTired]

Clean and polish the fuse clips.  Verify the clips can still pivot to align themselves with the fuse for maximum contact surface area between fuse and clip.  Check to see if the spring clips have lost their tension.  They should have a really firm grip on the fuse.  Check the solder connections on back of the fuse blocks.  Poor connections here can conduct heat to the fuse.

As you know, a fuse will melt when it's current rating is exceed.  The fuse is always generating some heat whenever current flows through it.  More current = more heat generated inside the fuse. 

The bike in stock form had plenty of margin between normal run current draw and the point at which the fuse melts.  3 ohm coils draw about 40% more power, through that fuse.  And, while it is not enough additional to melt the fuse outright, it will run in a warmer state than with the stock coils.  If you add contact resistance heating, poof.  In theory, you should increase the fuse rating by two or three amps.  That should get the normal run fuse temps back to where the factory designed.  Remember, though, that the factory used new, un-oxidized fuse clips, too.  Also, if you have added lighting upgrades that draw more power, these increases should be added to the fuse rating, too.