Topic: Suddenly dead, no 12v at all...

[dboblet]

So, this morning I get up and turn the key and..... nothing .... nothing at all.  No idiot lights, no resonse of any kind.
so, a bit panniced since I'm supposed to get on the road to get to work, I pulled out a multimeter and checked the battery... um...12.6 volts... I pulled the plug from the start switch and re-plugged it in and cycled on and off... nada
I pulled the fuse cover off and checked all 3 fuses, all good, and replaced them all anyway just in case... and ... nada
At that point I still had to get to work so I climbed into the fuel guzzling 4 wheeled box and headded off to work.
Any suggestions.  I'm thinking a bad ground?  Any other possibilities??
('78 CB750f )

[TwoTired]

Check the key switch and the plug connected to it.

[dboblet]

I pulled the plug on the key switch (twice) and no joy.  I'm intimately familiar with this key switch since I had a problem with it earlier.  I do not believe this is the problem at this time (but I reserve judgment until I take it off the bike, god forbid I need to.  Seems kind of strange it worked well last night when I parked it and it's dead now though... and the key switch is one of the common factors for the power system once you eliminate the fuse block.

I think I'm just going to have to start with a multimeter at the battery and work my way out to the keyswitch... or maybe the other way 'round.

[seaweb11]

kill switch?

[OldSchool_IsCool]

Kill switch wouldn't take out the instrument lights.  Ignition key switch would though.

www.oldmanhonda.com for some good wiring diagrams.

[ofreen]

A bad ground is certainly a possibility.  But so is a bad battery.  I've seen a few bad batteries show 12+ volts on a multimeter - until you put a load on it.   A test light is a handy thing to have.




Edit to add more free advice.

[Steve F]

I've had a key switch go bad, and it was because of internal resistance on the switch contacts made them so hot, that they meletd themselves into the plastic and recessed into the switch body plastic.  So the next time I went to turn on the bike, the contacts would no longer touch.

[scondon]

I've had a key switch go bad, and it was because of internal resistance on the switch contacts made them so hot, that they meletd themselves into the plastic and recessed into the switch body plastic.  So the next time I went to turn on the bike, the contacts would no longer touch.

    Good example of how "ran fine before I parked it" doesn't say much when it comes to electrical woes

[droopy]

I've had a key switch go bad, and it was because of internal resistance on the switch contacts made them so hot, that they meletd themselves into the plastic and recessed into the switch body plastic.  So the next time I went to turn on the bike, the contacts would no longer touch.

+1 opps +2 had 2 bikes do this ohm the switch out first the first one took me 6 hour's of tracking through wire to find it on the plus side all the connectors got cleaned 

[bradweingartner]

Tracing electrical woes is one of the hardest things for people to understand yet one of the simplest things to actually work on.

If you know you have 12v at the battery, go from there. Multimeter in hand, follow every connection through the wiring harness until you no longer have 12v. Viola!

Now because you can never be too sure of the ground as well, choose a non-battery ground for your testing. Easy enough, any Green wire should be golden.

  And have fun!

[Hush]

Remove both power and earth leads from battery and give them a good wire brushing.
Reattach and if that doesn't work, jump start from the "4 wheel box" if that doesn't give it power, crate it up and send it NZ care of Hush, I love an electrical challenge.

[dboblet]

Thanks.  I have a complete wireing diagram (thanks to this forum) and I'm actually a whiz at electrical stuff but won't have the time until Tuesday (grrrrrr). 

My thought is that it IS the ignition switch that is at fault.  I had to do a lot of repair to it to get it to work properly and until now it was flawless (after the repairs).  But it holds the #1 position on my possible cause list.

[Hush]

Get back to us when you find the fault, I do love a mystery too.

[dboblet]

And the cause is.....
After verifying that the battery was indeed fully charged, I took a piece of tin foil and inserted it over the entire fuse block (covering all contacts at the same time).  This had the expected effect of turning on the entire bike's electrical system with all idiot lights going on.  I test fired it but starting up the bike for a minute or so before proceeding along to find the root cause.

I pulled the ignition switch assembly off and took it apart.  Seems that one of the two spring loaded copper contacts got warm enough to melt the plastic base and became stuck with the spring compressed so it wouldn't make contact.  Also, one of the copper 'pins' it was supposed to make contact with was recessed further than the others.  This may have been a design (break before make - in electrical terminology). 

To fix, I lit a candle and holding the copper contact with needle nosed pliers, heated it to "very hot indeed" and then put it in place and wiggled it around to get it to melt the slot back to "close to" original shape.  The result was that the spring could now push the contact properly against the pins once again and when assembled, all was good.

The question remains as to why it got that hot in the first place.  I think it may have been because I was turning the ig switch on and off a lot when working on the bike... and maybe because I was repeatedly hitting the starter button (which as we know will cause heat build up ... somewhere... ).

So... I may buy one of those aftermarket switches I've seen as a result of this forum.... bit dear in price though.

[OldSchool_IsCool]

How corroded were the contacts?  Resistance=heat.

[dboblet]

Actually *all* of the contacts looked clean, though not necessarily polished.  There was no sign of 'burning' or pitting, or discolour at all anywhere.  Just blackened melted white plastic on the area where the contact was mounted by the spring.
No discoloration where the plug contacts were either so I can't figure it unless there was just a very large load for a short time, or heating caused by on/off surg from repeated switching on and off... my fault probably... Time will tell if it comes back.