Cant figure it out. Pls help!

[Muscle racer]

Hey guys,

I have another problem, and I can't figure it out.

2 days ago, I started up my bike to let it warm up.  As I approached it, it just died. No lights, which meant I no power.  I tried turning the key and nothing. Like the battery died. I left it alone for a few hrs, then came back and turned the key and the bike started. The next day, I tried again and it started up. I rode down the street and back then it died again. I figured it was the battery, so I took it out and charged it over night.

This morning, I put the battery back and still nothing. I turn the key but nothing. No response at all. I checked the fuses and their all good. I checked the key tumbler, made sure it was connected properly and it was.

What else do I check when theres no power? Could it be the ignition coils? I had them replaced last year, but with another original piece, so it's old.

Why did it die, then 4 hrs later, start up again?

3 days ago, it ran perfect. Now, it shows no signs of life!!!

Thanks again

Pls help, I have to ride to work later. Lol

[Half-Caf]

You may want to check out your kill switch. Taking apart the RH controls and giving the contacts a good cleaning may help.
Just my .02

[Re-run]

Coils will not affect power like that.
You need to look at the system from the battery to the ignition switch to the kill switch.
Regardless what your ignition does, you should still be able to get lights.
Start by checking battery voltage. If it is low, charge it up and then see if the bike starts. If it does, rev it up and see if the voltage to the battery rises. Should hit at 14 volts or pretty darn close once you hit 2000 rpm.
If the battery is ok, check your fuses. I know you said you checked them but they could be bad or have bad connections. Pull them all out and clean the holders till you see nice clean brass. Replace the main fuse with a new fuse. SOmetimes they look good but the filament is weak at the end and pulls apart when it warms.

From there, pull the connector off the bottom of the ignition switch and clean the connectors. Some contact cleaner would be good to get. Make sure it does not leave residue.

[LukeM]

You might also check your ground wire for your wiring harness. On my 79 750L, it was a green wire bolted to the frame under the seat. Your harness may be different.  I guess we need some clarification: when you say it dies, do you lose ALL power?  No headlight, no horn, no starter click, no tail light?

I fixed a '75 CB500/4 by tapping the guy's freshly rebuilt wiring harness.  Once he resolved the ground wire problem, it was ready and steady from that time on.

Good luck, don't lose heart, and I'll echo Re-Run's advice: start at the battery, and follow every connection.
It WILL run again.

Luke M

[Muscle racer]

I'll check the kill switch. The biggest of the 3 fuses, has some rust on the inside corners, but it's still in one piece.

It has no power. No lights, no horn, nothing. And I charged the battery over night.

I'll check every connection from the battery to the kill switch. Would I have to remove the gas tank?

Either way, I need to get it running.

Thanks for the quick resPonse.

[Muscle racer]

I think I might have figured it out.

I checked everything, and it all seemed right.

 I went back to the key switch. The back of it was kinda loose, so I pressed a screw driver against it,  to make sure it was in all the way and it started. I turned it off and ran inside to get my helmet, when I came out, it wouldnt start again.  

the back of the key switch just fell all the way off. I don't think it's supposed to that.

[TOOLS1]

I would check 3 things. Check, for voltage at the fuse box (both sides of the fuse). Check voltage at the ignition switch (again check, for it going in, and coming out). And the battery main ground. If in a pinch, you could hot wire the bike, by running a wire from the battery positive to the coils.
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[sgtslag]

To test the ground connection, first read the voltage across the battery terminals, directly:  it should read around 12.6 Volts, if the battery is fully charged.  Then, connect the red lead to the battery's Positive post, and touch the black lead to the engine block where it has bare, non-painted metal exposed (the spark plugs ground through the block, so this is an excellent approach):  you should read the same voltage, around 12.6 Volts.  Do this when the headlight will not turn on...  If you do this when the light works, you will learn nothing.  If the voltage reads low, or zero, then you have a ground connection problem.  Check the large, black cable coming off of the Negative terminal of your battery:  make certain it has a solid, tight, clean connection, at both ends (scrub with baking soda paste made with a little water, using an old toothbrush, to remove/neutralize any acid); measure the resistance (Ohmeter  function) on the heavy, black, ground wire, to be certain it reads zero, or less than 2 Ohms (checks for an open/broken wire, or bad crimped/ soldered connector on either end of the wire).  Cheers!

[Lucky]

You cannot see electrons, you have to measure them.
You need to use a volt ohm meter.

No electrical problem can be diagnosed until you have a SOLID
source of 12V.

Make sure your battery and its ground is good.
Make sure to measure between the ground post on the battery and the engine case.
also measure between the engine case and the frame for continuity on the ohms X 1000 scale.

BTW...A faulty kill switch would not make the lights go off. SKIP that.
FORGET the coils, They do not control the lights.
And if you still want to check their function just clip on an inductive timing light onto any
spark plug wire . No need to remove the plugs.

If the lights go off at the same time the engine quits that is a MAJOR clue.

[shinyribs]

Muscle racer wrote

I think I might have figured it out.

I checked everything, and it all seemed right.

 I went back to the key switch. The back of it was kinda loose, so I pressed a screw driver against it,  to make sure it was in all the way and it started. I turned it off and ran inside to get my helmet, when I came out, it wouldnt start again.  

the back of the key switch just fell all the way off. I don't think it's supposed to that.

Well,sounds like you found the loose connection! Cool. Now ride! Good job bro

Haha.I had to laugh picturing you "running " inside to get your helmet. Woo Hoo! It's running. I gotta go I gotta go!  Haha   I've been there. Thats a good feeling

[Muscle racer]

so i took this off the bike.

if the back comes off like this, does it mean its broken? the key turns but it doesnt do anything?

i wanna make sure its really broken before i buy another one.

[TOOLS1]

If it is not properly attached into the keyed part, it will not turn in the switch it's self. There is supposed to be a cover piece that locks it in. vintagecb750.com has them, but I would also check the switch by turning it with a screwdriver, and seeing if the bike runs.
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