Topic: 1978 cb750k- sudden battery death?

[Kev Nemo]

So this is new. I haven't been riding my bike as much as usual with the cold weather, but I have been running my bike at least once a week. I made sure to warm up my bike NYE for about 10min, rode over to my friend's with no issue and when I went back out to drive home a few hours later, it wouldn't crank and the headlight was dead. I charged the battery (charged quickly) and it still wouldn't turn over. I hoped it was just a bad battery, but after I put in a brand new battery, I had the same result (although the headlight came on for abit.) I checked the connections/fuses and no go there. I don't have a voltage meter to trace down a short (yet), but I'm wondering what I should check next-coil, etc?

[MötleyRöx]

What happened with the headlight?
How long is "a bit"?
And... did it go off suddenly or gradually?

[Kev Nemo]

What happened with the headlight?
How long is "a bit"?
And... did it go off suddenly or gradually?

1-it faded out completely
2-it came on for about 1-2 minutes, but very dimly. It would go out completely when I attempted to engage the start button.
3-Gradually. The first time I tried it on NYE, it actually tried to turn over for about 30 seconds and then died completely. With the new battery tonight, I could barely get the the headlight to come on at all.

[Kev Nemo]

Also, I checked all fuses and they were intact. I had a problem with this back in the Spring and it was a combo of the key ignition and bad wiring. I fixed it then and I haven't had an issue since. If it is the keyed ignition and I have to replace, what are my options there? Should I go with an aftermarket ignition or second-hand Honda unit?

[Yoshi823]

Sounds to me to be a short circuit diode in the rectifier.

Easily checked if only you had a multimeter.

[steam-powered man]

connections were checked, that's my first choice.  ah, maybe the battery you put in wasn't fully charged?  this would also give you the same condition. 

i'd sand/clean the battery connections again first (esp ground at the engine bolt).  then i'd pick up a radio shack multimeter (<$11), put the battery on a trickle charger overnight, then check voltage with the bike off, running at idle, then running at 3K.  i'd then note the battery voltage with bike off and check again in a few days, checking for drain.  good luck,
bobp   

[TwoTired]

FYI.  For this bike, when you push the starter button, it disconnects the headlight.  So, it going out is normal.

You need to know the battery state of charge.  Battery voltage or electrolyte specific gravity. To be certain it is fully charged.

Then you need to run the engine and measure the battery voltage at idle, 2000, 3000, 4000, and 5000 RPM.

Once you have this information, you can proceed from there towards diagnosis.

[Johnie]

Regarding your question on the ignition, if you have to replace it go with OEM Honda. There are a number of posts here with guys having problems with the aftermarket ignitions.

[andy750]

The 78s have a plastic connector behind the ignition barrel that hooks up the harness to the ignition - check thats making contact ok. If you need a new ignition barrel go with Honda - the aftermarket ones are not reliable.

good luck
Andy

[Kev Nemo]

Nope, new battery is fully charged through trickle charge-rule out that. Pressing the ignition button gets gets no response from engine, but kills the (very dim) light. I'll get a multimeter and see what I can come up with.

[Spanner 1]

Shorted rectifier is killing the battery lickety-split.....the rectifier is connected across the battery whether the ignition is on or not..
i.e.,  battery in bike = connected to rectifier......

[TwoTired]

Nope, new battery is fully charged through trickle charge-rule out that.

Nope you are making assumptions, the dearth of troubleshooting.
You can't rule out battery until the voltage is measured.  New batteries can fail, as can trickle chargers.  In fact, trickle chargers can ruin a battery if not used properly or rated properly.

Pressing the ignition button gets gets no response from engine, but kills the (very dim) light.

If the light is dim, it indicates low voltage.  I already told you that the starter button switch turns off the headlight while pressed.

I'll get a multimeter and see what I can come up with.

Now, THAT'S a good idea!

[Kev Nemo]

Shorted rectifier is killing the battery lickety-split.....the rectifier is connected across the battery whether the ignition is on or not..
i.e.,  battery in bike = connected to rectifier......

So is a recitifier rebuildable or should I replace it? If I replace it, where can I find one?

[Spanner 1]

Try this....put the ( charged ) battery in the bike with the rectifier unplugged. Read the voltage across the battery, reconnect the rectifier and watch the meter over a few mins. to see if the voltage drops as I suspect one or more of the 6 diodes in the rec. are shorted out.....
if so it's available from several vendors...do a 'rectifier' search here, lots of info. Good luck.....

[Kev Nemo]

I wanted to add that I also tried to kickstart it and had no luck there, either.

[Yoshi823]

Shorted rectifier is killing the battery lickety-split.....the rectifier is connected across the battery whether the ignition is on or not..
i.e.,  battery in bike = connected to rectifier......

So is a recitifier rebuildable or should I replace it? If I replace it, where can I find one?

http://images.google.co.uk/imgres?imgurl=http://www.osha.gov/SLTC/etools/electric_power/images/full_wave_rectifier_circuit.gif&imgrefurl=http://www.osha.gov/SLTC/etools/electric_power/illustrated_glossary/substation_equipment/rectifiers.html&usg=__d0WIbgD4u8ecJmhWhM0nhLdejV8=&h=283&w=300&sz=3&hl=en&start=1&um=1&tbnid=Gteb4U2S5eMoRM:&tbnh=109&tbnw=116&prev=/images%3Fq%3Drectifiers%26hl%3Den%26client%3Dfirefox-a%26channel%3Ds%26rls%3Dorg.mozilla:en-GB:official%26sa%3DX%26um%3D1

The OEM rectifier might be rebuildable...never tried.
The diagram shows a 4 diode type rectifier. The one on the Honda is a 6 diode type, as there are 3 outputs from the alternator ie.3 yellow wires take the AC voltage to the rectifier. By measuring with a multimeter set to about 2000 ohms, place one of the leads from the meter across between the red & any of the yellow wires.If you don't get a reading that way around, then reverse the meter leads & measure again. Do this between all of the yellow wires & the red output wire, then do the same thing between the yellow & the green/earth wire. If any of the readings show a short circuit in both directions, the meter will read zero ohms & this will suggest the faulty diode....I would think this will show between the green & yellow wires more than the red & yellow wires.
I think that the replacement units that are non-OEM are now enclosed in a single unit rather than the old style 'open to the elements' type.

[Kev Nemo]

The 78s have a plastic connector behind the ignition barrel that hooks up the harness to the ignition - check thats making contact ok. If you need a new ignition barrel go with Honda - the aftermarket ones are not reliable.

good luck
Andy

This was the issue, but I can't find this part online-any leads?

[andy750]

Glad to hear it. I used to use a zip tie to hold it in place. Once I did that I never had an issue again -well until the barrel itself gave up! But the connector held in place. Try that.

cheers
Andy

[Kev Nemo]

yeah-already been there and it lasted almost a year Someone linked me a replacement-cheap! http://www.regulatorrectifier.com/catalog/Honda-Switch-Base-Repair-Kit

[Kev Nemo]

Damn it-switched ignition/barrel and still killing the battery. I'm forwarding the mechanic this thread to he can check the rectifier.

[scottly]

Did you perform Spanner1's drain test? If you pull the main 15A fuse, you will eliminate the ign sw from the picture.