Author Topic: My + and - battery wires have only 1 ohms resistance..?????  (Read 1960 times)

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Offline Hondawggie

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If I turn the key on to my 1979 cb750F -- with the BATTERY REMOVED from the bike -- I'm getting almost a dead short when I measure resistance between the red "+" battery terminal wire and the black "-" or ground battery terminal wire.

That freaks the crap out of me.

Because think about it -- how drunk would you have to be to try this stunt:

- with the battery in the bike and connected to the harness
- and ignition on
- connect a 1 ohm resistor across the + and -  battery terminals.  Just to see some smoke and sparks.

You would need a HUGE, FAT 1-ohm resistor to handle the 12 amps of current between your battery terminals.

We're talking  ohm's law here,  (VOLTS / RESISTANCE) = CURRENT,
ie.  E/R = I.

Putting a 1-ohm resistor across the battery means you'd have (12 volts / 1 ohm) = 12 amps flowing through that resistor.

It better be a fatazz resistor to handle all that heat.

I mean, I've got a problem here, right?

If I switch the key off, the resistance between the red and black battery wires (battery disconnected) goes to infinite ohms.

But with the keyswitch on, battery disconnected and out of the bike -- I literally have only 1 ohm resistance between the red and black battery leads.


AS A SANITY CHECK -- I ran the same test on my '71 cb750k1.  Battery removed.
- key off: infinite ohms between the red and black battery wires.
- key on:  about 45 ohms between the red and black battery wires.

So my 1971 cb750k1 will have  (12 volts / 45 ohms) = 0.26 amps across the battery if I connect up the battery and turn on the key.

12 GOTT-DANG AMPS of current!

Someone tell me if this is normal on a 1979 cb750f.


Offline scottly

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Re: My + and - battery wires have only 1 ohms resistance..?????
« Reply #1 on: July 06, 2012, 07:39:27 PM »
Yeah, it looks you have a problem... with your K1. :o  Seriously, you should be reading a much lower resistance on the K1. If it has the stock mechanical regulator, the field alone would be about 7 ohms. If the ignition kill switch was on, and one set of points was closed, that would drop the reading to about 3 ohms, if both points were closed, about 2 ohms.
On the Dohc, the headlight and taillight are always on, and the filaments will read a lower resistance cold than when the lights are on. In other words, the resistance test you are doing is not really valid.
Don't worry about it.... 8)
Don't fix it if it ain't broke!
Helmets save brains. Always wear one and ride like everyone is trying to kill you....

Offline James T Kirk

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Offline Hondawggie

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Re: My + and - battery wires have only 1 ohms resistance..?????
« Reply #3 on: July 06, 2012, 11:37:58 PM »
Okay guys, thanks, that is a huge relief.   Dang ohmmeter, had me all worked up about it.

Thanks for the pointer to that dohc forum, will check it out.