Topic: Battery Current Draw on a '72 CB750

[jwalters]

Hello,

I have bene having a nagging concern that there is an electrical short somewhere on my bike.   I have several reasons for assuming this, but I'll spare you those details to get to the point and question.  I am going to try and lay out each current measurement with the bikes electrical "state".

State 1:  Current Draw from battery 2.4 amps
-Kill switch off (so the engine wouldn't run)
-Ignition On (key in the run position)
-Engine off
-No headlight, no status bulbs on (status bulbs = high beam, low oil, neutral, etc)
-basically no lights on the bike what-so-ever


State 2: Current Draw from battery 5.2 amps
-Kill switch on (engine is not on, but would fire if I kicked it over)
-Ignition On  (key in the run position)
-Engine off
-No headlight, no status bulbs on (status bulbs = high beam, low oil, neutral, etc)
-basically no lights on the bike what-so-ever


It makes sense that there is current drawn in State 2, because this is when the coils are being powered, but does the 2.4 amps seems correct for State 1?  Is there some other rectifier or something that is powered which I am overlooking?  If not, I know where to start looking for an electrical problem.

Thanks!
-jesse

[TwoTired]

In your state 1, the field coil of the alternator is drawing power to make a magnetic field and the instrument bulbs are on, depending on lighting switches.  (You don't say model and year of your bike and whether stock)

1.67 Amps = 7.2 ohms @ 12v
1 amp  = Four 3 watt instrument lighting bulbs  (These are approximates)
______
2.67 amp draw

Your state 2 adds one ignition coil.  5.2 minus 2.4 = 2.8 Amps  Using ohm's law, I'm guessing you have 4.2 ohm coil primaries or less with some extra resistance in your wiring connectors and switch contacts.

Cheers,

[BobbyR]

One very quick check for a short is to take a small 12V bulbwith leads attached, or you can us a  a volt meter. Put one lead on the negative post on the battery and  the other lead to the frame. If you have a short the bulb will light or the meter will register a voltage. Old time and low tech, but it works.

[jwalters]

I do say what model and year, its in the subject.  I also mentioned that the current values are taken AFTER I temperarily disabled the "status" bulbs.  But that does make sense about the alternator.  I was interested in whether the initial current draw was correct or not.  Seems like it is.  Thanks!

status bulbs = high beam, low oil, neutral, etc

In your state 1, the field coil of the alternator is drawing power to make a magnetic field and the instrument bulbs are on, depending on lighting switches.  (You don't say model and year of your bike and whether stock)

1.67 Amps = 7.2 ohms @ 12v
1 amp  = Four 3 watt instrument lighting bulbs  (These are approximates)
______
2.67 amp draw

Your state 2 adds one ignition coil.  5.2 minus 2.4 = 2.8 Amps  Using ohm's law, I'm guessing you have 4.2 ohm coil primaries or less with some extra resistance in your wiring connectors and switch contacts.

Cheers,

[jwalters]

Thanks Bobby, I probably should have mentioned that I am an Electrical Engineer, I was simply concerned about the where the initial current draw was coming from in my 1st configuration.  BTW, your negative terminal of your battery is tied to the frame so you should never see a voltage drop between the frame and negative terminal.
-j

One very quick check for a short is to take a small 12V bulbwith leads attached, or you can us a  a volt meter. Put one lead on the negative post on the battery and  the other lead to the frame. If you have a short the bulb will light or the meter will register a voltage. Old time and low tech, but it works.

[TwoTired]

I do say what model and year, its in the subject.  I also mentioned that the current values are taken AFTER I temperarily disabled the "status" bulbs.  But that does make sense about the alternator.  I was interested in whether the initial current draw was correct or not.  Seems like it is.  Thanks!

Oh yeah, the subject.  A shame you hid info like that...

Anyway, the bulbs in the list I provided are NOT the status lamps, but the four bulbs that illuminate the gauge dials.  These are powered directly from the ignition switch via the Br/W wire.  At least, that's how it is with the stock wire harness.

Cheers,

[jwalters]

You are correct sir!  My '72 has the abiltiy to completely turn off the headlight, I thought it turned those off too, but I'll go back and check.  Thanks!  From what it sounds like though, I think everything is ok, but if nothing else than curiosity, I'll go back and taker a peek!

Thanks again for all your help!
-j

[jwalters]

Yes, those backlights definitly go off when I don't have the headlight on.
-j

[BobbyR]

JW, You are not looking for a voltage drop with my test. You are looking for a short circuit. When going from Negative to Negative there should be no voltage present at the frame, engine block, or any other component normally found to be a ground.  If you have a short to ground you will find a voltage in one of these test points. I should have mentioned I am in the engineering department of a high speed rail car manufacturer, but I did not think it was necessary.  I am anxious to see what your findings are as i am interested in current draw on these bikes.

[jwalters]

I hear ya loud and clear on your method.  The bit of resistance in the frame would allow for a slight of voltage drop across the frame which could be seem in the bulb lighting up... got it.  I don't think I have a short (although I could be wrong) I was just curious about what the stock current draw was. 

I should have mentioned I am in the engineering department of a high speed rail car manufacturer, but I did not think it was necessary. 

BTW, I only brought up the EE thing, b/c sometime its nice to know if someone else has a similar background, allows one to cut through background material, I didn't mean anything insulting by it. Much like users on SOHC say they are or are not mechanics, etc.  Helps to know who your talking to!

Thanks again.