Topic: Electical Issue

[75modrat]

I have 12 volts from my battery (it is fully charged). but I am only seeing 8 volts at the positive lead for the coils. What could be the problem.

cb750K (stock motor, Dyna coils (not dyna ignition)).

[Hush]

More importantly, how is your bike going?

[75modrat]

Its not. Thats why I need to figure out this problem. I have the 3 ohm dyna coils and am running 1 ohm resistors. However, one resister is rediculously hot. I dont know if that is the problem or if there is another reason for the voltage drop.

Im trying really hard to get the bike fired up.

[bradweingartner]

Are either of your points closed? I'd wager to bet that the resistor that's hot corrosponds with the coil that the points is closed for.

[TwoTired]

Lets use ohm's law, OK?

With the points closed, and a battery at 12V. you have known 1 ohm and 3 ohm resistance in series with the supplied power.
1+3 = 4 ohms total.
12v /4 ohms = 3 amps current draw.  (E / R = I)

3 amps drawn through a 1 ohm resistor consumes 3 volts. ( R X I = V)

12 V - 3 v (dropped by the resistor) is 9 V, that you can expect to get to the coil in a perfect world.

I'd guess you have some more resistance in the path to the coils that would account for the other volt lost at the coil.  Could be contact resistance on the switches or connectors between coil and battery terminal.  You can find these devices with a wire diagram of your bike.  But, I don't think the 1 volt loss unaccounted for, is the cause of your bike not running.

Cheers,

[75modrat]

I extended my started cable by 1.5 feet. Now it does not work. My starter selinoid is functioning and I just fully charged the battery. This increased distance is not the cause of the starter not working, correct?

[TwoTired]

What size/gauge wire did you use for the extension?
The starter draws about 150 amps, assuming low resistance hookup.    The hookup must be capable of low resistance to deliver that 150 amps.

[75modrat]

I cut the original wire and soldered a new line in under the engine side cover. Used 8 gauge wire.

[TwoTired]

Does the starter motor terminal receive full battery POS terminal voltage when the solenoid is engaged?  Is the starter well connected to the engine case? Is the engine case well connected to the Battery Neg terminal?

[75modrat]

The area I am no longer sure about is the starter's connection to the case. I had taken the starter out a few weeks ago to run a new starter wire. Upon removal, I realized the cable is permanently attached. So I reinstalled (didnt think I could screw it up) and added soldered the new line into the old under the engine side cover. However, I have no idea what screwed up the starter. I am not sure about the starter's connection to the case. Is there any test I can do to verify without having to drain oil and dig into? Am hoping that I can find out what the resistance through the starter should be. Would that information even be enough to tell me whether that is the problem or not?

Frank

[TwoTired]

If the starter is electrically functional, it will have a very low ohms connection between the wire you soldered and the engine case metal.  The approximate ohms measurement?
12V / 150 amps equals 0.08 ohms  (R = E/I)

Few people have equipment at home to accurately measure such a low resistance.  Most would equate that to a direct short, or zero ohms.  Or, the same as reading any two probe points on the engine case.
You can make the measurement anyway, as a higher reading surely indicates a problem.

Circuits under active load can behave differently than open circuits.
You can possibly infer what the resistance reading is by measuring the voltage drop at the starter motor input.  You can then calculate the resistance, making an assumption of current draw...maybe.

Cheers,