[B_man_Al]

In post 36 I tried to explain what I did to determine the current and voltages associated with my 8BA coil/external ballast resistor combination on a 12V system. Key to the whole thing are two factors.

1-Do the measurements with the engine off, and the points closed. If you measure with the engine running you will get errors due to the voltages not being stable (points opening and closing constantly). If you have a scope, you can get accurate voltage measurements, but not with a multimeter.

2-Add a small series resistor ( Rsense) to allow the accurate calculation of the current, ( I coil = V Rsense/ R Rsenser) or do it directly. I used 0.18 ohms cause thats what I had, but any value that is 10x or more smaller than the coil + ballast will be fine. The lower the better to get better accuracy. This needs to be done, because the added sense resistor is designed to be stable with temp, and not vary all over the place like the ballast or coil resistance.

You can only calculate the current accurately if you know what the resistances are.

If you measure the current directly with a multimeter, then you don't need the sense resistor.

So the steps are:

1-turn off the engine, with the ignition on
2-measure the voltage applied to the ignition cct ... (ballast + coil ...nominal 6 or 12v)
3- measure the coil current , either directly or with a small sense resistor added.
4- Calculate the ballast resistance .... R ballast = V ballast/I coil
5- Calculate coil resistance ... R coil = V coil/I coil.

Check to see if results agree with the coil/ballast specs.

These need to be done quickly as the ballast and coil resistances will be changing quickly as they cool off.