whats the correct voltage on + coil termninal

on my 37 ford flathead i am redoing the igniton circuit (12v negative grnd) and have a 1.3 ohm ballast resistor in series with my 1.5 ohm primary resistance coil. at the coil + terminal i have a voltage reading of 6.33 volts and at the - terminal it was .43 volts with the coil fully hooked up to the stock points distributor. for some reason (which i have no basis for other then memory) i was expecting 9 volts at the + terminal and the 6volts at the - terminal. is the 6.33 volts at the + terminal correct? thks bob

bobs77vet said:

on my 37 ford flathead i am redoing the igniton circuit (12v negative grnd) and have a 1.3 ohm ballast resistor in series with my 1.5 ohm primary resistance coil. at the coil + terminal i have a voltage reading of 6.33 volts and at the - terminal it was .43 volts with the coil fully hooked up to the stock points distributor. for some reason (which i have no basis for other then memory) i was expecting 9 volts at the + terminal and the 6volts at the - terminal. is the 6.33 volts at the + terminal correct? thks bob

Click to expand...

If you're measuring everything while the engine is not turning, then it's a simple Ohm's Law DC voltage calculation. If the engine is running, any reading you get with a voltmeter is nonsense.

1.5/(1.5+1.3) X 12 = 6.4v with the points closed.

Your golden Bob. Drive it.:thumbs:

69427 said:

bobs77vet said:

on my 37 ford flathead i am redoing the igniton circuit (12v negative grnd) and have a 1.3 ohm ballast resistor in series with my 1.5 ohm primary resistance coil. at the coil + terminal i have a voltage reading of 6.33 volts and at the - terminal it was .43 volts with the coil fully hooked up to the stock points distributor. for some reason (which i have no basis for other then memory) i was expecting 9 volts at the + terminal and the 6volts at the - terminal. is the 6.33 volts at the + terminal correct? thks bob

Click to expand...

If you're measuring everything while the engine is not turning, then it's a simple Ohm's Law DC voltage calculation. If the engine is running, any reading you get with a voltmeter is nonsense.

1.5/(1.5+1.3) X 12 = 6.4v with the points closed.

Click to expand...

Unless you wanna get into inductive/capacitive reactance and wether the currents lead or lag ......put a scope across the + side of coil to ground and see the funny wave forms as in you think those points are a switch but it ain't no lightbulb.....just a comment for the NON geeks....

:smash::hissyfit::hissyfit::surrender::lol:

Actually with the engine not running but the ignition on, you should see battery voltage at the top side of the ballast resistor and dropping to about 6 volts at the end of the ballast resistor and at the + side of the coil if the points are closed, grounding the - side of the coil.

If the points are open, you should see battery voltage at the ballast resistor and the same at the + side of the coil AND at the - side of the coil.

The reason why is simple, since the points are open, no current flows and there is no voltage drop due to Ohm's law; when the points are closed, current flows so you'll see the respective voltage drops across the ballast resistor and across the coil.

Closed points should be at ground potential, but it is common for there to be a slight resistance across the points which may show as a fraction of a volt.

Larry you are dead on with exactly whats happening. I appreciate the help, bob

in fact i went home for lunch to check the sump pump after 8 days of rain.....and then slipped out to garage...with car turned off my battery measures 12.8 volts, the top of the 1.3 ohm ballast resistor measured 12.72 volts close to equal of battery, and strangely the other end of the ballast resistor showed the same 6.33 volts as the + terminal end on the coil. i guess since they are connected with a wire the reading is the same? i disconnected the wire that goes to the coil off of the ballast resistor and the voltage at this open terminal was 12.58. i dont think this was a good reading since the resistor couldnt go to ground other then through the digital volt meter. i did remeasure the resistance in the ballast resistor and it gave me the corrected resistance reading of 1.3 ohms. i guess i can put this to rest now. thks bob