Been Fiddling with Shnitz Box

[madmikeracing]

Was wondering why my voltmeter reads power on both the Positive top lug on my dyna.7 ohm coil and also on the Bottom lug of the same coil? I thought one was a ground? Im no wizard with electrical, I had my Fluke Digital Meter set on DC volts and one lead from negative to top of coil lug 13 volts and then the lug below 13 volts, I thought the bottom one was ground but I think im wrong again as usual. Im quite confused and didnt wish to try starting it until i was sure. I have a super eliminator shnitz ignition box hooked to it and dont wish to fry it , Im quite anxious as Iits a brand new engine and Im dying to start it up, Its a 1500 Paul Ghast 31 27 head g7 g13 cams ect with the other ? Normal Mods, Cant wait to take it out and spank it down my road. Thanx mike

 

[posplayr]

The ignitor alternating grounds and lets go of the ground(i.e. goes open). When it is grounded you see ground on the negative (or about 0.7V) when it is open you see the same as the positive terminal on the negative. Rotate the engine and confirm it goes up and down.

 

[pdqford]

Was wondering why my voltmeter reads power on both the Positive top lug on my dyna.7 ohm coil and also on the Bottom lug of the same coil? I thought one was a ground?

You just observed a phenomenon known as "open circuit voltage"! When the circuit is "open" there is no current flow, and therefor there is no voltage being dropped any place in the circuit. Hence, you will read source voltage any place in the contiguous portion of the circuit.

The instant the ignitor grounds that circuit, current begins to flow and voltage will be dropped across the different resistances in the "live" circuit. So if you have 13 volts at the voltage source, the current flow might cause a 0.25 volt drop in the circuit between the voltage source and the positive lug of your coil, so you would read ~12.75 volts at the top lug. The coil might consume 12.7 volts so you would read 0.05 volts at the bottom lug. And the ignitor and its ground might drop 0.05 volts. All the voltage available will be dropped in a live circuit.
But the second the ignitor release the ground, current flow stops and you will read "open circuit voltage".

If you have lousy connections in the power and ground portions of the circuit, you might drop 1.25 volts in getting current to the coil, and a lousy ground might drop another 0.75 volts, so that only leaves 11 volts left for the coil to do its job.

 

[madmikeracing]

Thank you guys, I was at a point where I was scared to turn it all on and try to start my new engine, That shnitz box cost me a pile of money. Usually I use a dyna with coils also some homemade ****, ha ha Any how I double checked all the wiring and gave it the starter, ignition off to prime engine with oil, (has external huge Pro Mod oiler ) Thing fired right up sounds good, Thanx learn something all the time here on this forum , Again thankyou

for the Help Mike