72 corvette ignition delimma
Le Mans Master
Joined: Aug 2006
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
See post #5 by Rebelyell.
He is correct that the ballast resister wire limits current not voltage.
Your typical VOM meter will only average the power because it cannot keep up as the points open and close.
AI is incorrect when stating it limits voltage.
See the sticky by 69427.
He is correct that the ballast resister wire limits current not voltage.
Your typical VOM meter will only average the power because it cannot keep up as the points open and close.
AI is incorrect when stating it limits voltage.
See the sticky by 69427.
Pro
Joined: Jan 2004
Posts: 653
Likes: 56
From: TX
While the intent of a resistor wire or ballast resistor is to reduce current to the coil, it has the complementary effect of also reducing voltage. To check for voltage drop at Coil+ with a typical meter: With engine OFF (not running) and key in RUN position with the points CLOSED. (however you should not leave the key in the ON position with points closed for very long.)
Tech Contributor
Joined: Jun 2004
Posts: 20,910
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
While the intent of a resistor wire or ballast resistor is to reduce current to the coil, it has the complementary effect of also reducing voltage. To check for voltage drop at Coil+ with a typical meter: With engine OFF (not running) and key in RUN position with the points CLOSED. (however you should not leave the key in the ON position with points closed for very long.)
Regarding the voltage drop, it's a constantly changing amount, and as mentioned, a voltmeter can't keep up, and will just give a rough average of things.
During the 15* of non-dwell, there is 12-13 volts at the coil C+, and that same 12-13 volts across the coil primary windings right after the points close, and then that C+ voltage ramps down to about 6-7 volts right before the points open, and then when the points open the voltage across the primary spikes up 100-200 volts at the C- terminal when the plug arcs.
An oscilloscope shows all this, but is pretty much overkill for most owners needs, and a voltmeter will give a sufficient brute force average voltage at the coil.
Pro
Joined: Jan 2004
Posts: 653
Likes: 56
From: TX
Regarding the voltage drop, it's a constantly changing amount, and as mentioned, a voltmeter can't keep up, and will just give a rough average of things.
During the 15* of non-dwell, there is 12-13 volts at the coil C+, and that same 12-13 volts across the coil primary windings right after the points close, and then that C+ voltage ramps down to about 6-7 volts right before the points open, and then when the points open the voltage across the primary spikes up 100-200 volts at the C- terminal when the plug arcs.
An oscilloscope shows all this, but is pretty much overkill for most owners needs, and a voltmeter will give a sufficient brute force average voltage at the coil.
During the 15* of non-dwell, there is 12-13 volts at the coil C+, and that same 12-13 volts across the coil primary windings right after the points close, and then that C+ voltage ramps down to about 6-7 volts right before the points open, and then when the points open the voltage across the primary spikes up 100-200 volts at the C- terminal when the plug arcs.
An oscilloscope shows all this, but is pretty much overkill for most owners needs, and a voltmeter will give a sufficient brute force average voltage at the coil.
No Oscope or convolutions necessary.
Thread Starter
Advanced
Joined: Apr 2022
Posts: 66
Likes: 7
From: Indianapolis
A neighbor of mine has a 1969 gto all factory original and I asked if I can take a reading on the positive side of the coil engine running and it was 9,4 volts. I'm not sure if someone has done some bypass wiring on this corvette but I'm reading 12.2 on the factory resistor wire black wire running. I ended up wiring in a ballast resistor and adding a 1 ohm resistor on the resistor ballast wire to the coil. Running on this 72' corvette now I'm running 9.3 volts with factory points. Thanks for all the help!
