Live Coil Voltage Levels: Pix

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nm9stheham

nm9stheham

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FWIW.... if you ever wondered what the voltages at the + and - of a typical ignition coil really look like when running, and why you get those odd 6 to 10v readings on your voltmeter at coil + for example, here are some oscilloscope pix from a running ignition system. An oscilloscope presents what is a real time graphing of voltage versus time on the screen; time runs left to right, and voltage on this pix runs 0 volts at the bottom and 16v at the top.

This is from a /6, stock Chrysler coil and points system, running at idle. The same types of waveforms will be seen with an electronic ECU system and a V8. (And my apologies for the poor pix.)

Pix 1 shows the coil - where it connects to the points. When the point are closed and is shorts the coil - to ground, the voltage is near the bottom or very close to 0 volts. This period of time is the 'dwell' of the points (or ECU). When the points open, that generates the spark from the coil , and the coil - voltage rises up to near 12v system voltage (13v in this case).

Pix 2 shows the coil + voltage. When the points are open, it also shows near to full system voltage. When the points close, then the coil + voltage drops to an intermediate voltage (approx 6V in this case); the exponential slope down to this voltage is the coil charging up for the next spark cycle. If your dwell (the time spent at the lower voltage) is too short, then the exponentially sloping charge is not completed and the spark gets weak.
 

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I hope you are using a high voltage probe, the voltage peaks to about 400V when the points open. Most standard probes are rated at 300V rms, on 10x setting, 150V rms on 1x.
 
Yep, old style Tek 10x probes being used.The ringing spikes were nowhere near to 400V in this case; the probe capacitance on these old probes is kinda high and probably is killing the spike peaks. (BTW, I am an MSEE, designed equipment for years starting at Bell Labs. Been around HV stuff plenty.)
 
...........and also partly why it explains how you can get the everlovin' $### $hocked out of yourself on anything on the coil neg, including the tach wire or the heat sink area on an ECU

Years ago, I used to have a 340 in an old Landcruiser FJ-40. For awhile I had an Mopar box on it. We'd get through the mud and up in the snow, and sooner or later.........get warm melted snow and mud in the "engine room" and on the ECU. Took me awhile to figure out "that mysterious miss" in the engine when wet

I finally moved the ECU and coil up under the firewall, and in fact bought one of the early MSD "plug and play" Chrysler replacement boxes. This was in the mid 70's AND I STILL HAVE the MSD!!!

Also, many guys don't seem to realize that these girls won't run (breaker points) without a good "condenser"
 
That condenser (capacitor) likely reduces the arc at the points that would energize the coil again, curbing spark. In electronic ignitions I build, no capacitor, just a IGBT. The IGBT does have a small amount of drain to source capacitance. The IGBT is a good switch. It also has avalanche protection that limits IGBT drain to source voltage to 390V. The strike voltage can be estimated by multiplying the coil - by the coil turn ratio, typically 75 or 100. The plug gap and other dynamic factors including compression, changes the strike voltage at plug.
 
KitCarlson said:
That condenser (capacitor) likely reduces the arc at the points that would energize the coil again, curbing spark. In electronic ignitions I build, no capacitor, just a IGBT. The IGBT does have a small amount of drain to source capacitance. The IGBT is a good switch. It also has avalanche protection that limits IGBT drain to source voltage to 390V. The strike voltage can be estimated by multiplying the coil - by the coil turn ratio, typically 75 or 100. The plug gap and other dynamic factors including compression, changes the strike voltage at plug.
Click to expand...
Good info to know. Sounds very interesting, Kit.
 
There is a lot of information to be found in checking the primary voltage at the coil with a scope. The pattern should be adjusted to show all cylinders on the screen, then look at each cylinder and make sure the coil voltage is being switched by a very "clean" and precise signal. You can make sure each cylinder is being triggered properly and whether or not there is any distributor wobble, bent shafts, bad lobes, or pickups.
I see people on the forums with misfire problems and they spend days chasing their tail trying to find a problem a scope will show in less than a minute.
 
That is very true. I added a bnc connector to a clamp on timing light CT. I then look at the primary signal, triggered by the CT pulse on #1.

No comments on my cheap light. :)
 

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All of mine are cheap, LOL. I have at least three I bought from yard sales.
 
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