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Discussion Starter · #1 ·
I know this has been discussed to death but not my situation. Running full msd ignition with 12v to the box. The coil is fed from the switch with the factory resistance wire still in. When I installed this 20yrs ago I called msd and they said leaving the resistor wire would be fine. If I recall right back then I had 8-9 key on volts at the coil and the engine ran good. Some reason I checked it today and get 3-4v and engine still runs good. If as is well known msd needs 12v to the coil why would it be running at all with 4? The meter is reading correctly. To test whether it runs "great" with 12 can I run P/N wire from the battery to the coil? If so do I leave the current wires attached while testing?
 

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The small red wire in the MSD harness is to trigger an INTERNAL relay that gives the box full battery voltage via the heavy red and black wires. If the voltage available at your coil positive harness wire turns on the box it is likely fine.

You can direct wire the trigger wire to the battery to test your theory...

Coil black and orange discharge hundreds of volts to the ignition coil via charged capacitors.

Green and violet are mag trigger wires. Don't hook violet to black striped violet on a DS distributor.

White wire is a points trigger or also for a pertronix or other self contained distributor's coil negative.


"In fact, the Digital 6AL delivers over 530 volts to the coil with up to 135mJ of spark energy for every firing! Increased output combined with MSD's proven multiple spark series is a win-win situation! The Digital 6AL is supplied with the wiring harness and the components you'll need for installation."








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Discussion Starter · #3 · (Edited)
I was mistaken the factory switch wire goes to the small red trigger wire on box, white wire not used. Black and orange from the 6al box to the coil. Maybe I am checking it wrong. Just placing the meter probes on the +- coil terminals?
 

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Given coil gets 500V DC from MSD, I wouldn't probe it unless your meter can handle that voltage.
 

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Discussion Starter · #5 ·
Well thats what I am being told but placing the + - probes to the + - of coil while running gives me 3-4 volts :unsure:
 

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FoMoCo,
While engine is running the capacitors inside the MSD are charging/discharging (a boost regulator) into the primary coil winding. So the average voltage may read less or your meter doesn't indicate it correctly. Is it an root mean squared 'RMS' type meter? 500V spikes may destroy the meter. That voltage is noisey and peaks are likely quite a bit over 500V. The waveform is a square wave with duty cycle dependent on engine RPM. So a DC voltage measurement isn't really applicable.

Hope this helps.
 

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Discussion Starter · #7 ·
I dont know if its a rms meter. For the heck of it I ran a wire from the battery to the red msd trigger wire. Engine ran the same and I got the same 3-4 volts on the coil. Rev the engine and voltage went up to 6ish. Certainly not causing issues but Im one of those 5Y root cause people that needs to know.
 

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The meter (DVOM) cannot process the square wave signal. Typically need a better tool... like an oscilloscope... to see the pattern and provide accurate info.
 

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Discussion Starter · #9 ·
The meter (DVOM) cannot process the square wave signal. Typically need a better tool... like an oscilloscope... to see the pattern and provide accurate info.
Thanks for the input.
 
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