Author Topic: Voltage at the coil  (Read 1102 times)


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Voltage at the coil
« on: December 28, 2018, 06:33:55 PM »
Bear with me for a second:

I just replaced an off-brand distributor with an MSD 8360 Pro Billet (their ready-to-run version which does not require the box). In checking for voltage at the coil I found 12v at positive side of the coil with the ignition switch in RUN... and 9 volts at start...just the reverse of what should be in place for a points-style original distributor. The MSD needs 12v at RUN.

I haven't pulled the harness nor separated the wiring (yellow and wrapped white?) yet and am wondering how the previous owner could have wired this thing backwards.

As I intend to install an original 480 distributor in the very near future resolving this issue is important.



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Re: Voltage at the coil
« Reply #1 on: December 29, 2018, 03:19:10 PM »
Think of electrical current as a water hose.
Valve off (ign sw off), no flow of water (current), max pressure (voltage).
Open valve (ign sw on), water flows (current flows), pressure drops, (voltage drops).

Twelve volts  with ign sw on IGN, and if point set is open, or module not working/spinning, max volts and no current flow.
If points are closed, then current flows, as circuit is complete, then lower voltage.
It may be true with yours, but a previously owned '67, needed the yellow wire from R terminal on solenoid to coil + for start voltage.
My previously owned '68's had twelve volts from ign sw START position, but yellow wire was still installed and I left it this way.
Sixty-nine is probably the same.

With nine volts during cranking, either/both, the battery is low charged or too small, and/or battery cables too small of gauge, min of 1 AWG and wire brush posts and clamps and attachment points on block.
My suggestion is externally charge battery overnight. Next day, check voltage on disconnected yellow wire and disconnected cloth covered white wire during cranking. Remember, start motor is a high current device and expect a drop of battery voltage, but not lower than nine volts, typically.
"Need more power, Scotty" comes to mind.

If white wire is 12 volts during cranking, you could, not required to do so, do away with yellow wire, but I would leave it and use it.
When connecting new 8360 distributor, tie back white wire and run a new 12 AWG wire from IGN spade on fuse panel, as this action will leave OE white wire for 480 distributor when installed.
A flashlight is a case for dead batteries.


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Re: Voltage at the coil
« Reply #2 on: May 12, 2019, 07:14:59 PM »
Also check the voltage at the battery terminals when cranking. If it is low there then you have a battery problem. If it is holding up there then look elsewhere for your problem.

68 Convertible w/327 275hp donor engine from a 67 Impala and TH350