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Discussion Starter #1
I tried to figure out why I have 12 volts on the + terminal of my coil while the car is running and about 9 volts when I start the car. I have looked at the wiring and it seems to follow the 65 mustang schematics. I used my Simpson 260 meter to test the wiring and the switch. I aqwam including a picture of the switch I have, I got it from this forum. I think the switch has an issue but I am not sure so I am asking you all for guidance in this matter. Thanks in advance.

Here is what I get when I check the switch contacts for continuity:

All pins are open, none shorted to each other, when the switch is in the OFF position.

Pins "B" and "D" are shorted as is the Post shorted to "B" and "D" in the Run position.

Pins "B", "C", and "D" are shorted as is the Post shorted to "B", "C", and "D" in the START position. This seems odd to me.

Pin "B" and the Post are shorted in the ACC position. This seems correct as I understand Pin "B" is the +12 always hot Pin.

As I mentioned above, any guidance would be much appreciated.
 

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Hello. :) I think that I would start by starting the car and then unplugging the brown wire from the 'I' post on the front of the starter solenoid, and seeing what sort of readings I was getting like that. :)
 

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Discussion Starter #3
OK, I will do that tomorrow morning and post the voltage reading. I have looked a bit on the net and finally found what looks like a table that shows the pin connection at the various switch settings and what I posted seems correct.
 

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Discussion Starter #4
Veronica,
I just tried what you suggested and here are the results. The coil voltage when starting the car is about 11 volts and now it stays at about 11 volts while it is running, not sure why I am not seeing the 9 volt and 11 volts as before. I remove both the "I" and "S" wires from the start solenoid, one at a time and saw NO difference in the voltage on the + terminal of the coil. I did check the battery voltage at the battery while the car was running and got the usual 14.6 or so voltage and when I measured the coil + terminal it was about 11 volts. This tells me that there is a difference between the battery voltage and the voltage on the coil. I have the Pink resistance wire as well as the 1 ohm ballast resister in series with the coil. Once we figure this out I will use the correct resistance.
 

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Hi again. :) The 11ish at the coil when starting is about right. Ideally, it would 12, but the act of starting the car drags the battery down a bit, so it's never really the full 12V. The reason that I suggested disconnecting the 'I' wire from the starter solenoid is because that wire is what supplies power to the coil when the key is in the start position. It is spliced into the the red/green wire that supplies power to the coil with the key in the 'On' position, but, that splice is downstream from the resistor wire, so, it's full 12V for starting and lower for running. A short in the starter solenoid can cause it have the full 12V all of the time. This is apparently not the problem for you. :) The 9V (or 12V, depending on how the car is feeling, I guess :) )you have at the coil is a bit high. Ideally, that should be less than 7V. The first thing that I would do is either A) replace the resistor wire, or, B) install an inline resistor, kind of like the old Mopars had, to knock the voltage down to where it is supposed to be a the coil with car running. I have heard people say that having the full 12V at the coil while running is a good thing, but that causes problems like point arcing and shortened life of points and coil. You are doing the right thing by straightening this out. :)
 

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It is not possible to get an accurate reading of the primary coil voltage while the car is running without an oscilloscope. This is because of Kirchoff's Law of Voltage. During the period of time between sparks, the circuit is open, so full alternator voltage is present at all points in the circuit. The voltage at the coil positive terminal of an eight cylinder engine idling at 600 rpm will fluctuate between 14v (when the circuit is open) and 7v (when it is closed) 2400 times per minute, or 40 times per second.

To see what the operating voltage is while the engine is running, do the following:
- Remove the wire from the coil negative terminal. (The small one going to the distributor.)
- Place a jumper wire from the coil negative terminal to the engine block or similar chassis ground.
- Turn the key to "on" and measure the voltage at the coil positive terminal. You should read approximately half battery voltage. (Assuming you have the stock setup of a 1.4 ohm resister wire and 1.4 ohm coil.)

Do not leave the ignition on for any longer than it takes to get a reading, as the constant current flow through the coil can overheat it if left that way for a few minutes.

You are probably reading 11v because your analog Simpson 260 (or any voltmeter, for that matter) cannot possibly switch between 7v and 14v 40 times pes second, so you are reading a sort of average.
 

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Discussion Starter #7
Veronica,
Thank you for your insight. I have already trashed a new MSD coil and a set of points because of this issue. That is based on my mechanic and what I have been reading on this great forum. I do have the 1 ohm ballast resistor in series with the Pink wire and the readings I mentioned are with the TWO resistors in series and this has me a bit confused because with all this in-line resistance I was really surprised to see such a high voltage reading.

Joe,
Thank you for your insight as well. You are 100% correct that without a scope my Simpson meter is integrating the voltage. I will do as you suggest tomorrow morning and see what type of readings I get. I will post them right after making them. I am learning a lot from all of you and look forward to solving this issue. Have a good evening.
 

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Discussion Starter #8
OK, I did not have a good day with the car. I had trouble reinstalling the ignition switch so I could do what Joe suggested. Well to make a long story short I ended up messing the guts of the ignition switch all in pieces so I now have to order a new switch and I might as well order a new tumbler as well. I have made several steps backwards on this issue. As soon as the new switch is installed I will make the voltage measurements and post them for you advice. Take care.
 

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Discussion Starter #9
Got the new ignition switch installed and traced the wiring from the starter solenoid to the firewall connector. The pins are OK but I have an extra wire in the harness, it is a heavy black wire that is attached to the left side of the starter solenoid, connected to the battery. It does not show up on the schematic. I checked the wires on the coil and the go to the correct places. I get about 0.5 volts across the ballast resistor. I was thinking of increasing the ballast resistance, put another one in series to see if that will drop the voltage down enough while the car is running but not enough so it will not start. I am going to talk with a few people tomorrow that might provide more insight. Any other suggestions would be appreciated.
 

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Discussion Starter #10
I have traced all the wires associated with this issue and they go where they are supposed to go. I unplugged the gauge harness at the firewall and checked each wire to see if it was shorted to another wire and they are all OK. I verified the start relay is functioning correctly, no shorts etc. So the starter relay is OK, the wires are OK, the ignition switch is OK but when I start the care I still get about 8 volts in the start position and 14 volts in the run position. Any suggestions would be great since I do not see which wires I would like to switch to fix this issue. I have already smoked a set of points and the same happened to my MSD coil.
 

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Discussion Starter #11
I remeasured the voltage on the coil + terminal with the - terminal grounded, per Joe, and my reading was 6 volts with the key in the run position. This makes sense because the battery is reading 12 volts and the coil resistance is 1.5 ohms and the resistance of the Pink wire is 1.4 ohms so the voltage drop on the + terminal should be half of the battery. Thanks for all the help you provided.
 
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