I was messing around in the truck today with dash out and happened to touch the pink(or faded red) wire coming from the ignition switch and it was HOT! felt the harness it goes into and even it was hot. shut the truck off and started doing some checking and none of the wires under the hood felt warm just under the dash. I then measured the voltage at the coil and it was only 3 volts? This thing does have a msd coil in it, (already had the coil on the shelf and the old one was shot when I brought the truck home) and I know that the msd coils are designed for 12 volts all the time. Clipped my buddy's coil from his f100 to the terminals on mine and I was getting the normal 6-7 volts like the factory ones should. Should I just bypass the resistor wire so it does not burn up? I am not driving this truck right now, just start it to move in and out of the shop sometimes, but I don't want to cause any damage to the other wiring if this thing decides it wants to melt or something. Eventually I will put a 6AL box on it once I get the new engine built, just don't see the need when its just a stock 302

 

No, use the correct coil. Those "hot" coils will destroy your ignition module over time.

Don't be so quick to assume the problem has gone away. You need to make sure there's not some sort of short anywhere, or wiring fault like somebody tying a load to the 'I' terminal of the starter solenoid (common way to burn up the ballast resistor). Measure the primary winding resistance of the "hot" coil to make sure it really was responsible for pulling all that extra current through the ballast resistor.

 

ok, checked the msd coil and it is at spec .7 ohm. I grabbed my old leakey stock one and checked it, it has 2.2 ohm. The resistance from the coil + wire terminal to the back of the ignition switch is 1.3 ohm. With the truck running i could get 13.9v at the back of the ignition switch, and 9.5 volts at the msd coil with the voltmeter connected to batt ground and + on coil, and 2.5v across coil posts. with the old coil i had i got same 13.9v at switch, 11.6 volts at the coil with the voltmeter connected to batt ground and + on coil, and 4.9v across coil posts. The ballast wire was still warm with the stock coil but not as warm seemed like. I did not see anything tapped into the 'I' terminal anywhere under the dash or engine bay. How do those numbers sound? not sure what the stock coil resistance is supposed to be.

 

I assume this is a Duraspark II ignition system? You haven't actually stated what you're working on. All statements below reflect Duraspark II.

It's not valid to take DC voltage measurements at either terminal of the ignition coil with the engine running because you're not looking at a DC waveform. It's preferred to take these measurements with the key in RUN and the engine stopped. That's because the ignition module will ground out the coil, letting primary current flow.

When you take measurements this way (the proper way), the voltage across the coil posts should be very close to the voltage at the positive terminal of the coil with respect to ground. That's not what you're seeing now. The negative terminal of the ignition coil can reach 400 volts instantaneously when the ignition module releases (yes, that's just the primary side). Your meter doesn't know what it's looking at.

That being said, the voltage at the positive terminal of the coil with respect to ground will not likely change much between the way you measured it and the way I've specified. The trend you've described makes sense; lower primary winding resistance will pull the voltage at the positive terminal of the coil lower. However, the voltages themselves are quite high, and the resistances you've stated do not support these voltages mathematically. These voltages are also different than what you described in your first post; I don't understand what has changed.

The primary winding resistance is specified as 0.8 to 1.6 ohm, however in practice you will likely see readings toward the higher end due to contact resistance and meter leads. The ballast resistance you measured (1.3 ohm) is correct.

 

sorry, it is Duraspark II. Thanks for the tip about the truck being off, I will re-do the readings and see if there is any changes. In the first post the readings were coming across the coil posts only, and with my buddy's good factory coil and him looking at the meter. Not sure if he just said the it was 6 volts or if it was closer to reading what my factory coil was, next time he comes over this way I will snag it again and see what kind of numbers I am getting with his coil.