Coil votage

[Mellvis]

After checking my ignition system I found that I have a full 12 volts at the coil power wire. All I've been reading on here is that it should be 8 volts. As near as I can tell everything is original, even the color of the power wire. I've been driving it this way since 2010. This is the second set of points and condenser since then. Seems to me too much voltage would tear up the coil rather than points and condenser, right? Would it simply be easier to put in an old style resister to correct the voltage?

[Blackwaterforge]

Did you check it with the engine running?
You should 8volts when it's running.

[64 f100]

Unless someone has put an aftermarket col on the truck with built in resistance you need 8 volts at the coil. As to jumping in a resistor that would be up to you. The resistance wires sometimes go bad as do all resistors at times. As to running the coil for any length of time with 12 volts. Eventually if not already will cause the coil to run hot which will affect the output of the coil. May not go totally bad for a long time. Could cause some starting problems at times and efficiency problems also. As to burning the points , I have no idea if the coil voltage may affect points, but bad condenser , etc. may have an affect. I eliminate points entirely on all my trucks . Huge benefit to electronic ignition. Run much better with a petronix .

Rich

[Toyz]

Yep, common question. Check with engine running or with wire from coil to points grounded, either via closed points or independent grounding. Seldom will you find a truck with original wiring setup sending excess voltage. The nature of resistance wiring is that if there is a failure, voltage is reduced or not present. There are rare exceptions, of course, but I am fairly certain you will find reduced voltage under the described testing conditions. Just do a momentary key-on test, though, to avoid overheating. And, one more edit: yes, full battery voltage, which actually could exceed 14 volts, will damage points.
Paul

[Mellvis]

Ok, my check was done without the engine running. What causes the voltage to be a full 12 while not running but back to 8 when it is running?

[Toyz]

Probably have to get Daniel to correct me here, but, if the only drain is that minute amount which passes through the meter; there is not enough resistance to actually drop the voltage, thus you will see full voltage until current flow (to ground) is sufficient. A probably poor analogy would be a pinched garden hose with a valve on the end. Full pressure would be available at the valve regardless of the restriction until the valve is opened. Pressure would then drop off because of the flow restriction at the pinch.
OK, Dan, or another of you elecmatricians, straighten me out here!
Paul

[64 f100]

I have never had to start the engine to get the voltage reading. Voltage drop is in the resistance wire, regardless of running or draw. You should only see 8 volts give or take a little at the coil coming in. As to the resistance wire, it can break down and let more voltage through . I'm sure there is spec on the resistance to check the wire, but I don't know what it is offhand. .

[Toyz]

You do not have to have the engine running; in fact, Ford's procedure makes no note of checking while running, probably due to the increased voltage while running. I believe Ford's standard acceptable voltage, engine not running and primary output jumpered to ground, is 6.6 volts or less. Thus, if you see less than 8 volts under running conditions, I would find that acceptable. It's just a quick and easy test. If the coil primary output is not grounded, you will see near battery voltage thru the ignition supply. If the points happen to be closed at time of checking, then the primary is grounded. It's easy to verify this; just put a piece of paper between the points to interrupt the ground, then read your coil + voltage.
As I stated earlier, resistance wires CAN fail and allow excessive voltage; my observation is that normal failure mode on these trucks is a break or breaks in the wire allowing little voltage to pass once the start circuit is inactive. This often results in an engine which will readily start, but will die after the key is released from start position.
Without knowing how many miles and what conditions Mellvis has driven his vehicle in five years; my initial thought would be that correct static testing will yield the correct 6.6 or less volts; if not a calibrated replacement wire is readily available with no need for concern as to splicing in a resistor of correct ohm value.
Paul