[Herb Hagan]

If memory serves me correctly, the positive wire to the coil is a resistance wire. You should only see 9 VDC to the + coil when the key is in the run position. George K. should be able to verify this.
Good Luck,
Herb

If it ain't broke, don't fix it!

[george kujanski]

No, most people are stuck on the 8 or 9 volts numbers when measured while the car is running. I also used those numbers in previous posts but around 6 volts static is more accurate. The important thing is that the voltage should be noticably lower than battery to indicate that the resistor wire is in the circuit. While running, the points are opening and closing, giving you an average voltage in-between the 6 volts (static, not running, points closed) and battery voltage, static, points open). Did you try the previous coils yet? Also, try wiggling the wire harness while running to see if the missing gets worse, might be a defective resistor wire? Get a resistor block for a MOPAR from the auto store , they use a separate resistor instead of resistive wire, connect one end to the battery, the other to the coil (+) terminal and try running with that, see if there is an improvement. You will have to disconnect the resistor to shut off the engine.
These are all inexpensive things to try if you still think it is ignition related.

Borrow an HEI and drop it in, see if your problems go away.

Regards, Georg

[george kujanski]

Question: if you put on one of the previous coils does the problem temporarily get "fixed"?

Secondly, are you sure that the "miss" is ignition related? Is it a regular, periodic, miss such as from a plug or wire, or is it more random, sporadic like a vacuum leak or fuel related? Is the timing mark erratic when using a timing light?

If you are convinced that it is ignition related, the quickest way to find that is to find a place with an ignition scope. You will see an ignition problem right away.

On one hand you say replacing the coil works ok for a short time but the measured ignition parameters look fine. Try the previous coils & see if the problem is the same or changes for a short time. IMO, if all the other ignition parts are in good shape, I'm thinking your problem is elsewhere.

In any case, don't get tunnel vision and prematurely rule out other possibilities. Keep an open mind.

Regards, Georg

[stu]

If the high voltage path was shorting out, would that cause the secondary winding to fail over time? Maybe that is a clue. Also, is the coil ground OK? It relies on the case making a solid connection to the block, right? (No separate ground lead)

[george kujanski]

John, taking the wire off the coil and measuring voltage will measure battery voltage. The coil and resistance wire must be connected WITH THE POINTS CLOSED to form a complete circuit, which then forms a voltage divider giving you ideally about 8 volts (give or take; it won't be battery voltage).

I believe on a '68, similar to my '67, there should be two wires to the (+) side of the coil. One wire is plain, and is used to provide battery voltage to the coil for better starting, The second wire is the resistance wire, and active in the RUN position; the first wire is dead in the run position. This feeds slightly less voltage (the 8 or 9 volts above) to the coil which makes the points last longer.

Now if you only have one wire, and it is not a resistance wire, that is not correct. Use your multimeter, if it has a volts scale to measure the coil (+) voltage with the key in the RUN position and the points closed. You may have to bump the engine over a bit to insure the points are closed, or, alternately, use a jumper to ground the (-) side of the coil to make sure. Do not maintain this connection too long, as the coil is drawing current constantly. Also, when you disconnect the jumper wire, the coil will fire, so watch out for high voltage; keep away from the high voltage terminal. A reading of 8 or 9 volts (less than 12.6 in any case) indicates you have a proper resistance wire in the circuit. Measure with the positive meter lead to the (+) terminal and the negative to engine block.

If all above is OK, I can't explain why the coils are going bad, if indeed they are. More likely, the problem is elsewhere as John says, and you should look elsewhere if your resistances are ok.

The grounds shoould be braided bare wires bolted from the rear of the block to the firewall sheet metal somewhere.

Regards, Georg

[BryanH]

johnta1, nope, all I have are the (+) and (-) posts and the high voltage (?) line going to the top of the distributor.

[BryanH]

Last night I replaced the coil with a previous one and had no change in the way the car ran. I also tried wiggling the wires, and that did nothing.

Then I put my timing light on each plug wire as suggested by engineer, and all looked good except cylinder number 8, which was very sporadic. It looked like it was not firing more than it was firing. Then I put the light on the coil high-voltage wire, and it looked like there was a miss there too. Then I swapped the cylinder 8 and 6 plug wires to see if the plug wire was bad, but 6 stayed good and the miss stayed on 8.

The miss on cylinder 8 made me think the distributor cam might have a bad lobe, but the fact that I could see a miss "upstream" from there at the high-voltage wire makes me wonder. Is it possible for faults to cause bad readings "upstream"? Could a bad spark plug in no. 8 cause the bad readings I saw at both the plug wire and high-voltage wire?

Any help appreciated.

Thanks,
Bryan

[george kujanski]

Stu: it's strange, all right. All problems can usually be explained, however. The HV return is thru the battery feed. The coil schematic shows the opposite end of the secondary side of the coil connected to the (+) terminal.

Bryan: if the lobe is worn, meaning the high spot is closer to the base diameter, your dwell, at least for that cylinder, is already increased. The problem in that case is that the points barely open, if at all.

Dwell adjustment is a compromise. On the one hand you want max dwell (more time closed) to build up the field in the coil, especially at higher revs. You also want the points to open as wide as possible to minimize the arc when they open. These are conflicting requirements, however, so the compromise is about 30 degrees, which gives an open gap of about .018, as I recall.

I also find it hard to believe that the lobe would only be worn in the #8 position. More likely the gear or a lot of play in the shaft bushing, perhaps.

Regards, George

[stu]

So where does the secondary coil return to? Just curious, I always assumed it was tied to the case but maybe it returns thru the battery feed.
All the ideas seem like they would making a contstant misfire, wonder why changing coils without doing anything else fixes it? Strange.

[Sun Tuned]

Oops! Definitely the wrong post. 10 new switches wouldn't fix a bad cylinder. Well that's what I get for trying to eat, chase the child, and not pay attention. Glad you got it fixed.

[BryanH]

I haven't had time to try the previous coils yet. I should have time to try that tonight along with moving the wires while running.

Thanks,
Bryan

[BryanH]

Well, I replaced the no. 8 plug and the car runs OK now. Not as good as it could, but I suspect I need to change the other plugs as well. Plus I will check the distributor, etc. as described above.

It still seems strange that replacing the coil made the car run good for a while. Maybe it will remain a mystery.

Thanks everyone who helped me with this. I learned a lot.

Bryan

[BryanH]

Thanks for all the help! I have learned a lot from this post.

[BryanH]

Could the fact that I'm only seeing 6 volts at the coil instead of 8 or 9 be part of the problem?

Thanks,
Bryan