[BryanH]

Help! I am having a problem with 1968 GTO 400 ignition coils. They only last a short time (less than 20 miles) before the car starts to miss and run poorly. When this happens, I can replace the coil and the car will run fine ... for a little while. Everything is stock. I think it is hooked up correctly (pos to the wiring harness and neg to the distributor). I'm pretty sure I am using the correct coil. I've bought several from NAPA, and they all have the same part number and have text specifying they are to be used with an external resistor. And I had the same kind of coil in the car for a long time with no trouble until this problem developed. Does anybody have any idea what could be causing this? Thanks, Bryan.

[BryanH]

Help! I am having a problem with 1968 GTO 400 ignition coils. They only last a short time (less than 20 miles) before the car starts to miss and run poorly. When this happens, I can replace the coil and the car will run fine ... for a little while. Everything is stock. I think it is hooked up correctly (pos to the wiring harness and neg to the distributor). I'm pretty sure I am using the correct coil. I've bought several from NAPA, and they all have the same part number and have text specifying they are to be used with an external resistor. And I had the same kind of coil in the car for a long time with no trouble until this problem developed. Does anybody have any idea what could be causing this? Thanks, Bryan.

[BryanH]

By the way, the coil seems like it might go bad when starting the car. If I put on a new coil and drive around, it will start to miss just a tiny bit by the time I shut the engine off. But the next time I start it, it runs real bad.

[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]

Yep, the harness wire/wires go to the (+) side of the coil and you should measure 8 volts or so (instead of battery voltage, 12.6) at the (+) terminal, with the key in RUN, AND THE POINTS MUST BE CLOSED. With the points open you will read battery voltage.

So, if the resistance wire is good, the above will be true.

I have to ask, how do you know that the coils are going bad? Usually if you operate a coil that needs a resistance wire without a resistor, the points will deteriorate quickly and cause ignition problems. This may be accompanied by coil deterioration, but the points condition would be a indication. Also, make sure your engine-to-chassis and grounds are in good shape, because the return path for the coil secondary current is thru these groounds.

Regards, Georg

[george kujanski]

BTW, I assume you have the coil mounted in the OEM location, away from additional sources of heat, such as headers, etc.

SOmetimes, while chasing down problems, the problem magically gets fixed for a while because perhaps cables are moved, etc. Check to make sure you do not have an intermittent coil wire, etc. that gets "fixed" when you replace the coil.

Regards, George

[BryanH]

Thanks for the information.

George, I think the coils are going bad because the car runs fine when I replace them. This is without changing the points (which look fine to my non-mechanic eyes), so I assume the points are OK. I also checked the dwell the last time I changed the coil, and it was about 29.6, pretty close to the 30 degrees I set it to when I last replaced the points.

Is it possible for a resistor wire to go bad so that it no longer provides enough resistance?

I think the coil is in the stock location. It is next to the distributor. I'm not sure what the bracket is mounted to, but I think it would be the back of the intake manifold, the back of the LH head or the back of the block.

I have checked the coil connections, so I don't think there is an intermittent connection problem. Also, this is repeatable, another thing that makes me think it's not an intermittent connection problem, as the same connection "fixes" probably would not happen repeatedly.

Forgive my ignorance, but I'm pretty much a novice machanic. If you could tell me the proper way to check voltage at the (+) terminal, it would be appreciated. I have a multimeter that I use to check dwell, can that be used to check voltage?

Also, where are the grounds located that I should check?

Thanks again,
Bryan

[johnta1]

You don't have any vacuum lines or anything else contacting the coil do you?

Any other wires running to the coil?

johnta1
Pontiac Power RULES !!!
www.wallaceracing.com

[BryanH]

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

[johnta1]

Take the wire from the '+' post off and use a voltmeter to see what the voltage reading is with the key on. Should be around 9 volts.

I would change the points.Has to be cheaper than changing coils all the time.

I really think you have a problem elsewhere.
Might have a bad ignition wire,bad electical connections in distributor, or even a vacuum leak.
Also the cap may be carbon tracked.

Do the plugs show any fouling?

johnta1
Pontiac Power RULES !!!
www.wallaceracing.com

[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]

Thanks everybody for the information. I'll check everything and see what I can find.

John, yes it is expensive replacing coils! I need to figure out what's going on.

George, my car does have the 2 wires going to the (+) side of the coil.

Thanks,
Bryan

[BryanH]

Last night I measured the voltage at the coil with the key in the run position and the points closed, and the reading was about 6 volts. That seems a little low, but is it low enough to cause problems? The reading with the points open was about 11.75.

I also measured resistance in the coil. The primary circuit was about 1.2 ohms and the secondary about 6.7 ohms. I compared those to another coil that I think is good and the good one was 1.5 on the primary and 6.7 on the secondary, so it seems like my "bad" coil is probably OK.

I looked at the ground strap from the block to the firewall, and it is there and looks OK. Is there anything more I should do to check it?

I double checked all the coil connections, and all the ignition wire connections, then fired the car up, and it still misses. The car ran smooth and strong in late March when I first replaced the coil for the last time, so that makes me think the points, plugs, ignition wires, distributor cap and rotor are all OK. In fact, the cap, rotor and points are practically new. They are the first things I replaced when trying to fix this problem after it started last fall.

Any additional help would be appreciated.

Thanks,
Bryan

[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

[BryanH]

Thanks for the help.

I'll try a previous coil to see what happens.

The miss is regular like an ignition problem. But it doesn't feel like one dead cylinder, it feels like they are all affected. Last fall when this started, I replaced the points, cap and rotor. The points helped a little but not too much. Since the plug wires and coil were fairly new at that time, I checked to make sure the carb was getting fuel and it was. I looked for vacuum line problems but didn't see any. That's when I replaced the coil just to see what would happen and voila, ran great.

Maybe I should find a place with an ignition scope and let a pro check it out.

Thanks,
Bryan

[george kujanski]

You could go back to the basics; check compression, attach a vacuum gauge, etc. to get a measure of overall health of the engine. Once my '67 started running erratically. I searched all over the place, checking everything. Turned out to be a worn Quadrajet throttle shaft bore.

Regards, George

[engineer]

With the engine running, hook a timing light to each spark plug wire individually and look at the light, if it pulses steady and strong, go to the next wire, if you find any that the light is weak or the pulses are not consistent, That cylinder has a ignition problem, either plug, wire or cap or flat lobe on distributor. That is the easy way to check for a miss.

[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

[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

[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