[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]

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

[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

[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.

[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

[BryanH]

If a distributor lobe is worn, is it possible to compensate for that by setting the dwell longer than spec?

Thanks,
Bryan

[george kujanski]

Stu: I believe there is no internal ground connection in the coil. The case is not connected electrically to the coil inside.

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.

[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]

So the problem appears on 8, then look at that plug, the no. 8 terminal on the top and inside the cap, the rotor contacts 8 okay? Looks okay, then look at dist. cam lobe in no. 8 position? flat, shiny, etc. compared to the other sides? Dist. shaft tight up and down, no rocking side to side? good ground to dist. plate under cap? etc. Your problem has to be downstream of coil.

[george kujanski]

Well, if you actually see a miss in the coil HV wire, that indicates that the problem could be in the point system, i.e., the points themselves, the lobe, the gear, etc., Since the opening of the points is what actually causes the coil to fire. Anything that affects the instant of time the the points open will cause the coil to misfire. since you say it is one cylinder only. Also, the dwell being too short, (points not closed long enuf) will cause a misfire at the coil output.

Another way you would see a misfire is if the coil is actually developing voltage ok, but the plug gap is too wide for the spark to jump it AND the wires, etc are in good shape insulation-wise so that the spark does not jump anywhere else. Most inductive-pickup timing lights are triggered by the spark CURRENT in the wire. if there is no spark CURRENT,i.e. (no spark discharge) then the light may not flash, the cause being a possible bad plug.

As before, good hunting. Regards, Georg

[BryanH]

I forgot to mention that I replaced the distributor cap with a previous one too, which did not help.

What about the miss that I could see at the high-voltage wire? Could that be caused by a bad plug or distributor or anything else "downstream"?


Thanks,
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]

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

[Sun Tuned]

How about the ignition switch?

[george kujanski]

Well, if your observations of cyl.8 are correct, you have determined the following: the coils themselves don't seem to be going bad; cylinder 8 is the problem; the plug wire for cyl 8 is not the problem.

Ok, so the possibilities are: a fault in the cap for #8, (carbon track,etc.) bad plug, plug gap too wide, ( forces the coil to develop higher voltage for that plug which either it cannot do, or the voltage developed is leaking off somewhere else since the plug is not firing. Example: say the bad plug needs 20kV to fire, the coil develops 18kV which is too much for the insulation on the wire, boot, or cap. The spark will jump or leak off at the weak point and never jump at the plug.

So the spark energy may be leasking off elsewhere, such as the boot to close to the head, etc.

Check those things, check the gap, temporarily replace the plug with another one, see if the problems stays or moves to another cylinder, you get the idea. Remember, the above is based on the assumption that your observations of #8 are valid.

Good hunting, Regards, George

Hey, on your idea of a bad lobe, what about a bad distributor gear, also? Could cause a bad dwell variation at one point in the rotation.....

[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]

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

[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

[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