1993 Ignition Coil

GrandAm 1993 SE 6 cyl.

This car uses the wasted spark, as far as I understand it uses only 3 coils to 6 cylinders. Each coil side goes to one cylinder's spark plug, so when one cylinder is on top of compression, its pear is on exausting. It means that two spark plugs receive high voltage at the same time.

Well, last month the car started to present a problem that seemed to me to be a failing cylinder, it got worse, in the middle of the highway it looked like to have only one cylinder working.

The mechanic found a bad ignition coil, replaced it, everything back to normal. 10 days later the same sympton, failing cylinder.

With a highly isolated plier I removed each of the ignition wires from the ignition coil while the car was idling. 5 of the coil metalic contact was jumping a strong spark to the neighbor wire. One had nothing.

The failing coil was exactly the one the mechanic replaced.
Removed this coil and the next for electric comparison.

According to documentation, the coil is a simple power transformer, has a primary isolated from the secondary. The primary is connected to the power control module, the secondary is connected to two spark plugs via the wires.

Measuring resistance, found out that the primary of both has a very low resistance, it should have. The secondary of the failing coil shows around 8k ohms, while the good coil shows around 6k ohms.

Now, there is the problem. Measuring the resistance from any secondary pin to any primary pin, on the good coil it shows open circuit, it should be. The bad coil shows 18k ohms of resistance. It means that there is a short circuit between the secondary to primary and it also means a spark can be jumping back to the ignition module. Ouch.

What intrigues me, is that one side of the failing coil's secondary is effectively generating a strong spark, you can feel the engine failing when the wire is removed. The other side of the secondary present no spark and do not change nothing in the engine idling. Of course it means that side of the secondary is shorted to primary, right?

Now, question:

It is possible for something being damaging the coil? I question this cause the same position bad coil. Is it possible for a new coil to fail after 10 days? Yes, it is possible I guess, but what are the chances?

Mechanic is far away, will waste more time and gas to go there to retrieve the 1 year warranty on the part, than go to AutoZone and buy a new coil for $20.

Comments?

Check the ignition module under the coils. This could be going bad and causing the coils to die. It is possible for a coil to go that fast but without something causing the problem I doubt that the coil would go by itself. Also, check for bad wires. A bad wire with too much resistance can damage the coils very easily.

I got the same thought. For two coils to short circuit inside, it is just because the high voltage spark can't go anywhere so in time it just start to poke holes through the insulation. The 18k ohms between secundary and primary is a prove of that. I went directly to measure the wire for cylinder #1, it was completely open. A short run to AutoZone, 20.99 for the coil plus 25.00 for a new set of wires fixed the problem and gave me extra few HP's (probably). Didn't want to go through replace the spark plugs today, the physical position of the plugs #2, #4 and #6 are horrible at the V6 engine, you need to have thin arms and children hands to get in there with the wrench and all. Next time. Problem solved. Now will be a discussion with the mechanic, he needs to pay for the new damaged coil... it was his fault not to check the wires...