The ignition coil is two copper windings around a common iron core - essentially a step-up transformer. The ignition module puts current into the primary winding, and the voltage induced on the secondary winding fires the plug.

Ignition coils fail in the same ways most transformers and electric motors fail: electro-mechanical forces cause micro-movement of the copper windings. Eventually a winding (single loop of the coil of copper wire) loosens. A wire can break by fatigue (total failure of the ignition), or it can rub on a neighbor wire, the iron core, or a support piece. Rubbing wears through the insulation. You can short out individual turns in the coil winding. That lowers the output voltage, but the coil will still work until more turns short out. You can short out between the primary and secondary windings, and have internal arcs give you the christmas tree effect.

I replaced both of mine at 60,000 miles after having the dreaded christmas tree effect on the day after Christmas on a trip to the middle of rural Mississippi. Both coils ohmmed out with spec, but replacing them cured the problem. Sometimes you can't find high-voltage arcing between the primary and secondary using a voltmeter. Individual windings will check out OK even though the part has failed.

Age degrades the insulation, and use loosens the coil internals over time. It is time for anyone with original ignition coils to get replacements on order.

 

What is the second part listed below?

 

I am not sure where you got that, Pelican?... the PET has just the 944 part number listed for #11. And the part I received matches the prior thing, too.

 

Yup. Pelican. Must be for another model/year?

No worries. I've ordered the 944 part.

Any idea if Pelican is shipping OE or OEM? Their price is better than Sunset! (at least last time I checked Sunset it was $77 each).

 

I'm not sure the difference, but they are Bosch parts just like the original, and they come in a bosch box.... they look the same except the can of the coil is shiny silver versus black.

I think I'll order two more before they sell out!

 

My thoughts exactly!

$100 Plug Wires, oops! I mean $50 coils don't come along every day.

 

55 left in stock

 

They are Bosch but they are not the same as the OE Bosch. Whoever is supplying Pelican is buying "Made in Brazil" Bosch coils which have a habit of crapping out after a few months if they work at all. To save $25 its not worth the headache! The "Made in China" Bosch parts also look the same but they will not last anywhere near as long as the parts that are made for the US market. This comes directly from the DSM (Bill Marchetti) at Bosch's aftermarket division.

 

"Eventually a winding (single loop of the coil of copper wire) loosens."

That's one mode of failure. The others are:

1. Eventual high voltage breakdown of secondary to primary, i.e. 35KV on the secondary, e.g. via case cracks.
2. Loss of dielectric capacity of the oil insulator,
e.g. engine heat & I squared R losses.
3. Mechanical fatigue of the terminal connections.
4. etc.

 

Thanks... interesting! Learn something every day!!

Now, do the two small "ignition modules" that are attached to the heat sink on the coils ever "wear out" as well? I've never read of anyone needing to replace those.

 

"Now, do the two small "ignition modules" that are attached to the heat sink on the coils ever "wear out" as well?"

Very rare! They're well designed and cheap (~$100).

 

The car should run without problem with either coils disconnected, one at a time. I do that test from time to time as a broken distributer belt can cause great harm to the engine if the rotor is static and the same spark plug does all the fireing.
First of all check your distributor belt, easiest done by removing the caps and observe that both the rotors are turning when you run the engine starter. Also check that both rotors always points against the same sylinder lead.