surprising short life of ignition coil

[tmctguer]

yesterday, while driving my '73 911E on the freeway, i had that horrible "loss of power" experience where you know the car has just shut down. i was lucky to be able to get it over to the shoulder before losing forward momentum (the beauty of going 85 mph......).

i had it flat-bedded home at the unfreakin' believable cost of $157 for a mere 28 mile ride..........i woulda PUSHED it home if i knew it was that much !

anyhow, i got it home and had no spark. i had recently fixed the small fraying copper wire that grounds the distributor plate, so i double checked that to make sure my repair had held. it had -- still no spark.

i pulled the high tension lead out of the bosch ignition coil that i had changed approx. 9 months ago and checked for spark there. virtually non-existent. i checked the connector leading into the coil. it had a little corrosion on it. i cleaned it, checked again for spark, and again it was virtually non-existent.

i pulled out my ancient bosch coil (which i believe to be the car's factory unit), swapped the wires, and checked for spark. the old coil had spark in DROVES. so i swapped coils. upon removal of the new coil, i turned it over and found a thick waxy substance leaking from the bottom. upon replacement of the coil, the car fired up again.

any clues as to why my new coil would have only lasted 9 months? it was the correct part number for my car. just a fluke??

also, with a bad coil in my hand, what tests can you do with a multimeter to confirm the status of an ignition coil? i want to try to do these tests on a bad and good coil so i use the results for future reference.

opinions on my coil's short life are much appreciated. also, comments about coil tests with a multimeter are welcomed, too.

[Early_S_Man]

Oh, the potential value of carrying a spare (old) coil in your spare parts 'kit' ...

Was the new coil one of the shiny, aluminum-cased Bosch units made in Brazil?

Sometimes you can test the resistance of the primary and secondary windings ... and find a discrepancy in the secondary winding value that is supposed to be from 650 - 790 Ohms. The primary is seldom the problem, and is supposed to be 0.4 -0.6 Ohms, and any checks with a digital multimeter should be done after changing to a fresh battery ... many DMM's show dramatic inaccuracy with an old battery! Also, the coil may not show bad resistance readings, as it may be breakinking down --- shorting or arcing ... only under load!

[tmctguer]

S-Man, you're right about the Brazilian vintage of the coil. i opened my shop manuals (imagine that !) and performed the 2 tests on the coil specified. both were within spec.

it must have failed under load. looks like i'll try my luck again with another silver coil and just keep my fingers crossed.

[Early_S_Man]

You might want to try a used coil from the Pelican parts board, or eBay. I recently got a used SC coil for $35 and it works fine with Bosch or Perma-Tune unit on the bench ... nice, fat, blue 10.0 mm spark!

You may want to contact the supplier of your shiny Brazilian unit ... it may have a one-year warranty. I am beginning to wonder about those aluminum-cased coils!