Diagnosing an electrical problem
by Pat Yanahan, May 2009

Ever since I acquired my 1964 356C I was having a problem that had the symptoms of fuel starvation.  With my trusty private mechanic Gordy Smith and counsel from Tom Funk we tried everything.  The issue was that when the engine was just started it would run great but after warming up it just did not have any power under acceleration.  After sitting cooling down for half an hour or so it started and ran just fine until it warmed up again.

So Tom adjusted the carburetors (when it was cold) and we tore around the neighborhood by his shop and it ran perfectly.  But on the toll road driving home it warmed up and exhibited the same problem.  So over a period of a year we did the following:

  1. Replaced the spark plugs
  2. Rebuilt the fuel pump
  3. Replaced the ignition wiring
  4. Rebuilt the carbs
  5. Replaced the gas lines
  6. Adjusted the valves
  7. Exchanged the distributor with one that was known to be good
  8. Asked everyone if they had any idea of what may be the problem

Now I hate to admit that my undergraduate degree is in electrical engineering from one of the big universities.  You will soon see why.  Driving home from one of our club events in Wisconsin I was just limping along the toll road, frustrated with the poor performance of my C coupe.  It was running around 3,000 RPM when it was time to turn on the headlights due to the setting sun.  As I pulled out the light switch the engine coughed and again exhibited the "fuel starvation" symptom.  A few more ons and offs with the lights told me that this was an electrical issue, not fuel starvation.

As we were driving home I pondered my basic transformer theory from my freshman year and thought:  Isn't the ignition coil a transformer and maybe the spark plugs are not getting enough high voltage when the engine heats up?  So the next day I removed the ignition coil (newer looking Bosch Blue coil) and measured the primary and secondary resistance with my ohmmeter.  Everything was within specification.  But the physical location of the coil on the fan housing gets fairly warm after driving a while so I tested the primary while heating up the coil on the bench--still within spec.  Then I let it cool down and repeated the secondary resistance with the heat gun on the coil.  As the coil heated up the secondary resistance eventually dropped in half telling me the secondary was shorting out when in a hot engine compartment.

Sure enough when I let the coil cool, the secondary resistance went back up to normal.  Obviously the coil was working fine when cold but shorted across the windings when under normal heat in the engine compartment, lowering the voltage to the spark plugs.  A new coil was installed in five minutes and now it runs like a C engine should both cold and hot.  So to all of you taxpayers who helped me through college under the GI Bill--thank you.  I may be a little slow, but I did eventually figure out this challenging and very basic technical issue.