View Single Post
  #21   IP: 24.152.131.220
Old 09-23-2011, 07:41 AM
ndutton's Avatar
ndutton ndutton is offline
Afourian MVP
 
Join Date: May 2009
Location: Torrance, CA
Posts: 9,619
Thanks: 198
Thanked 2,208 Times in 1,425 Posts
If we accept Pertronix's advisory as gospel, you are correct Kelly, anyone with an alternator that charges at more than 12 volts (that's all of us) and have a 3 ohm coil have trouble in our future.

So why the difference between those of us with and without overheating coils? I'm guessing there are variables from boat to boat:
  1. Charging voltage - the higher the charging voltage the higher the amperage and the farther away from the specified maximum.
  2. Ammeter or volt meter - those long ammeter wire runs that produce voltage drop while bad for the battery may be helping the coil.
  3. Engine run time - the longer the run time the hotter the coil. Heat build up is cumulative. I believe heat related coil damage is also cumulative.
  4. Oil or epoxy filled coil - Pertronix's own literature says oil filled coils are more heat tolerant.
  5. Internal coil resistance - How accurate is the 3 ohm figure from coil to coil? 1/2 ohm either way can make a big difference.
I have the Pertronix ignition and their 3 ohm oil filled Flamethrower coil that is performing well. I'm adding a ballast resistor ASAP as a precaution and will measure coil temps as soon as it's done. I've already measured the temps on the current system so I'll have something to compare.

As far as what size resistor, I'm figuring the charging voltage from my fixed point alternator at a worst case scenario of 15 volts (no ammeter run), will measure the internal coil resistance and calculate the target system amperage at 3.75 amps (see edit) (1/4 amp margin of safety). Assuming the coil is at precisely 3 ohms (before actual measurement for the sake of this example only), I'll need a 1 ohm resistor.

Coil temperature measurements will be the acid test but I'm kind of excited that finally there's something solid to follow instead of grasping at clouds.

Anyone else interested in making coil temperature measurements, it would be best if the measurements were made both before and after the addition of a resistor for comparison.

edit:
Later on in the testing process I arbitrarily decided to lower the target amperage to 3.4 amps or 15% below the maximum for a wider margin of safety.
__________________
Neil
1977 Catalina 30
San Pedro, California
prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others

Last edited by ndutton; 10-04-2011 at 11:59 PM.
Reply With Quote