Posted by Bob on March 31, 2010 at 08:40:25 from (69.178.228.68):
In Reply to: coil resistor posted by Mark W. on March 31, 2010 at 05:13:05:
Here's the straight FACTS.
Coils vary in primary resistance depending upon part number, manufacturer and application. Some are as low as 0.5 Ohms, some close to 5 Ohms.
For acceptable breaker point life at a specific system voltage the combination of coil primary resistance and any added external resistance must be such that primary current (ignition "on", points CLOSED, engine stopped) is in the 3 Amp to a maximum of 4 Amps (give or take a little) range.
Less than 3 Amps will PROBABLY give a weak spark, much over 4 Amps will be tough on breaker points.
If you can, when you get your new resistor installed, MEASURE the actual primary current and see where you're at compared to that range.
NOTE... besides different resistance VALUES, ignition resistors may be either fixed resistance or "PTC" (positive temperature coefficient).
With FACTORY systems, generally a FIXED resistance is used with systems that have a "starting bypass" (a separate terminal on the starter switch or solenoid that provides full battery voltage to the coil during cranking).
For FACTORY systems NOT using a "starting bypass", a PTC "ballast resistor" is used. When the ignition is switched "on" the resistor has a relatively low resistance to aid in starting, then it's resistance quickly drops to a higher value as it heats up to prolong point life and cause less heating of the coil.
NAPA, Standard Ignition, BW, etc., have parts guides that will show the resistance for a given part number and whether it's fixed or PTC.
Finding a counterman who is willing to grab that guide and have a look is another matter.
Also, there's "true" 12 Volt coils on the market that have enough primary windings so that no external resistor is needed. NAPA IC-14SB is ONE example of such a coil.
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