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Hi, fellas and gals! I've noticed something odd on my car, a 1979 450SL: In the positive lead to the ignition coil, there's a ballast resistor, mounted on the fender well just behind the coil. Now, I know from years ago, that a ballast resistor reduces excess sparking at the points and therefore reduces wear and pitting - fine. But in the later cars like mine which have the OEM breakerless ignition, what's the use of the ballast? Seems to me if I removed it, I'd get easily more voltage to the coil and hotter spark as well. I've checked that ballast with my DVM and there's between 4-5V drop across it; that's a fair bit to be sure.

Anyone have any ideas on this? Years back, in the 70s, my dad and I always liked using CD ignition systems, Mallory and the like and dad always got rid of the ballast for more coil voltage. The AM radios sounded like heck, but Dad didn't care...more spark, better MPG! he'd say, rest his soul!

Sandy Gerli
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When was the last time you replaced the points?:rolleyes:
Until I consulted my schematics, I wouldn't remove anything that had a 4-5VDC drop unless the car would not function. And the problem was traced to that source.
 

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1998 SL500, 1959 220S, 1970 280SL
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A lot of people replace points with a Pertronix or Crane unit, and then change the coil to one that can handle 12V. Your system is spec'd out, I believe, to run on lower voltage with a 12V feed only when cranking. If you bypass the resistor, you will probably destroy your coil very quickly. I'm not sure what would happen to your igition unit.
 
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