12V Conversion coil question for John T or others

Question- all the points-type car engines I've ever worked on had a ballast, either a separate one or in the wire itself. Do all these, mostly 12v systems, actually have a 6v coil thus requiring the resistor? I'd always been under the impression that the resistor was required to reduce voltage to the points to keep 'em lasting longer.....and when we'd convert a GM points system from points to HEI, we'd run a new wire that bypassed the original wire that had a resistor in it. So, does a normal 12v system need a resistor with a 12v coil or not???

NOOOOO OO a 12 volt system that uses a full true 12 volt rated coil (some call internally ballasted) DOES NOT USE EXTERNAL BALLAST.

A weakness in point ignitions is excess current would cause premature point burn up so the current is limited to 4 amps or under. Therefore since a 6 volt coil only has say 1.5 ohms resistance you need the other 1.5 ohms of external ballast so the points current is limited to 4 amps. A 12 votl coil already ahs 3 ohms of primary resistance so no extra balalst is used.

John T

In Bob M's diagrams it shows to use a light between one of the alternator terminals and the switch. In a different author's 12v conversion explanation, both using the GM 10-SI alternator, the other guy said to use a diode to prevent feedback when the switch is off.....

Will the light in Bob M's diagram do the same thing?

The 10SI was originally designed to use the light. (It's charging system warning light idiot light in your car.) However a diode properly wired up will provide an equivalent current feedback limiting function as a warning lamp.

Personally I prefer the warning lamp. It will light immediately should the alternator quit working or if the fan belt breaks.

I use 2 resistors. one for the coil and one for the alt. saves a lot of headache down the road

That all being said, Ive seen plenty of those alternator set ups WITH NO RESISTANCE WHATSOEVER (or a diode) and they all worked fine NO PROBLEM. Its my belief HOWEVER the idiot light or 10 ohm resistor or more can provide some degree of current limiting protection for the alternators internal circuitry which no or low (1.5 ohm ballast) resistance does not.

I guess THE BOTTOM LINE in my humble opinion as far as safety and to still allow the alternator to excite would be in order of these preferences:

BEST INCANDESCENT Idiot Light of around 40 ohms or so cuz it limits current PLUS is a charge indicator

SECOND BEST A resistor of at least 10 ohms up to maybe 40 to 50 or so ohms

THIRD A diode to prevent run on

FOURTH To use NOTHING

Any agreement or disagreement ????? ??

John T

There are some people that run without and have no trouble. Whatever you decide will be just fine.

The idiot light Bob n others use also has a high resistance, probably higher even then 10 ohms I recommend, so it may be even better as a safety current limiting protection device PLUS it serves as an idiot charge indicator light.

CAUTION some lights like an LED can be way toooo much resistance and not allow enough current for excitation THEY WONT WORK The ones Bob uses are still incandescent I believe????? ??

Whatch think Bob ????? ?????

John T

>Whatch think Bob ????? ?????

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Right on John! The warning lamp must have a cold resistance in the range of about 10 to 40 ohms.

Less than 10 ohms it will backfeed enough current to fire the coil after the ignition is switched off. And greater than 40 ohms (eg. an LED) will not draw sufficient current to "flash" the alternator at startup to cause it to begin charging.