Running a coil on full 12 volts

[Bob Hoffmann CLC#96]

Guys,
Trying to help a local guy. He has a 68 DeVille. Had a fire at the ignition switch. It appears when it got rewired the coil resistor wire got replaced with a regular wire. There is no external resistor in sight. I understand that causes early point failure. My question is HOW early & the best way to resolve this issue. Thanks in advance.
Bob

["Cadillac Kid" Greg Surfas 15364]

BNob,
Short of replacing the resistance wire the easiest thing I can think of is using something like a tri-5 chevrolet ballast resistor.  The attached site is Rock Auto. They list four options.
Greg Surfas


http://www.rockauto.com/catalog/raframecatalog.php?ck[ID]=0&ck[idlist]=0&ck[viewcurrency]=USD&ck[PHP_SESSION_ID]=4jhspk864a4e6ap9jai5140hq1

[Classic]

Some coils have an internal resistor while others rely on a resistor wire or an external resistor to limit the current flowing into the coil from the car's 12-volt supply.  Your local NAPA store, etc. should have both kinds.

[R Schroeder]

Correct me if I'm wrong, but if it has a normal wire to the coil, what is wrong with running a 12 volt coil ?
I understand the voltage is dropped to save the points , but for someone driving it a few thousand miles per year it shouldn't hurt.
I understand that there is two wires feeding the coil. One for full 12 volts at the start, and the other wire comes in when your in run position.
For that reason I don't think a internal coil would work.
Please let me know if I'm wrong. Would like to understand why this would be a problem.
Roy

[Philippe M. Ruel]

The voltage is not primarily dropped to save the points - this is the condenser"s task, and could be corrected with a correct condenser. The voltage is dropped to save the coil.

More precisely, the voltage is lifted (the resistor shorted) at the start to ensure proper ignition operation, to compensate for battery voltage drop at the start.
Then it goes to about 8V because an external resistor coil is in fact a 6V coil, that will bear 8V but will eventually fail if submitted to 12V for too long.


An internal resistor coil will work on 12V. It is of a less sophisticated design, with an internal resistor featuring little resistance when cold - ie at first start in the morning, with a cold battery prone to voltage loss when starter may take seconds to start a cold engine. The start of a hot, flooded engine with such a coil may be harder.

[R Schroeder]

OK, I see now how that system works. I didn't realize it was a 8 volt coil to start with.
It starts on 12 and then dropped to correct the voltage to match the coil.

Couldn't he run a 12 volt coil with the solid , non-resistance wire , feeding it then ?

So, it isn't early point failure, its early coil failure.

I did find this wiring diagram which cleared it up for me, but it would have been better if it stated the coil was an 8 volt.

Thanks Roy

[dplotkin]

More precisely, the voltage is lifted (the resistor shorted) at the start to ensure proper ignition operation, to compensate for battery voltage drop at the start.
Then it goes to about 8V because an external resistor coil is in fact a 6V coil, that will bear 8V but will eventually fail if submitted to 12V for too long.

More precision is required...the purpose of the resistor (ortherwise called a ballast) is to limit current through the coil primary windings and not voltage. The effect is a voltage drop that is easy to measure and that is why many speak of the purpose of the resistor as to drop voltage. A voltage drop is a result, but limiting current is the purpose, in exactly the same way a ballast works with a fluorescent lamp.

What Philippe says above is true, but for the sake of precision I add the aforementioned.

Dan

[Wayne Womble 12210]

You want to get into a complicated analysis of a simple looking circuit, you have entered the right place. Yes, the voltage is dropped by the resistor, but ONLY when the points are closed and the engine is not running. Otherwise the ballast resistor is a current limiter, and will read V12 volts when the pulses are going on. It is more complicated than I care to get into.

The best thing to do is add a ballast resistor. 

Some reading material:  http://www.corvetteforum.com/forums/c3-tech-performance/2986717-ignition-physics-for-regular-guys-like-me.html

[bcroe]

The Corvette guy seems to have it all.  Probably here, we only need to be concerned how to
make it right, not how to redesign it.  If you aren't familiar with an oscilloscope, redesign
is out of range. 

I'll just add, at maximum rpm you could just have a coil & points to reach the right current
(and stored energy).  But when you slow down to idle, current would just keep building up
far above what the components could take.  So the resistor is added as a limiting factor, and
some adjustment made to still get decent top end.  In truth the points are the biggest
limiting factor, and the rest of the system (including the increase from 6V to 12V) must be
set up to protect those  +&%^ points.  The HEI (with electronic dwell) is so much better. 

Voltages inside a working system aren't fixed, but can be seen on a scope.  Bruce Roe

[TJ Hopland]

Yep current limiting is what the resistor does.    One quick way to find out about that is if you don't have one with a points system turn the key on and wait a couple minutes.   50/50 chance the points are closed.   If they are and there is no resistor you will burn the coil out and stress the rest of the wiring. 

In most cases of this era GM there were actually 2 wires at the + terminal of the coil.  Some other makes and eras that connection was in the harness somewhere.   One was the one from the starter and the other was the one from the key via the resistor.    I would go with the external resistor that was listed earlier.    You then need to figure out which wire is which.   Separate them, turn the key on.   The live one is the one that needs to connect to one side of the resistor.  The other wire goes to the other side of the resistor which then connects to the + side of the coil.    That way you still get your 'boost' when cranking.