# Ballast resistor not dropping voltage to coil



## Freddy_Krugerrand (Aug 19, 2012)

1966 GTO. Purchased a new ballast resistor and it is not dropping the voltage. Both at the coil and at the other end of the resistor I am still getting 12 volts and it's frying my points.

Only upgrade from stock is a single wire alternator but I can't see how that would stop the voltage from dropping across the ballast.

Any help would be appreciated.
-alex


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## Fuddy (Aug 7, 2018)

Freddy_Krugerrand said:


> 1966 GTO. Purchased a new ballast resistor and it is not dropping the voltage. Both at the coil and at the other end of the resistor I am still getting 12 volts and it's frying my points.
> 
> Only upgrade from stock is a single wire alternator but I can't see how that would stop the voltage from dropping across the ballast.
> 
> ...


Ballast resistor goes from ignition switch to coil +. 
Voltage across battery (fully charged good battery, car not running) should be about 12.5 volts. Running (depends on lots if factors) should be roughly 14 volts.
So one side ballast to ignition switch on position. No power off or accessory.
Other side to coil +. Coil -- to distributor (which of course points, condenser to ground).
Since cranking voltage is low (maybe 9-10 volts), from coil + ...that side should go to starter solenoid. 
So key on (motor NOT running) should have 12.5v one side ballast. Other side ballast 12.5v if points are open...about 10v points closed.
Be sure coil is for external ballast.
Be sure car doesn't have a resistor wire, ignition circuit.
Ballast resistance should match coil recommendation.
Be sure (This most likely is problem) that the coil + side only gets power through ballast resistor and starter solenoid. 
When car is running if you measure, say, 14v across battery you should have that on ignition switch side of ballast (may have a small voltage drop), other (coil +) side of ballast should be about 4v less.
Also important...known good condenser. New doesn't mean good.

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## PontiacJim (Dec 29, 2012)

Freddy_Krugerrand said:


> 1966 GTO. Purchased a new ballast resistor and it is not dropping the voltage. Both at the coil and at the other end of the resistor I am still getting 12 volts and it's frying my points.
> 
> Only upgrade from stock is a single wire alternator but I can't see how that would stop the voltage from dropping across the ballast.
> 
> ...


If the car is stock and you are using points - which I read as you are, I don't think you should have a ballast resistor (but I don't own a '66 either). Your car should have 2 wires paired together at the + side of the coil. One wire is 12Volts - used when starting the car. The other wire is your factory resistor wire and is typically a cloth cover wire. This operates when you snap the key back from "start" to the "run" position. This wire supplies about 9Volts to the coil so as not to burn out the points. Both of these wires run off the key switch.

There is no wire that runs to the starter on a '66 other than the purple wire to the "S" terminal and is used to activate the solenoid for cranking. The other lug, which I believe is labelled "R" is not used on the earlier cars. This lug is only energized when you start the car and will disconnect power as soon as you snap the key back to the "run" position.

That said, is your factory resistor wire missing or burned up? If it is, then I might replace it back to factory. It will come off the firewall connector block on the engine side. A wiring diagram will show this and your key switch connections. 

You can use a ballast resistor, but you are still in need of a 12Volt source to start the engine that will shut off when you put the key in the "run" position just like the key switch is designed to do, otherwise you will be overriding the ballast resistor and continue to get 12V at the + side of the coil - and this may be happening now?

You will still need 2 wires going to the + coil, a 12V supply to start the engine and the resistor/ballast source when the key is in the "run" position. Here is where you could run a wire off the solenoid "R" terminal to the + coil to get your 12V supply to start the car which will disconnect once the key is snapped back to the "run" position. Then you will need a constant 12V source that is activated by the key switch or toggle switch to provide 12V and be able to be shut off to disconnect the power source when you turn the car off. This is where the ballast resistor comes in. Run that wire to one side of the ballast resistor and then run the other side to the +coil. This should give you 12V going into the ballast resistor and drop down on its other side to the needed 9V that will go to the +coil.

Pretty sure that should cover it, but I am no wiring expert. :thumbsup:


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## Freddy_Krugerrand (Aug 19, 2012)

Stock, the car has two wires coming from the ignition to the coil. As I understand one is used to start the car and the other is a resistance wire that is used while the car is driving. 

If you switch to a balast resistor what do you do with that original starting wire that gets a full 12 volts. Unless I am misunderstanding something.

And why can't you use the original starting 12 volt wire off the ignition to the coil to start the car with a balast resisitor? Why do you run a new wire off the starter? I'm confused.


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## Fuddy (Aug 7, 2018)

What you said is absolutely correct "Stock, the car has two wires coming from the ignition to the coil. As I understand one is used to start the car and the other is a resistance wire that is used while the car is driving. "
On my wife's Pontiacs, since I was an electronic technician, we decided best to eliminate that resistor wire, ignition switch to coil + with a ballast resistor mounted wherever convenient (firewall or near coil).
Regardless of how the wires are run, the idea as you said is crank position coil + gets full power. Running it goes through resistor. So using meter or test light, one wire goes to resistor from ignition (on). Other side resistor to coil +. Also here a wire that gets power cranking, either from ignition switch (crank terminal) or where it goes which is starter solenoid.
It boils down to (imho) how stock you want it to be, convenience, if things have been changed in the past, etc.
Years ago a friend brought his 69 GTO 400 here, it wasn't running right. Running I measured 3-4 volts to coil +. Obviously there was high resistance with resistor wire or high resistance connection. I replaced resistor wire with copper, used a ballast resistor, wired correctly. 
Needless to say it picked up lots of power.

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## the65gto (Oct 9, 2008)

Fuddy said:


> What you said is absolutely correct "Stock, the car has two wires coming from the ignition to the coil. As I understand one is used to start the car and the other is a resistance wire that is used while the car is driving. "
> On my wife's Pontiacs, since I was an electronic technician, we decided best to eliminate that resistor wire, ignition switch to coil + with a ballast resistor mounted wherever convenient (firewall or near coil).
> Regardless of how the wires are run, the idea as you said is crank position coil + gets full power. Running it goes through resistor. So using meter or test light, one wire goes to resistor from ignition (on). Other side resistor to coil +. Also here a wire that gets power cranking, either from ignition switch (crank terminal) or where it goes which is starter solenoid.
> It boils down to (imho) how stock you want it to be, convenience, if things have been changed in the past, etc.
> ...


Ditto. That is how I have my ignition wired. I removed the resistance wire. In the event I change from points, I would bypass the external resister, but at present, it serves as a test point.


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