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Ballast Resistor Shows 12 Volts

Started by Coupe Deville, January 28, 2014, 01:20:36 AM

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Coupe Deville

Hello everyone. I installed a new coil in my 57 Sedan. However when I mesure the voltage on ethier side of the ballast resistor it shows battery voltage(12 volts). I have been told that a ballast resistor should put out about 9 volts. Is that correct? Is this possibly why the old points were extremely burnt.
Any help is appriciated. I need to get this car off the Privious owners driveway soon.

Thanks

-Gavin
-Gavin Myers CLC Member #27431
"The 59' Cadillac says more about America than a whole trunk full of history books, It was the American Dream"

cadman49

gavin..me again.  sounds like you have a internal resistor coil. a lot of people do this because they are hard to tell apart from an external resistor coil,which you need. the ballast resistor will be 12v on one end and 5v-7v on the other. I have a 2.5v on my cars and they work great. remember,its not the points that fire the plugs,its coil saturation,triggered by the points,which are just a switch,much like the ones in your house..(.p.s  that pertronix ignition works really great)..r.king

TJ Hopland

If there is no load on the resistor (points open or engine not running) you won't read much of a voltage drop.   The bigger the load the more the voltage will drop.   

I suspect Cadman meant 2.5 ohms, not volts.    2.5 ohm was a common resistance for this type of application. 

The basic concepts of what is going on is just Ohm's law which is simple multiplication or division of amps, volts, watts, and resistance.   Where it gets complicated is with loads that are not constant (like an ignition coil) and loads that may change resistance as the operate or heat up like a ballast resistor and or ignition coil.  That's where you get some math formulas that are a little more complicated.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Dave Shepherd

All internally resisted coils I have seen are marked as such on the coil case.

Coupe Deville

The coil I bought says on it, Mush use a external resistor. So when the points touch that creates load and drops the voltage. I tought that was the job of the resistor to drop the voltage? Just wanted to ask because I didn't want to fry my new coil.

-Gavin
-Gavin Myers CLC Member #27431
"The 59' Cadillac says more about America than a whole trunk full of history books, It was the American Dream"

curly

Quote from: Coupe Deville on January 28, 2014, 08:15:56 PM
The coil I bought says on it, Mush use a external resistor. So when the points touch that creates load and drops the voltage. I tought that was the job of the resistor to drop the voltage? Just wanted to ask because I didn't want to fry my new coil.

-Gavin
TJ Hopland is correct.   The resistor will read battery voltage on both sides if the points are open.  When the points close, the primary circuit is complete to ground through the resistor and coil. Without a ground, there is no circuit, so there is no current (amps) flowing for the resistor to resist.

T Lewis

Coupe Deville

Oh...I see. Thank you that makes scence. I Did not take into consideration that it was the ground wire too. Sorry for all of the questions. I recently got a very soon deadline to meet to get this car out of its current resting place. I would like to drive it out and not tow it out if I could. Looking more closely at the ignition system other pictures of 365s from the late 50s have a external condenser mounted on the coil bracket like this picture. My car currently does not have one. Do I need one of these?
Thanks
-Gavin
-Gavin Myers CLC Member #27431
"The 59' Cadillac says more about America than a whole trunk full of history books, It was the American Dream"

curly

That is for radio noise suppression, not needed to run the engine

T Lewis

Wayne Womble 12210

If checked with the points open it will read battery voltage because its not a completed circuit and there is no flow of current. At steady state with the engine not running and the points closed, the reading should be somewhere in the 6-8 volt range.  However, if the car is running, the voltage will climb back into the battery range again because now the circuit does not follow ohms law.  The resistor now serves as a current limiter and not a voltage limiter.   

TJ Hopland

There is no escaping Ohm's law, its kinda like all those other laws like Newton's, we are sort of stuck with them.  Your volt meter is completing the circuit just like the coil does.  Volt meter just does not present enough load on the resistor for most of us to notice.   It is there and if you had a sensitive enough meter on the proper scale there will still be a drop.

I believe technically speaking current limiting is what the resistor is doing.  Voltage is just the easiest thing for most of us to measure and it does drop because of Mr Georg Ohm who figured out this whole relationship between volts and amps.   At one point in my life I did actually understand what was going on in a conventional ignition system and it was a lot more complicated than I had originally thought.  That was stuff I leaned quite a few years ago and have not really had to think that hard about since then.   I do fairly regularly use Ohm's law but its to figure out simple things like how many amps does a 1800 watt worth of light bulbs draw.  If you are curious the answer is at 120 volts its 15 amps.           
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Wayne Womble 12210

You are absolutely correct, the meter does complete the circuit, but it is intended by design ( very high impendence ) , not to significantly affect the conditions in the circuit.

You are also correct that the dynamics of the ignition system is very complicated for such a simple looking circuit.

Jeff Rosansky CLC #28373

I guess I should shut off a few bulbs!!
Jeff
Jeff Rosansky
CLC #28373
1970 Coupe DeVille (Big Red)
1955 Series 62 (Baby Blue)
Dad's new 1979 Coupe DeVille

Coupe Deville

Hello everyone. I went over to the car today and put my multimeter on the coil. I turned the motor over by hand until the points closed. I got +10.3 volts. Is this exceptble in the range?

Thanks

-Gavin
-Gavin Myers CLC Member #27431
"The 59' Cadillac says more about America than a whole trunk full of history books, It was the American Dream"

Wayne Womble 12210

Sounds a bit high.    Check it above and below the resistor.   

You could also check the ohm value of the resistor and the coil.

Coupe Deville

Hello everyone. The resistor works as it should. It doesn't show 12 volts anymore, its all sorted. The problem I have now is this. When cranking the car coil did not have 12 volts. It had almost no power when cranking. The car started and was running fine for the longest it ever has. I shut it off to check the oil. I went back in and tried to start it. No luck. There was many more attempts to start after that but then we noticed we were not getting full voltage to the coil. I have a wiring diagram. I am thinking the problem has to be with the yellow wire going to the top of the ballast touching the coil wire. I would have thought that if something went wrong the wire would have no voltage not just little voltage. Anyone els had this problem?

Thanks

-Gavin
-Gavin Myers CLC Member #27431
"The 59' Cadillac says more about America than a whole trunk full of history books, It was the American Dream"

J. Gomez

#15
Gavin,

If you look at the diagram the yellow wire goes from the starter solenoid to the ballast resistor terminal which has the coil (+) wire connected, this place a direct 12V from the starter solenoid up to the coil

Once the engine is running and the ignition key is in the “ON” position the voltage now comes from the ignition key pink wire going on the opposite side of the ballast resistor. The voltage on the other side of the ballast resistor to the coil will be about 9V-10V.

Hope this helps..!
J. Gomez
CLC #23082