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Old 11-30-2009, 01:22 AM   #1
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Default coil? do i need to add a ballast resistor?

i was planning on strapping a better coil into my '82 242t, was going to run a crane PS6o. i need to know if i need to add a ballast resistor, or it just a bolt in?
is there anything else i need to know here? im used to repairing cars, or full upgrades, this is a bit new for me.
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Old 11-30-2009, 01:26 AM   #2
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ballast resistors are for cars running points. i got the same stuff when i installed my msd coil. just toss it and wire it up like stock.
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Old 11-30-2009, 01:35 AM   #3
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Originally Posted by nextproject View Post
ballast resistors are for cars running points. i got the same stuff when i installed my msd coil. just toss it and wire it up like stock.


read the instructions, if your replacement coil does not have the correct OHM range as the stock, a resistor might be needed.....

on my 242 with LH2.2 and EZK, my Accel coil needed a resistor...my Mallory coil did not.
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Old 11-30-2009, 01:43 AM   #4
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Quote:
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ballast resistors are for cars running points. i got the same stuff when i installed my msd coil. just toss it and wire it up like stock.
No that is incorrect. Ballast resistors are used as current limiting devices even with some electronic ignitions. There is one on the original posters 240 turbo. You need to measure the primary resistance of the coil you want to use. You may need an additional ballast resistor so it works properly with the stock ignition system which is electronic not a points system.

The stock ignition box is designed to work with a .9 ohm ballast resistor and the coil I have measures 1.8 ohms. So total resistance is 2.7 ohms with my setup. The stock spec is probably about 2.5 ohms to 2.9 ohms or there about. You want to keep the primary resistance very close to that otherwise you'll burn out the ignition unit from excessive current.
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Old 11-30-2009, 02:17 AM   #5
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No that is incorrect. Ballast resistors are used as current limiting devices even with some electronic ignitions. There is one on the original posters 240 turbo. You need to measure the primary resistance of the coil you want to use. You may need an additional ballast resistor so it works properly with the stock ignition system which is electronic not a points system.

The stock ignition box is designed to work with a .9 ohm ballast resistor and the coil I have measures 1.8 ohms. So total resistance is 2.7 ohms with my setup. The stock spec is probably about 2.5 ohms to 2.9 ohms or there about. You want to keep the primary resistance very close to that otherwise you'll burn out the ignition unit from excessive current.

useful information, i know there was some here! i realized this forum has a lot of conflicting opinions, and people who like to give bad info.
....thank you for helpful info!
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Old 11-30-2009, 07:01 AM   #6
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if you have to run an additionaly resistor in-line with your 'performance' coil, then there is an extremely high chance that your spark will have less energy than it did with the stock coil.
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Old 11-30-2009, 07:40 AM   #7
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Originally Posted by 740Weapon View Post
if you have to run an additionaly resistor in-line with your 'performance' coil, then there is an extremely high chance that your spark will have less energy than it did with the stock coil.
this, lol i was thinking the exact same thing.
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Old 11-30-2009, 11:30 AM   #8
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Originally Posted by 740Weapon View Post
if you have to run an additionaly resistor in-line with your 'performance' coil, then there is an extremely high chance that your spark will have less energy than it did with the stock coil.

all i need the "performance" coil as you put it to do,is match stock at minimum. i can grab a number of aftermarket coils locally, but would have to order, or grab a pic n' pull used one to run stock.

...may be wrong here, but the way i understand a coil.. measuring resistance wont tell you a bit about its output, simply because you gain resistance with more windings, but more windings are needed to produce a higher output?
with that said quality comes to play, so if you take two coils, the "hotter"/functioning coil, would be one with resistance close to stock specs, but with a higher winding ratio?

as for the actual resistor... you guys do know that a "ballast resistor" isnt so much for limiting voltage like a regular resistor, but for making sure it's constant,fluctuating voltages can burn out a coil.
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Old 11-30-2009, 11:43 AM   #9
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in the good olde days coils were designed to operate at a reduced voltage to start the car while cranking.

so on say a chevy, the coil is powered directly from the starter durring cranking when voltage is low but through a resistor durring normal operation to save the ignition system.
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Old 11-30-2009, 05:19 PM   #10
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Quote:
Originally Posted by 740Weapon View Post
in the good olde days coils were designed to operate at a reduced voltage to start the car while cranking.

so on say a chevy, the coil is powered directly from the starter durring cranking when voltage is low but through a resistor durring normal operation to save the ignition system.
Even though you wrote that first part incorrectly. I know what you mean. This is also exactly the setup used on a 240 kjet cars with Bosch electronic ignition. It's a constant dwell design system which uses a ballast resistor for current limiting on the primary ignition circuit. While starting the coil gets 12V from the starter then once started and the key is turned to the run position. The coil gets a reduced operating voltage through the ballast resistor.

A lower resistance than stock in the primary ignition circuit will quickly burn up the ignition box on these systems. I know from experience. If you want an increase in voltage with these systems and you keep the primary resistance about stock. A coil with a high turns ratio will give you more output than stock. Some high output coils that have very low primary resistance will need an additional ballast resistor to bring the primary back to the proper resistance. But if they have a high turns ratio the voltage output increase is there.
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Old 12-01-2009, 01:38 AM   #11
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now this makes sense. i appreciate it dl242gt, its been so damn long since Ive had to deal with this.
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Old 12-02-2009, 01:09 PM   #12
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well im glad i posted my misinformation here! how does one go about measuring this stuff? luckily, i kept the ballast resistor and the original box so if i need it i can install it now. how would i know what the stock resistance is? step by step please, never measured coil resistance before (i know i know, dee de dee)
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Old 12-02-2009, 03:41 PM   #13
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The ballast resistor resistance is printed on it. I would also measure it to make sure it is within spec. Next you measure the resistance of the primary of the coil. Measure from terminal 15 to terminal 1. Put the ohm meter on the 20 ohm scale and hold the tips of the probes together to make sure you know what the resistance of the leads is so you can deduct it from what you measure. The stock resistance specs are printed in the Volvo ignition book on k-jet.org.
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