Bad ballast resistor

[Bill D (TX)]

I have a 1947 8n, front mount 12V. With no sparkies, I finally figured that the ballast resistor went bad. I replaced it, got sparkies but noticed the new ballast resistor started smoking. Any ideas why the ballast resistor would go bad? Is smoke from a new resisitor normal? By the way, when the sparkies stopped, I changed from points to electronic ignition. Thanks.

 

[Bob]

It's NORMAL for oily residue from the manufacturing process to smoke off of a new ballast resistor.

HOWEVER, what kind of a coil are you using and what additional resistors (if any) are in the circuit?

Do you have a way of measuring primary current?

Over 3 Amps to up around 4 Amps is OK, more and the resistor, coil and EI module are all subject to losing more some than they can tolerate.

 

[Bill D (TX)]

I am using a 12V square box. There are no additional resistors in the circuit. I have a cheap multi-meter but have not checked the current. Note that I have had this tractor for over 10 years with no additional resistors in the circuit. The previous ballast resisitor was placed about 5 years ago.

 

[Bruce (VA)]

There are a few reasons why you can run a 12v square coil w/ only the OEM ballast resistor in the circuit & not burn up the coil, good luck being the primary reason.

It used to be before the "Land of Almost Right" started making coils that you could count on a 12v frontmount coil as having 3 ohms of internal resistance & the 6v coils as having 1.5 ohms or less. Thus, thanks to Ohm's Law, you could calculate what additional resistance you needed in the circuit to limit coil current to 3.5 amps. So, you will need to measure the internal resistance of that alleged 12v coil & see what it is. A digital multi-meter has two probes & a switch. Set the switch on resistance. Put one probe on the top of the coil & the other on the pigtail at the bottom. It will give you a reading in ohms. Lets just say it reads 3.0 ohms. Your OEM ballast resistor (which you must use) is about 1 ohm hot. A coil a 3.0 ohms, plus the ballast resistor at 1 ohm (hot) gives you 4.0 ohms resistance in the circuit. Your 12 volt alternator puts out 14.5 volts. You need to determine current (amps). 14.5 v divided by 4.0 ohms gets you 3.6 amps; close, and probably safe. And, that is why you may not need any more resistors. But, and this is the problem......what if the coil is only 2 ohms? Do the math. 14.5 volts divided by 3.0 ohms gets you 4.8 amps! Not good! And, if the coil is less than 2 ohms (and some are) it will fry quickly and take your EI along w/ it. So, to get it to 3.5 amps, you need another resistor in the circuit. Either that, or keep spare $30 coil around & be prepared to replace that EI as well.

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