Coils frying

[Dean E]

I have 8n that was conv to 12 volt.First thing was coil that was 6v melted than put new 12v coil on it and everything worked fine.after a few starts that coil melted also.What should I check to make sure the coils quit melting.Thanks for the replys Dean

 

[Bruce (VA)]

What you have experienced is a costly lesson in the practical application of Ohm's Law.

Until you find out the resistance of the coil, you will be playing a very expensive guessing game regarding the value of the resistors you will need in the circuit to keep from melting the coil.

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 1.0 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 & probably ok. 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. So, to get it to 3.5 amps, you need another resistor in the circuit. Either that, or keep spare $30 coil around.

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