? about Coils

Brad Gyde · Jun 21, 2010

Hey guys,

I've read many times odds and ends about coils.. and I know I have read there is a way to check to see if a external resistor is required or not.

could someone refresh me as to how?

Needed a coil for a combine, and yesterday while rummaging in the drawers I found one that appears to be new (still sealed in a plastic bag).. but has no markings on it.. and even being young, with a decent memory, I have no memory of buying it, or what I would have ever bought it for.. Maybe I bought it just to have it, or maybe it came in a box of stuff from a auction, or maybe it was given to me.. I have NO idea.. but before I fry it I figured I would ask. (no sense in ruining 20 bucks when I can ask a question free, and at worse look a little stupid)

I know it has something to do with resistance, if I remember right (and I can't seem to find the post I remember reading that in)

so if someone could refresh my mind and tell me what to do, that would be great..

Thanks in advance.

Brad

Bob · Jun 21, 2010

In round numbers, coils with up to about 2.5 Ohms primary resistance are 6 Volt coils (can be used on 12 Volts with the appropriate resistor).

Coils with a primary resistance more in the 3 to 3.5 Ohms range can be used on 12 Volts WITHOUT an external resistor.

An easy test is to measure the AMPERAGE draw of the ignition system, switch on, engine stopped, points closed. Generally, the current needs to be in the 3 - 4 Amp range. Much over 4 Amps shortens point life and makes the coil run HOT. If it's BELOW the 3 Amp range, spark is gonna be weak.

Brad Gyde · Jun 21, 2010

So for primary resistance would I check between the positive and negative terminals on the coil?

for the amperage, where would I want to check that? at the switch itself, or?

All the electrical "fun" is new to me.. I always been able to take some wire and make stuff work, but now I'm going more for reliable and less on the "hey it'll work for a minute"

Every day I get just a little older, and every day a breakdown is less fun.

Brad

Bob · Jun 21, 2010

"So for primary resistance would I check between the positive and negative terminals on the coil?"

Yes, with an ACCURATE Ohmmeter.

"for the amperage, where would I want to check that? at the switch itself, or?"

The primary amperage will be the same measured ANYWHERE in the primary circuit, between the ignition switch and the coil or between the coil and the breaker points.

Brad Gyde · Jun 21, 2010

Thanks Bob,

I'll do some checking and see what I come up with.

Brad

Bob · Jun 21, 2010

Just remember the Ohms values I posted fo 6 Volt coils or "true" 12 Volt coils are a "ballpark figure", kind of a generic average, and consumer-grade Ohmmeters aren't REAL accurate at such low values.

The Amp values are a good and more accurate way to verify what's going on.

MORE primary ignition Amps are acceptable for pulling tractors or hotrods where short-term performance outweighs point burning/life or coil heating, but the 3-4 Amp range is where you want to be at for working tractors/daily drivers.

? about Coils

Brad Gyde

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Bob

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Brad Gyde

Member

Bob

Well-known Member

Brad Gyde

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Bob

Well-known Member

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