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Re: 1948 8n-- no spark

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08-22-2006 05:27:52

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Phil.. 3 ohm is a standard primary value for a 12v coil. You can buy off-the-shelf round can coils likethis.. the NAPA IC14SB

The square coils that are labeled as 12v are about 2.5 ohm coils.. thus the need for the 1/2 ohm dropping ressitor.

The oem ballest resistor is for use with the 6v square coil. If you run the 6v square coil on 12v, you need the oem ballast resistor, and a dropping resistor like the 8ne10306.

Do some online research.. It's easy to find graphs showing coil ohm values and the resistors used to make them work. They all fall at 2.8-3.2 ohms... Putting 3ohms in the middle.. etc.


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Phil (NJ,Az,Sask)

08-22-2006 14:07:03

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 Re: 1948 8n-- no spark in reply to souNdguy, 08-22-2006 05:27:52  
Soundguy, [quote] Phil.. 3 ohm is a standard primary value for a 12v coil. You can buy off-the-shelf round can coils like this.. the NAPA IC14SB[/quote]

The IC14sb is a low RPM coil used on old Jeep & Checker Motors, 4 & 6 cyl engines. 3 ohms is not a normal automotive 12 coil. The hi-performance coils will be 1 ohm or less.

The round coil is oil filled and robust but the front mount is not, although some believe that little elves are responsible in redistributing melted tar back to the top of the coil or that some other unknown phenomena will overcome gravity. The physical properties of overheated Copper are well known, brittle, discolored & resistive (in spots) This is responsible for most household fires when a new receptacle is installed, because of a loose overheated connection, without re-stripping the old wiring to good copper. (oxidation is not the same as discoloration)

The OEM 6 volt Front Mount normal 6 volt thermal dissipation is 9 watts, with Ballast. If the ballast is bypassed watts = 49 and this results in over heat damage.

A coil with 2.5 ohms & .5 resistor will need to dissipate a MAX of 57.6 watts (4.8 X 4.8)x 2.5

A coil with 2.5 ohms & OEM Ballast will only need to dissipate a Max of 23 watts (3 X 3)X 2.5

If the tractor "starts" & runs with the OEM ballast you have a ~ 3:1 thermal advantage.


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uh . . . Dell (WA)

08-22-2006 18:08:02

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 Re: 1948 8n-- no spark in reply to Phil (NJ,Az,Sask), 08-22-2006 14:07:03  
Yeah Phil.........3:1 thermal advantage and WEAK SPARKIES. You know better.

You know the "infamous ballast resistor" is nominally about 1.4 ohms at operating temperature. (it ranges from about 0.4 when really cold to about 1.7 when really hot) and when I went to school, 2.5 + 1.4 = 3.9. This means a reduced current flow in the coil primary winding and resultant WEAKER magnitic field to induce the high voltage secondary. Its all about electo-magnetic theory. Its AMPS that make magnetic fields.

Interesting that you claim the NAPA IC14-SB is a low rpm coil for 4-cyl Jeeps and 6-cyl Checker cabs (Continental Motors flathead six?, I can't remember). Does that mean that our recommended useage as a real 12-volt coil that uses NO RESISTOR on 2000-rpm 4-cyl N-Engines is outta specs?........respectfully, Dell

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Phil (NJ, AZ,Sask)

08-23-2006 01:14:48

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 Re: 1948 8n-- no spark in reply to uh . . . Dell (WA), 08-22-2006 18:08:02  
Dell, Your getting closer in your math but you mistake low current with the right (limited) current. The ~4.0 ohm (coil + Ballast) results in 3 Amps circuit current starting & 3+ running at 14.5 volts. The 3 Amps will blow your socks off.

The original 12 coil was closer to 4.0 ohms and your .5 ohm resistor was a good solution, an oem ballast doesn't work, but since they changed the coil to 2.5 ohms PPL are burning out coils. The only way that a coil will overheat is if the current is TOO high and that is a design fault, Wrong Components.

The choice is actually simple. If you are wrong the coil melts resulting in $$ damage. If I am wrong the tractor won't start & run but “no” damage is done and you can change it.

RE: 3 ohm round coils.. If the coil can dissipate the heat, and it can, why would anyone disagree with using that coil? My comment was to "Sounder's" 3.0 ohm comparing of the front mount coils ability to dissipate heat, of 58 watts, to that of a "proven" round coil that dissipates 70 watts. (Worst Case).


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08-23-2006 05:53:41

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 Re: 1948 8n-- no spark in reply to Phil (NJ, AZ,Sask), 08-23-2006 01:14:48  
Phil.. A point... I didn't say that 3 ohms was a standard automotive coil.. to be fair.. i did say it was a standard primary resistance value.

I'm trying to locate my GM balalst resistor and coil cross reference chart. All the coils and ballast resistor combo's on that chart range from a total primary resistance value of 2.8 - 3.2 ohms.. with the vast majority of them setting right at 3 ohms.

Are you taking into account duty cycle?

Also.. your remark about jeeps?.. I recall these engines being evaluated for a military jeep evaluation.

As for the thermal advantage.. at what point do you start sacrificing 'reliable' starts and running in adverse conditions.. like lowered voltage due to worn and dirty connections.. etc.

For reference.. that last 12v square coil I gutted was potted solid to the top.. looked like epoxy potting. Note.. this was a plastic encased coil.. not a metal can encased one...


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08-23-2006 05:01:09

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 Re: 1948 8n-- no spark in reply to Phil (NJ, AZ,Sask), 08-23-2006 01:14:48  
Put the math to the proof then.. How come there are so many 12v front mount couils and half ohm resistors running around perfectly fine?


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