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12V 8N ignition resistor ?

Butch(OH)

Butch(OH)

Well-known Member
Hi all, been on this site a long time but first post on this forum. I have a 8N in the shop with front mounted distributer that owner converted to 12V and it needs a resistor in the ignition circuit. NAPA has three different ones with various ohms. Which one works best in this application? Thanks in advance!
 
(quoted from post at 05:51:48 02/24/17) Hi all, been on this site a long time but first post on this forum. I have a 8N in the shop with front mounted distributer that owner converted to 12V and it needs a resistor in the ignition circuit. NAPA has three different ones with various ohms. Which one works best in this application? Thanks in advance!
Click to expand...
hatever it takes to obtain 4 to 5 (max) amperes through the coil steady state (not running).
 
(quoted from post at 02:51:48 02/24/17) Hi all, been on this site a long time but first post on this forum. I have a 8N in the shop with front mounted distributer that owner converted to 12V and it needs a resistor in the ignition circuit. NAPA has three different ones with various ohms. Which one works best in this application? Thanks in advance!
Click to expand...

Obviously a resistor is cheaper than a new coil but a new coil is still pretty cheap. I'd put a 12v coil in that didn't need a resistor and then never have to worry about resistors again. NAPA P/N IC14SB is about 25 bucks and won't need a resistor in the circuit.
 
See tip # 30. Your best bet is a 12 volt coil. If that's not possible, then follow JMOR's advice. Measure the internal resistance of the coil, assume .3 ohms (cold, not running) for the required OEM ballast resistor and buy the additional resistor that gets you to the max current JMOR provided.

See why a 12 volt coil is the easier fix? ;)
75 Tips
 
OK I am not a Ford N guy. So,, there is a 12V coil available for the front mount I take it? In the tips it says something about the "OEM" resistor? that must be used even if I use a 12V coil? Tractor electrics have been messed with extensively prior to it hitting my shop. There is a NAPA ICR11 hung behind the dash that is burnt and open. Have seen that previously when the key was inadvertently left on. Guess I will get the meter out and do some checking. Thanks!
 
" that must be used even if I use a 12V coil? "
" there is a 12V coil available for the front mount I take it?"

Yes to both.

Check out the pic.

You can get the OEM ballast resistor and 12 volt coil from this site or nnalert's.
ballast_Bruce_zpsboeikaki.jpg

75 Tips
 
OK thanks for that!. The factory resistor has been bypassed- I saw it but thought it was for low high charge rate such as use with a 3 brush generator. Didn t spend any time following old wires.

Butch
 
(quoted from post at 09:26:27 02/24/17)
(reply to post at 09:36:32 02/24/17)
Click to expand...
Whatever it takes to obtain 4 to 5 (max) amperes through the coil steady state (not running)."

JMOR, Please explain more to some of us who are not a knowledgeable.
Thanks,
Click to expand...
et I=4.5amps, Let Rcoil=0.5 Ohms (measure yours). Assume 14v battery voltage when running at speed.
Rtotal=Rcoil + R resistor, then Rresistor=Rtotal-Rcoil

Rtotal=E/I=14/4.5=3.1 Ohms

Rresistor=Rtotal-Rcoil=3.1-0.5=2.6 Ohms

And if you use both the 12250 OEM resistor from a front mount plus another resistor, the the other resistor would be 2.6-0.6=2 Ohms. Note: I used 0.6 for the resistance of the 12250 OEM resistor at running temperature, as compared to Bruce's quoted 0.3 Ohms cold. Thus the greater current for cold start up, even with battery at 12.5v vs 14v.
Sorry you asked?
 
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