The Stovebolt Site Forums The Shop Area The Electrical Bay Ignition Coil Resistor?

Since I have converted to 12V on my 54 truck, I need to add an external resistor before the coil. classicparts has them for 17-24 but can't tell me what resistance they provide (I'd like to go to NAPA as it would be MUCH cheaper). Any suggestions for a resistance or a part number? Thanks in advance

alla same, just get one from NAPA for a 56-60 Chevy truck, or you could get an internally resisted 12V coil Napa #IC14 or Echlin ECH IC21

Bill

so the Ohms of resistance don't matter?

all the same, there is no variation, they're designed to cut the voltage from 12 to about 9, no idea what the specific ohm rating is and I doubt any vendor knows without actually measuring one

Bill

I deleted my earlier response, agreeing with Bill; however, after I went to NAPA:

NAPA Echlin ICR11 (1.35 Ohms)
NAPA Echlin ICR13 (Approx. 1.8 ohm)
NAPA Echlin ICR23 (1.2 Ohms)
NAPA Echlin ICR34 (1.4 Ohms)

And a few others, with a variety of resistance values.

yea, I saw the different Ohm ratings, and I just didn't know if it made a difference as to which I chose!

yes, other makes use ignition resistors, even into the 80s
"For a list of specific cars that this part is compatible with, please visit our Buyer's Guide."
there will not be a variety of possibilities for an old Chevy truck, like I said, ask for one for a 56-59 Chevy truck, it'll be what works

Bill

NAPA 1956 Chevrolet truck - 1.82 ohms

Yep, thats the one I got after work. I found it after the first time Bill told me what to look for.

I was later just wondering about the different Ohms, learning things here.

Thanks Tim and Bill

This is how the resistance affects current.

Current is what you are trying to keep low enough to save the points and the coil.

Volts = Resistance X current. This is Ohms law.
So just change it to represent Current instead of Volts, and you get...Volts (divided by) resistance = Current Just some math to make you nuts..:) Now on to the part that matters.

1, 6 volts (divided by) 1.2 ohms (the smallest resistor in the list from (tcleaderman thank you Sir)=5 amps.

Still with me?

2, 6 volts (divided by) 1.8 ohms (the biggest listed) = 3.33 Amps.

All other values will be in this range.
This is the current that goes to the coil and the points. And is what you would get if the coil was those resistance values. Any way it is a "limit" for the points. Because that was what they were designed for. So when we change to 12 Volts we need to get the current down to these "limits" to prevent overheating the coil and the points. So to guess, and it is a guess, that the coil is 2 Ohms, to 1.5 Ohms. Let us try to get the current in the same range with 12 Volts.

1. 12 Volts (divided by) 1.2 Ohms (resistor) + 1.5 Ohms (coil)= Current. 12 Volts (divided by) 2.7 Ohms ( just added the resistor to the coil)= 4.44 Amps

Yes I cheated a bit but that is OK. The worst real world is the smallest resistor and the smallest coil.

2. 12 Volts (divided by) 2 Ohms + 1.8 Ohms = Current. As before, 12 Volts (divided by) 3.8 Ohms (again I just added the points and the coil) = 3.6 Amps. So the worst case is 4.44 Amps and the best case is 3.6 and the "limit" is...5 Amps to 3.33 Amps and surprise...we are within those "limits" The values are just not that critical. After all we just want the Truck to run for a long time. Hiperformance guys have different goals in mind. I hope this is helpful.