resistor for ignition coil

I am converting from mag to automotive ignition coil. Coil I have says use external resistor. It doesn't say what ohms. I have a resistor which looks like 2 resistor in one ceramic block and no value on it. One side reads 1.3 ohms and other around 5. Then I found one I am familiar with and it says 75 ohm on it and I checked it and that is what my meter says. The 75 ohm one looks like what I always saw on firewall in 50's and 60's. However google doesn't say anything about a 75 ohm one, all much lower like about .75 to around 7 ohms. How can I tell what to use?

When I do the 6 volt to 12 volt thing I use O'reilly's part #VR-1 it is nice and small and easy to hide if you want and mounts in a way that help it keep cooler

In cases where you use a 6 volt coil on a 12 volt system, the external series ballast is close in ohms size to the coils primary (between its + and - terminals) that way it drops 6 volts leaving 6 across the 6 volt coil i.e. a 50/50 voltage divider.

Typical (not all) 6 volt coils have from say 1.25 to under 2 ohms primary resistance (a lot are 1.5 ohms) so you want a ballast near that (i.e. 1.5). Many (not all) typical ballast resistors are from 1.25 to 1.85 or so ohms NOT 5 AND CERTAINLY NOT 75 OHMS.

Some of those old Chryslers had a dual ballast, one for starting the other for running, so use the side thats around 1.2 to 1.8 ohms is my advice.

BEFORE ANYONE HAS A CALF these are typical values (Im talkin old tractors here mind you) not alllllll used them but regardless if you have a 12 volt tractor and use a 6 volt coil you want to drop 6 volts at the ballast and if its ohms are close to coils ohms YOU WILL DO THAT.

Many coils labeled "12 volts requires external ballast" are in reality more like 6 volt coils and they MUST have the ballast just like they say or they could overheat

John T

You didn"t say whether you had a 6v or 12v electrical system. if it"s 12v and if your coil says to use a resisitor than you ought to check the primary resistance in coil. It"s probably ~1.6 ohms. So you"ll need a resistor that"s no more that 1.6 ohms (hot) to limit the primary current to 4 amps max. The easier thing to do is to buy a IC14-SB coil from NAPA and you won"t need a resistor in the circuit.

If you have a 6V system and the coil is 1.6 ohms primary resisitance than you don"t need a resistor.

You didn't say if "automotive ignition" means points or electronic. If you're going electronic you may as well get a new high energy coil with an internal resistor.

Resistor is supposed to match the specific coil and how many cylinders the engine has.

The ballast resistors are thermally and load reactive and not a fixed resistance. They allow more spark energy when you need it, and less when you don't in order to preserve points-life.

Here are a few NAPA numbers that will work good enough for the coil mentioned.

ICR13 NAPA 1.8 ohm resistor(when cold-checked)

IC23 1.2 ohms (mostly used in Mopars)

ICR11 1.35 ohms rated, but usually reads 2.8 ohms when cold.

ICR34 - 1.4 ohms rated

Also, not from NAPA:

Hot Spark brand - HS14BR - 1.4 Ohm heavy-duty, external ceramic ballast resistor
30 watts. Typically reduces current to coil and points @ 4,000 RPM by about 0.5 amps or about 2.0 volts

Lucas 3BR- rated at 1.3 ohms

Accel ACC-150250 1.35 Ohms

Standard RU-4 1.35 Ohms

Standard RU-23 1.2 ohms

Standard RU-37 1.4 Ohms

If you have 12 volt battery for starting use Napa's coil with builtin ballast resistor P/N IC14SB for about $18.00. Hal

Toro,

STOP with the "old mechanic's wive's tale", already!

There's NO ballast resistor built into that coil.

It simply has more primary winding turns to be able to operate on 12 Volts WITHOUT the need for a power-wasting resistor.

I stopped at a parts house and explained about the 12 volt tractor was turning the points purple and pretty often...I needed a resistor. The parts guy looked in the book and came back with a box and threw it down and told me that it probably wouldn't work on a Massey Ferguson cause it was for a Toyoto and smiled. ohfred

That is just a coil with a fixed primary resistance of 3.25 ohms.

The IC14 at NAPA is $50. The $19 coil is a Chinese version, NAPA # MPEIC14SB

True enough, but why acuse a ballast resistor of being "power wasting?"

A high fixed-resistance coil with no ballast resistor can be truly power wasting when you're cranking on a cold engine that won't start at sub-zero temps.

A good ballast resistor matched to the correct coil gives you the best of both worlds when set up correctly. It allows full system voltage at cold cranking, and then tapers back to only what is needed for good running, to prolong the life of the ignition points.

Why do you suppose 95% of all USA cars and trucks 1950-1970s with gas engines and ignition points used the ballast resistor and coil system??

Ford began using a conventional 6-volt, externally resisted unit from 1949-'55 and
then converted to a 12-volt, externally resisted unit from 1956 to 1974.

General Motors vehicles used the same 6-volt non-resistor ignition coil from 1923-1954.
They converted to 12-volt in 1955 and the coils were 12-volt and externally resisted.
The same coil was used from 1955 right up until 1975.

Chrysler converted to 12-volt in 1956 and they used an externally resisted coil, which
was used right up until 1978, even in the electronic ignition-equipped vehicles. Point
distributors required a two-terminal resistor with 1.2 ohms of resistance, whereas
electronic ignition distributors used a four-terminal resistor with the 1.2 ohms primary and
a secondary circuit of .5 ohms resistance.

American Motors circa 1956-1959 cars used the GM externally resisted 12-volt coil. The 1960-'62 V-8 models used a Prestolite Distributor with a Chrysler externally resisted coil (and resistor), etc., etc. They continued to use the GM ignition coil with external resistor until 1974 in USA cars. Canadian were often different.

Actually, Chrysler used a ballast resistor which was bypassed when the starter was engaged. That's why some aftermarket starter solenoids have a spare terminal, for by-passing the resistor.