Ballast resistor query

[Rebel]

Hi all,

One of my heater hoses going into the firewall recently split soaking the ballast resistor in antifreeze / coolant. After replacing the hose and starting it again, the ballast resistor started smoking. It looked like this when I took it off

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BB4F7329-B3AB-4B00-ABDD-E028B2DDF65F.jpeg (169.85 KiB) Viewed 949 times

Fitted a spare that I have which is slightly different in design and looks like this

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The original resistor has 1.12 ohms stamped on it, the spare is 0.8 ohms. The metal plate on the spare one gets too hot to touch after a couple minutes, I have no idea whether the original got hot or not as never had cause to fiddle with it before now.
I know that when it reduces the voltage it’ll generate heat, the original one was all ceramic though. The new one has a metal bracket to hang it by, this is the part that gets very hot.
Is the new 0.8 ohm resistor the wrong size ?
Will the metal bracket heating up scorch / damage the paint on the firewall or worse ?
Do I just need one that’s all ceramic like the original one ?
Or
Am I just being a pussy, and should fit the spare one and forget about it

I have electronic ignition, the orange box style

TIA

[Dave999]

The 1.2 ohm one or bigger is usually used with a points set up and complimentary points coil
the 0.8 one is often used with orange or chrome box (and appropriate coil)

yes they get hot...less so when using the car body as a heat sink, i.e bolted down but they get hot

resistor and coil are usually a complimentary pairing, with the resistance of both combined, set to limit the current that the points or mopar electronic box has to switch off and on.

the 0.8 one would usually be used with one of mopars' performance focused igntion boxes and the coil that came with it.

so in theory if your coil was purchased to go with the ornage box 0.8 ohm should be fine

a the moment you will be getting more spark power than you previously had, but a hotter coil especially at low rpm when the dwell is long

but i am assuming you had the wrong ballast for the coil before


mopar boxes were designed for specific jobs
for example a chrome box with a 0.25 ohm ballast is supposed to be good to 10,000 rpm in a drag race situation i.e 10 -12 seconds and not much idling
if you fitted it to your car and drove down the A1 for 5 hours that box or coil will suffer and one will fail well in advance of its intended life span

if you used the 0.8 ballast with it you basically turned the more expensive chrome box set up into an orange box equivelent
bigger balast = lower current = leass heat in coil and box and a smaller rpm range
Basically more lilkley to survive none race use in a street car

ornage box is a nice compromise between performance and street

should be ok


Dave

[Rebel]

Thanks Dave,
As usual, an informative and educational reply.

My car was originally using a points coil set up when I bought it with the 1.12 ohm ballast resistor. When it was changed to electronic ignition, I don’t think the ballast was changed as well ( as far as I remember ), must admit I didn’t know the resistors came in different sizes though.

[Blue]

Yes the ballast gets hot but if it’s doing it’s job the coil will remain cool. The ballast was there to protect the points, the stock coil is only designed to run constantly at about 7 volts and the ballast drops the voltage to that. I don’t understand why Chrysler didn’t just do away with the ballast and fit a 12v coil when ignitions went over to electronic.

[Rebel]

Thanks Blue

[Dave999]

Think the whole staying with the ballast when electronic came about was due to power handling capability of the power transistor
and the fact that on a 8 cylinder a 12 volt 3 ohm primary coil probably would have caused some rpm range issues that would not be seen in the standard 12 volt coil application namely 2, 3,4 cylinder engines
will be something to do with switching time, frequency, the impedence change caused as frequency increases, limiting useful RPM range

buzzing a coil off and on at some frenqucy will casue it to erm...... push back.... and that is linked to the coil, its windings, the magnetic field and the resistance of primary and secondary

avoiding a coil configuration with characteristics that get in the way of what you need to do in respect to rpm is key to the design.

4 cylinder needs half the sparks of a v8, they can run a 12 volt 3 ohm primary resistance, v8 not so much ... 6000 rpm on the 4 cylinder is 12,000 rpm on the v8 as far as the coil is concerned i.e in motor cycle territory..

coil packs or coil on plug solved this issue....

Dave

[Blue]

The majority of aftermarket electronic ignition’s don’t use ballast resistors. I guess it was all a bit new fangled when Chrysler started to use it in the early ‘70s or more likely it was just cheaper to stay with a ballast…