Over Heating Ballast Resistor on 57 250n hp Fulie

[David C.]

Help, What would cause a Ballast Resistor to run very hot??
Back ground--It has 12.3 volts going into it and approximately 9 volts and 4 amps after the resistor.
The Resistor measures 1.7 ohm resistance.
I have tried different Resistors and coils with the same result.
The points (2) and condenser are new as is all of the car wiring.
The Battery is also new from Restoration Battery.
The car seems to run fine but the Resistor does get hot enough to smoke!

Any ideas would be greatly appreciated.

Dave

[John W.]

Re: Over Heating Ballast Resistor on 57 250n hp Fulie

You are drawing way too much current on the distributor side of the ballast resistor. You may have a bad condenser or a short to ground maybe in your coil. 9 volts and 4 amps is 47.6 watts. P=Vsq. / R or 47.6=81 /1.7 Way more than that resistor was designed for. You battery voltage also seems low at 12.3 volts. Good battery fully charged should be around 13.6 volts. Shut the car off and check your battery voltage. Without the draw from the distributor it may go up. Since the ballast resistor is fed through the ignition switch it also may be getting hot along with your wiring harness.

[Jim L.]

Re: Over Heating Ballast Resistor on 57 250n hp Fulie

You are drawing way too much current on the distributor side of the ballast resistor. You may have a bad condenser or a short to ground maybe in your coil. 9 volts and 4 amps is 47.6 watts.

Your calculation is not right. There is only 3.3 Volts drop across the ballast resistor, based on the OP's measurement. If there really is 4 Amps passing thru the ballast (and I'm a little dubious), it's dissipating only 13.2 Watts. That's not horrible. But yes, it will get hot. It's supposed to.

About that current: If the ballast really does have 1.7 Ohms resistance, then for a 3.3 Volt drop, the current is more likely to be a little less than 2 Amps. On the other hand, if there really are 4 Amps flowing through 1.7 Ohms, then the Voltage drop across the ballast would be 6.8 Volts, not 3.3 Volts measured. So, you see, something doesn't add up quite right. My bet is that the actual current is less than 4 Amps.

Jim

[Duke W.]

Re: Over Heating Ballast Resistor on 57 250n hp Fulie

How are you measuring? Engine idling? Engine off, but ignition on and points closed? It makes a difference.

Steady state current may be four amps, but with the engine running it's not a DC circuit due to the points cycling.

Battery voltage should be about 12.6, ignition off, and 13.5 to 14.5 engine idling with minimum electrical load.

Your data is not meaningful unless you provide full context, and it's normal for the ballast wire to be too hot to touch.

Any oil or paint on the ballast may cause smoke, so make sure it's clean. The original ballast was 0.3 ohm.

Duke

[David C.]

Re: Over Heating Ballast Resistor on 57 250n hp Fulie

Duke,
The primary coil resistance is 1.2 ohm
The Ballast resistance is 1.7 ohm
The static (not running) voltage is 12.7
The running voltage is 13.3
The measured outside temperature of the ballast got up to about 150 F after 10 minutes at fast idle.
I agree the ballast runs hot maybe I do not have a problem but I do not remember it being that hot to the touch.

[Michael W.]

Re: Over Heating Ballast Resistor on 57 250n hp Fulie

150F while running sounds about right to me.

[Duke W.]

Re: Over Heating Ballast Resistor on 57 250n hp Fulie

I don't see a problem, either. The reason why the resistor is mounted in a ceramic block is because it gets hot. Also, when running at fast idle for 10 minutes it will probably get hotter than on the road since there is little air flow through the engine compartment.

BTW, the wire is a special alloy whose resistance increases with temperature, which limits current in extreme conditions to protect the points and keep the coil from overheating.

Duke