The Stovebolt Site Forums The Shop Area The Electrical Bay Ballast Resistor

What exactly does a ballast resistor do? On my 3100 with a 235 it wouldn't start. Changed out the ballast resistor and it still wouldn't. I then bypassed it and it started fine. Thoughts? Thanks

Are you using a 6 volt or 12 volt battery?

George

I've never seen a ballast resistor on a 6 volt system, so I am assuming you have a 12 volt system.

In a 12 volt system a ballast resistor reduces the amount of voltage (and therefore current) in the primary winding of the coil while the points are closed. This reduces the heat in the coil and the current handled by the points, making them last longer.

It also reduces the amount of voltage available from the coil to fire the spark plugs. That isn't a big problem once the engine is running, the generator (or alternator) is charging and the battery is fully charged. BUT (big but) when the battery voltage is lower while the starter is engaged (especially in the winter cold) the ballast resistor reduces the voltage to a lower value, reducing the spark even more. To get around this problem a "bypass" circuit provides battery voltage directly to the coil primary, which results in a better spark while trying to start the engine.

The common way to bypass the resistor is with a separate "start" position on the ignition switch or the 4th terminal on a starter solenoid if you have one. On older vehicles (you don't give the year of your 3100, Tricordati) with a "stomp starter" you can replace the switch on the starter with one that has an extra terminal on the side and attach that terminal to the coil primary connector.

It's a 1949 with a 1958 235 12 volt. To get it started I just bypassed the ballast resistor but seems to run rough.

Your starter solenoid should look like the picture below. The terminal at the top has the positive battery cable connected. The "S" terminal on the right has a wire connecting to the start terminal on the back of your ignition switch. The terminal on the bottom connects directly to the starter motor. The "R" terminal on the left should be connected to the + terminal on the coil. This will supply the "bypass" necessary to get 12 volts to the coil while the engine is being started.

Just curious: Your coil isn't marked

12 v
No resistor needed

is it?

Some coils have a ballast resistor already attached to them...like the one below. If yours looks like that you won't need the resistor.

No my coil doesn't have a resistor attached.

Thanks I will check when I'm back at it this week.
If I just leave it bypassed, what's the harm?

The harm is the points will go bad in less time. The coil will run hotter, which may have an effect on its life. The upside is you will have a stronger spark until something fails.

You cannot leave it bypassed. Your points will fry in short time. That may be why it is running rough now.

George

And today condensers are horrible quality...just pathetic junk. Some fail right out of the box and others wait until the engine heats up to operating temperature. Still others wait to fail until you're a few miles from home. That's why I came up with the alternative condenser a few years ago.

Jon what's the "alternate condenser"?

So could my problem be the condenser? How would that affect the ballast resistor?

You have dirty or burned points. Measure the voltage at the coil terminal that goes to the distributor with the ignition switch in the "run" position, engine stopped, and points closed. If it's more than about 1/4 volt (250 millivolts) clean or replace the points. Bypassing the resistor is allowing enough current to flow through the burned points to make the engine run. That won't last for long.
Jerry

Please take a look at the sticky at the top of this section. You'll see what the whole project has been all about. Basically it is a condenser designed to last for something over 200,000 hours.