66 ignition ballast resistor

[Gary S.]

In trying to troubleshoot my problem in my post yesterday about engine stumbling on the highway, I measured the voltage from my ballast resistor to the coil. With the ignition switch on and the engine not running I get 12+ volts. I thought that the purpose of the ballast resistor in my car was to step the voltage down to 6-9 volts for delivery to the coil. I had always believed that this was to preserve point life. Is this normal? What should the voltage be to the coil?

Thanks, Gary 21316

[Jim T.]

Re: 66 ignition ballast resistor

If you just turn on your ignition and check it quickly and stop the check, the ballast resistor is working like it is designed to work. The voltage will drop the more time it is in use, or should drop as it was designed to.

[Ed Jennings]

Re: 66 ignition ballast resistor

Gary, Ballast resistors usually fail open. I sort of doubt that that is your problem. To answer your question, they are supposed to put out about 8 volts +/- a bit. Where are you checking the voltage? The resistor should read about 1.4 ohlms across it. You probably can't read this low a value accurately if you don't have a digital ohmmeter. My 67 327/350 had a similar problem a few months ago, and it turned out to be trash in the carb. Too lean a mixture can also cause backfires. What you have described is actually "afterfiring". Good luck.

[Wayne M.]

Re: 66 ignition ballast resistor

Gary --- Your distributor points were probably open when you put the voltmeter to the downstream side of the ballast resistor. There has to be current flow for the resistor to "resist". Try bumping the engine until the points are closed. You'll feel heating at the resistor. Then put the voltmeter to each side, and note the difference.

[Gary S.]

Re: 66 ignition ballast resistor

Wayne, thanks for the comment. I guess I will check the voltage coming from the ballast resistor while the engine is running. That should give me a good check. Thanks, Gary

[Gary S.]

Re: 66 ignition ballast resistor

I will check the resistance on the resistor using my digital mulitimeter, before I put in my neighbor's known good coil from his 73. I checked the voltage at the double pink wire coming off of the resistor going to the distributor and got around 12 volts at both areas. I will check it with the engine running just to make sure. I read in the GM manual that during start the voltage is 12 but drops to 6-8(?) during running. I am not certain that it is fuel related because it didn't bog down or mush out, it just quit cold for about 1/2 second with absolutely no life for that split second. I had a fuel pump fail on me and it only exhibited its failure during climbing turns or under real hard acceleration. I will check the output of the fuel pump and the fuel filter, though. Thanks Gary 21316

[Dale Pearman]

Re: 66 ignition ballast resistor

All you guys are dead wrong about the ballast resistor. It's a CURRENT regulating device and the voltage drop across it is dependent not only on the curent through it but also on the temperature of the resistive element. Heat is liberated into the surrounding atmosphere dependent on ambient and thermal eficiency. The most stressful condition for the resistor is 0 RPM with the points closed. The coil core is saturated and current at it's maximum. The least stressful is at high RPM when coil saturation does not occur and the AVERAGE curent is lower than saturation current. In this state the resistance is lowered, allowing a higher current to pass. The resistance value is all over the place depending on temperature. The 1.4 ohms is measured at 75 degrees F.

If you don't beleive me, set up a digital ohmmeter with disposable leads to the resistor and put a propane torch to the main body of the resistor. See what happens! The resistance will go UP UP UP!

What saves the points is the capacitor in the distributor. Current cannot be started or stopped instantaneously in an inductor. When the points open, this current has to go somewhere or it will jump across the points thereby burning them. Current through a capacitor CAN be changed instantaneously so this is where the current goes. (until the cap acquires a charge) Then the cap discharges through the inductor in a reverse direction. A decaying oscillation occurs with the time constant determined by the coil resistance.

The ballast resistor ain't got nuttin to do with saving points!

Varooom!

[motorman]

Re: 66 ignition ballast resistor

i can tell you from my trying it, if you leave out the resistor you burn up the points in a hurry. we used a bypass switch to cut out the resistor for drag racing. i later installed a dwell extender using a SCR to close the points electronically before they closed mechanically. this way you got total saturation of the coil at high rpm. this worked till solid state ignitions came out.

[Dale Pearman]

Ballastresistorless!

Of course! No ballast resistor-no current limiting. (except for the coil primary resistance) Now when the points open a much larger current must be passed by the capacitor which quickly becomes charged. The capacitor, coil, ballast and points are ALL carefully engineered to be compatable. If twice the energy is stored in the inductance of the coil due to more primary current, and the points are opened, a capacitor engineered to store only half this energy will do only that. The remaining energy is dissipated in the ioized gap between the points. The points rapidly fail. (you have a helluva spark however for a while). We used to add a dry cell in series with the battery to accomplish the same result, leaving the ballast in the circuit. Points don't last this way either.

Varooom!