Ballast Resistors Explained...hopefully

[Dan D.]

Re: Ballast Resistors Explained...hopefully

Jerry, it sounds like your battery is low. The 11.4 you read should be the battery voltage, and up over 12 volts. if that was so, then your 6.4 volts would be 7 or 8 volts, which would be normal.

-Dan-

[Richard G.]

Re: Ballast Resistors Explained...hopefully

I was going through my parts stash today and found this: GM # 1957154 / Delco D1110
Since we have this thread going I will add some pictures of one.

[John S.]

Re: Ballast Resistors Explained...hopefully

Since GM eventually figured out that a higher resistance was better doesn't it make sense to use the later part numbers in my 55 instead of the 0.3 ohm part?

with the single point distributor in 55 it makes more sense to me to use the 1.8 ohm part and I do not think you'll ever have to worry about changing points. for rapid acceleration in a 55 you will never have any problem with the 1.8 ohm part. at 70 mph I am at 3000rpms and 80mph approximately 3500 rpms. a 55 will never have to worry about turning 7000 rpms. to get a accurate reading of ohms always use a digital meter and if bench testing tighten the screws to get a accurate reading. there are some ballast resistors that look like the chevy resistors and read 2.3 ohms, so it is always good to use the digital meter because of it's accuracy.

[Brad V.]

Re: Ballast Resistors Explained...hopefully

NPC 307 and 328 released the #1957154 (1.8 ohm) resistor on 1/3/64 for L-76 (365hp) and L-84 (375hp) engines, replacing the #1931385 (0.3 ohm) resistor, along with the #1115087 coil, replacing the #1115091. For the rest of the '64 model year, all Corvettes with point ignition used the same resistor (#1957154) and the same coil (#1115087).

Reading this thread as another poster noticed both the 1963/1964 Judging and Spec guides are incorrect. Are the NPC 307 and NPC 328 posted somewhere for reference or is the 1964 AIM notes of this change sufficient to update the guides?

1964 AIM, Section L76, Sheet 1, along the RH side of the page, under "The Following Parts Install the same as R.P.O. L75"
Revision Record Change Date: 1-13-64
Change 7: 115087 WAS 1115091
Change 8: 1957154 (1.8 ohm) WAS 1931385 (0.3 ohm)

[Richard M.]

Re: Ballast Resistors Explained...hopefully

Follow-up question-----On my 63 fuelie, the voltage reading on the ballast resistor post to the distributor reads 6.4 volts with just the key in the "on" position. The other ballast resistor post hooked to the pink wire coming out of the dashboard reads 11.4 volts, again with the key in the "on" position. Are these "normal" readings?

Thanks

Jerry

Jerry, it sounds like your battery is low. The 11.4 you read should be the battery voltage, and up over 12 volts. if that was so, then your 6.4 volts would be 7 or 8 volts, which would be normal.

-Dan-

Jerry, Dan's spot on, your battery is a bit low and needs a charge of may have a weak cell or one on it's way out.

At rest with IGN on you should be at around 12.6 Volts on a fully charged battery.

One more thing though, on a stock Ignition system your readings on the Ballast resistor output(the wire to Coil+) can vary if the ignition points in the distributor are OPEN or CLOSED.

If OPEN, you will likely see the same voltage on both Ballast terminals because there is NO Load on the resistor as the Coil- is disconnected from Ground.

If CLOSED, this now Grounds the Coil- and so there is now a current flow from the Ballast output to the Coil+, therefore a LOAD exists through the Ballast resistance and the voltage will drop across it and you'll see a lower voltage on that side, somewhere around 7 to 8 volts based on which Ballast is used.

Rich

[Dan D.]

Re: Ballast Resistors Explained...hopefully

Let me add one more thing to what Rich has said. With older, used ballast resistors the terminals on the back side of the resistor can become corroded and have high resistance. You need to look at the backside and make sure the screw terminals are clean and bright where they contact the resistance wire. There is the dissimilar metals problem in play here. I have seen this before.

Rich is right about whether the points are open or closed. If they are open there is no voltage drop across the resistor. If they are closed, then expect to see 7 or 8 volts.

-Dan-

[Gene M.]

Re: Ballast Resistors Explained...hopefully

I was going through my parts stash today and found this: GM # 1957154 / Delco D1110
Since we have this thread going I will add some pictures of one.

Your photo is a case where the NOS part does not represent the production line part.

[Jack H.]

Re: Ballast Resistors Explained...hopefully

On voltage readings, remember the voltage(s) you measure depend on whether the engine is started/running or not. With the engine running, the alternator should be putting out charge (bat voltage of approx 12 VDC will be pumped up to circa 14 VDC from charging depending on engine RPM).

If the engine isn't running, then what you measure will be relative to how the points came to rest when the engine stopped. If the point are in their open position, then current isn't flowing through the ignition primary circuit and there will be no IR voltage drop division between the ballast and the coil primary...