Ballast Resistors Explained...hopefully

[Troy P.]

Re: Ballast Resistors Explained...hopefully

Good answer. Sounds very sensitive.

Reminds me of the right hand rule and pnp transistors...all things they tried to teach me in my Naval electrical engineering class 40 years ago.

[Dan D.]

Re: Ballast Resistors Explained...hopefully

Hi Jack,
To compensate for meter lead resistance you can short the meter leads together, note the resistance, and substract it from the reading obtained with the ballast resistor.
Some of these new cheap DVMs are amazingly accurate. I bought a $4 DVM recently from Harbor Freight, just to have in the garage so I dont have to go down stairs to the workshop every time I wanted to measure something.
I measured several 'well used' ballast resistor resistors recently. They have been laying around the shop for the last 30 or 40 years. I measured 1.9, 1.9, 2.1 and 0.5 respectively. So the first 3 must be the 1.8 ohm resistors and the 4th one a real old 0.3 ohm resistor. I obtained these readings using the HF meter, lead resistance subtracted out of course. I then measured them on my good Fluke 8012A meter (low 2 ohm scale). The readings were amazingly close.
I'm sure GM changed to the 1.8 ohm resistor to increase point life, as you say. But the trade off is spark voltage at high RPM. I think it is related to the article in last summers Restorer magazine on 'Run the Red Line, where .050 additional rotor gap limited high end response. For FI engines the coil changes from the 091 to the 107. I believe this was done to increase high RPM spark voltage. At 7,000 RPM the coil charge time is only 70uSec. No where near enough time to develop full spark voltage.
Jack, I have been reading all your posts with great interest. You are an amazing hard drive! I hope I get the opportunty to meet you in person some day. Best regards -Dan-

[Troy P.]

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?

[Dan D.]

Re: Ballast Resistors Explained...hopefully

To increase point life - yes. The 1.8 will be better.
To turn 7000 RPMs - No. The 0.3 will be better.
For originality - The 1.8 has to be made to look like the original. There have been many posts recently concerning the bracket tooling and even the ceramic details. Additionally, replacement 1.8s do not look exactly like original 1.8s. Seeing how every GM car built had a ballast resistor for many years running, tooling degraded over time, probably was re-tooled several times, and there may have been more than 1 supplier. -Dan-

[Michael J.]

Re: Ballast Resistors Explained...hopefully

.
At the time of cranking (when the starter is operating), a separate wire gives the points the full 12 volts, bypassing the ballast resistor. When you release the key from "start" to "on", all the power to the points now has to flow thru the ballast resistor, preserving the points. If the BR fails, it fails "open" causing the engine to shut down....meaning no juice gets to the points"

Interesting info, as I have been told before by people here that leaving the key on "On" would burn the points. Turns out that is wrong.

[Duke W.]

Re: Ballast Resistors Explained...hopefully

I think that can happen, especially if the points are old and already in marginal condition.

Duke

[Troy P.]

Re: Ballast Resistors Explained...hopefully

Very informative. Thanks for the insight.

For a car that doesn't get driven much I'd rather have the rpms than the point life. Probably should change the resistor on my 63 Fuelie too. I suppose its a "point life" version. Seems to me it has a black dot or a stripe on it.

[Jim T.]

Re: Ballast Resistors Explained...hopefully

Based on the information of burning the points with the key left in the on position and the engine not running and the points closed, I would not leave my ignition on with the engine not running in my 69 or 70. I have heard stories/rumors of the points burning and actually sticking together when one would leave the key in the on position with the engine not running for a long enough period of time. If the points are open, there is no physical contact of the points with the car's ignition on. Having resistor wire from the fuse block to the coil, I question how hot the resistor wire would get? This wire is wrapped in the wiring harness and getting to hot could perhaps create a problem.

[Dan D.]

Re: Ballast Resistors Explained...hopefully

If you leave the key on with the engine off you will have constant current flow through the ballast resistor and points, providid the points are closed when the engine stopped. It is also 100% duty cycle. When the engine is running the points are closed only 33% of the time. So do not leave the key on for very long. You can burn the points and the ballast resistor as well. PLus you can drain the battery. -Dan-

[Scott S.]

Re: Ballast Resistors Explained...hopefully

You can follow the usage trail through the Assembly Manual and NPC's (Notice of Production Change) - it goes like this:

'56 - Early '63 all used the #1931385 (0.3 ohm) resistor; carbureted cars used the #1115091 coil.

TSB #DR-577 (2/27/63) described the February 4, 1963 production change at St. Louis to the new #1957154 (1.8 ohm) resistor on all 250hp and 300hp engines, " to eliminate point burning during cold-weather operation". The #1115091 coil continued on all points applications. It also noted that the Service parts would be identified with a black dot (for the old 0.3 ohm resistor) and with a blue stripe (for the new 1.8 ohm resistor).

The #1957154 (1.8 ohm) resistor continued to be released for 250hp and 300hp engines for 1964 start of production, with a new coil (#1115087).

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).

For '65 (and through 1967), all Corvettes with point ignition used the 1.8 ohm resistor, but the #1115087 coil was replaced by the #1115202 coil.

John,

I noticed this in the 1964 AIM today, Section L76, Sheet 1 along the RH side of the page (under "The Following Parts Install the same as R.P.O. L75"), and the Revision Record gives the change date of 1-3-64, saying the Ignition Coil WAS 1115091.

If I understand correctly from the TDB archives, the 1115087 Ignition Coil is much easier to find these days than the 1115091 Ignition Coil. If the 1115087 Ignition Coil was used for L76 from approximately January 3rd on (estimated VIN 8192 according to the Corvette Birthday Book), then both the 1963-1964 Judging Guide and the NCRS 1953-1967 Corvette Specifications Guide are in error. Both show only the 1115091 Ignition Coil for 365hp & 375hp, and 1115087 exclusively for 250hp & 300hp.

[Wayne W.]

Re: Ballast Resistors Explained...hopefully

Actually when the car is running it acts as a current limiter and not a voltage reducer. Its sort of involved in theory and I will not try to explain it, but running a volt meter at the resistor output will not read 6 volts but closer to 12 on a 12V system. Voltage doesn't burn points, current does. Surely at a steady state, with the engine not running and the points closed, there is a voltage drop, but thats not the way it is while running.

[Wayne W.]

Re: Ballast Resistors Explained...hopefully

This is a pretty good explanation by Mike (69427) over on the other forum.

A resistor limits current. It does not set up a regulated voltage for a coil. The coil sees (system voltage) 12v at the start of every dwell period. Every time. It does not see 6 volts or 8 volts or any other number except 12. An ignition coil is not a resistance. It is an inductance (which means V=IR doesn't apply), with a resistive component. The primary current waveform is predominately controlled by the coil's inductance, and the "voltage" one measures is just an unavoidable side effect of the need to have a series resistance to limit the peak current in crude mechanical switching circuits. And as such, measuring anything on an operating coil with a voltmeter yields nonsense. It's a continuously varying current and voltage waveform, and to get anything resembling a true accurate measurement requires an oscilloscope. Period. The only time a volt/ohmmeter should be used anywhere near a coil is when the engine is not running. This will allow you to remove the effects of inductance from the equation. This will tell you what the resistances are in the circuit, and if the points are open or closed, but little else.

[John H.]

Re: Ballast Resistors Explained...hopefully

John,

I noticed this in the 1964 AIM today, Section L76, Sheet 1 along the RH side of the page (under "The Following Parts Install the same as R.P.O. L75"), and the Revision Record gives the change date of 1-3-64, saying the Ignition Coil WAS 1115091.

If I understand correctly from the TDB archives, the 1115087 Ignition Coil is much easier to find these days than the 1115091 Ignition Coil. If the 1115087 Ignition Coil was used for L76 from approximately January 3rd on (estimated VIN 8192 according to the Corvette Birthday Book), then both the 1963-1964 Judging Guide and the NCRS 1953-1967 Corvette Specifications Guide are in error. Both show only the 1115091 Ignition Coil for 365hp & 375hp, and 1115087 exclusively for 250hp & 300hp.

Scott -

That's correct - they don't show the breakpoints in usage.

[Domenic T.]

Re: Ballast Resistors Explained...hopefully

Troy,

All explained very well by the guy but one thing that that was not really said (I think) was that the biggest draw on the battery is at start when the starter is cranking.

The total voltage drops during start and if it weren't for the wire on the solonoid giving 12V to the coil during the cranking, the start would take a while as the 6V to the coil would be reduced during cranking causing a hard start.

A good clue that the coil didn't get the 12V boost was if the engine started as the starter was released.

DOM

[Gerald C.]

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