Ballast Resistors Explained...hopefully

[Troy P.]

I would appreciate it if an ignition systems expert could explain the function of and details on ballast resistors. Some of the questions I have are:

1. What do they do?
2. Why don't the 6 cylinder/6 volt cars have them?
3. Why are there so many different types - plain, marked with a dot, marked with a stripe, etc.? Many part numbers.
4. Are all the Delco resistors without any ancillary marking the same?
5. If not, how can you tell if a resistor in your possession is the right one for your car since they don't have part numbers marked on them?
6. I assume you can test the resistance but where can you find info on the resistance each application needs...265, 283, 327, high and low HP, etc.?

Thanks in advance for your consideration and education.

[Mike M.]

Re: Ballast Resistors Explained...hopefully

I would appreciate it if an ignition systems expert could explain the function of and details on ballast resistors. Some of the questions I have are:

1. What do they do?
2. Why don't the 6 cylinder/6 volt cars have them?
3. Why are there so many different types - plain, marked with a dot, marked with a stripe, etc.? Many part numbers.
4. Are all the Delco resistors without any ancillary marking the same?
5. If not, how can you tell if a resistor in your possession is the right one for your car since they don't have part numbers marked on them?
6. I assume you can test the resistance but where can you find info on the resistance each application needs...265, 283, 327, high and low HP, etc.?

Thanks in advance for your consideration and education.

1) they drop the voltage at the points to 6 volts once the ignition switch comes off the crank mode. during cranking, 12 volts to points. if the points had 12 v to them during the run phase, you'd be replacing points every 5K miles or so. 2)no need to drop to 6 volts on the 6 bangers. their entire electrical system is 6 volts. 3)many configurations/markings because they were used by GM from 1955 thru about 1974. different suppliers of the resistor's bracketry would help explain the different bracket configurations. there was no cut-out under where the wires attach to the resistor from 1955 thru 1957 and probably beyond. by the midyears, there was a cut-out under the wire attaching part of the porcelain block.4) no. 5) test their resistance. 6) don't knopw answer to that one. good luck,mike

[Jerry W.]

Re: Ballast Resistors Explained...hopefully

Had same questions....i found this in archives

When the engine is running, you can get by with a lot less current at the points. That's the purpose of the ballast resistor. If the points were to receive full battery current/voltage at all times, they would only last a few hundred miles before needing replacement.
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"

[Duke W.]

Re: Ballast Resistors Explained...hopefully

Go into the archives and search for posts on this subject by the late Dale Pearman. He described it as an "analog computer", and I think he was correct.

IIRC Dale had an engineering degree from MIT.

One of the ballast's functions is to keep the primary circuit/points/coil from overheating, and it does this by using wire that has increasing resistance with increasing temperature.

The 0.3 ohm ballast that was used on some engines through '64 provided more spark energy, but had a reputation for burning points. I was never OE after than - replaced by the 1.8 ohm ballast. These are the approx. resistances at room temperature.

Duke

[Bill I.]

Re: Ballast Resistors Explained...hopefully

Then and in that event, would a jumper between the 2 screws allow you to at least get it running? Thanks, Bill

[Troy P.]

Re: Ballast Resistors Explained...hopefully

Great info guys. I will look up Dale's posts.

So in the meantime is it correct to say there were only two different resistances used and I should be able to test them with an ohmeter to see which they are?

[Bill M.]

Re: Ballast Resistors Explained...hopefully

Then and in that event, would a jumper between the 2 screws allow you to at least get it running? Thanks, Bill

Just move the "run" wire from its side of the resistor to the other; no jumper required.

My '59 was unhappy with the full 12 volts, but it ran.

[Mike M.]

Re: Ballast Resistors Explained...hopefully

Then and in that event, would a jumper between the 2 screws allow you to at least get it running? Thanks, Bill

i keep a jumper wire made up out in the shop. it comes in handy when the spark plugs get wet with gasoline as in flooding. often times the 12 volts to the points will generate enough voltage to fire the flooded engine without removing and cleaning the plugs. we also used the jumper trick back in the 60's for 1/4 mile runs. unsure if it lowered the et's all that much. mike

[Troy P.]

Re: Ballast Resistors Explained...hopefully

I'm guessing that cars started out as 6V and someone decided that's the voltage points should run at for optimum life. So when they switched to 12V systems they kept the 6V compatible points rather than re-engineer them to be more durable under 12V?

Military vehicles are 24V...well, at least they were in my day. So do you suppose they had/have special ballast resistors to knock 24V down to 6V?

[John H.]

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.

K-66 Transistor Ignition-equipped Corvettes didn't use a ballast resistor; the T.I.harness had a built-in length of special resistance wire instead.

In '68, the ballast resistor disappeared, and was replaced by a length of woven fabric-covered special resistance wire from the fuse block to the coil that served the same function until 1975, when the HEI ignition system was introduced, powered by a full 12-volt feed circuit.

[Troy P.]

Re: Ballast Resistors Explained...hopefully

Great info there!!!

Now since I have a 55, do you happen to have the data on that one too? Or can you surmise an answer from your vast understanding of the issue?

[John H.]

Re: Ballast Resistors Explained...hopefully

Great info there!!!

Now since I have a 55, do you happen to have the data on that one too? Or can you surmise an answer from your vast understanding of the issue?

Troy -

I'm not familiar with those details on the '55; Roy Braatz should have that information.

[Roy B.]

Re: Ballast Resistors Explained...hopefully

OK this is it 55 (0.3 ohm)

[Troy P.]

Re: Ballast Resistors Explained...hopefully

OK, what confuses me is the 55 is 0.3 ohms, per Roy, as is the 56-63, per John. But the part number on Roy's box differs from the part number John provided. I'm guessing they look exactly the same too. If so, what could the difference be?

One thing I was trying to figure out is if the NOS Delco resistor from my 56 Cadillac Eldorado Biarritz, which is not in a box and doesn't have a part number on it, is correct for my 55 Vette. Sounds like they should have the same resistance, which I can test for.

[Jack H.]

Re: Ballast Resistors Explained...hopefully

One thing I was trying to figure out is if the NOS Delco resistor from my 56 Cadillac Eldorado Biarritz, which is not in a box and doesn't have a part number on it, is correct for my 55 Vette. Sounds like they should have the same resistance, which I can test for.

Measuring a resistance as low as 0.3 ohms can be tricky! You're at (or beyond) the precision of most common multi-meters here. Plus, the intrinsic resistance of your meter probe wires can 'swamp' the accuracy of your measurement...

Another way to get there with accuracy, is to connect the ballast to a load (say, several tail lights tied together) and run power up the circuit. Measure the voltage dropped across the ballast and then measure the current flowing through the ballast.

The V= IR equation rearranges to R= V/I and you can compute the resistance of the ballast pretty accurately...