Fried points

[Michael K.]

What would cause points to get fried after a very short time (minutes) ?What should be checked? This is a dual point in a fuelie if that matters. Mike

[Bill I.]

Re: Fried points

Extreme voltage, check coil and ballast resistor. Bill.

[Ernie F.]

Re: Fried points

Maybe the ballast resistor!

GM used the resistors in such a manner as to drop the voltage to the points.

When your car is started the resistor will send 12 volts to the point set only to drop as the resistor heats up. If the ballast resistor is not working properly or bypassed, you will eat up points pretty quickly. .

[Richard S.]

Re: Fried points

i fried 3 sets of points before figuring out that coil was getting to hot. i was using correct .2 ohm black dot ballast with mallory coil then with generic chevy coil and it fried points after couple minutes. i then switched back to generic pep boys ballast and generic coil and it sloved problem. i do have a repo 091 coil on order and hoping it is matched correctly to .2ohm black dot ballast.but wont know until i try it out.good luck

[Bill C.]

Re: Fried points

Just tossing in my .02 here,

1) Ballast resistor limits CURRENT, not voltage

2) Excessive CURRENT is what causes points to heat up and fry, creates excessive arcing which creates excessive resistance which creates more heat and round we go......

3) Excessive CURRENT is what causes excessive heat build up and damages coils

4) Ohms law states as voltage increases, current (amps) drops, and vice-versa

Excessive voltage will make your lights brighter and cause other maladies but it will NOT create any heat, it will actually reduce it proportionally

Our systems only start with 12 volts, so developing a lot of heat do to high current draw is something that will happen real fast, ever feel how hot a battery cable gets after excessive cranking? Voltage does not do that. Thats why we doubled voltage systems back in 55 to reduce current draw by the same 50%

Good reasons to keep all wiring and connections in good shape, less resistance equals less heat!

[Ray C.]

Re: Fried points

I had the same problem on my 1961 270-HP with the dual point distributor. I was using a reproduction .03 black dot resistor. I now use a Chrysler after market resistor at about .06 ohm and have not had a problem with point burn since. It was also recommended that the condensesor be changed when you change the points.

Ray

[Jim L.]

Re: Fried points

4) Ohms law states as voltage increases, current (amps) drops, and vice-versa

This is incorrect.

Ohm's law, in equation form, is: I = V/R

In words, if you increase the Voltage in a circuit, the current flowing in the circuit will increase proportionately.

Excessive voltage will make your lights brighter and cause other maladies but it will NOT create any heat, it will actually reduce it proportionally

This is also incorrect.

Heat is power. Power dissipated in a circuit is calculated from: V^2/R

(That's the square of the applied Voltage divided by the circuit resistance.)

So, if you increase the Voltage in a circuit, the power, as heat, given off by the circuit will also increase.


Jim

[Duke W.]

Re: Fried points

I had the same problem on my 1961 270-HP with the dual point distributor. I was using a reproduction .03 black dot resistor. I now use a Chrysler after market resistor at about .06 ohm and have not had a problem with point burn since. It was also recommended that the condensesor be changed when you change the points.

Ray

It's 0.3 ohms. The P&A catalog lists it as .03, but that is an error that has been in the book for over 40 years.

It would help if the OP would post the coil mfg./part number, and use an ohmmeter to measure coil and ballast resistance. There may be a compatibility issue. Also an idle current reading with an inductive ammeter would be useful.

It was not uncommon for early Corvettes equipped with the 0.3 ohm ballast to burn points, which is why it was dropped from production by 1965.

In most cases, point life can be extended by using the conventional 1.8 ohm ballast to reduce primary current. It will reduce ignition energy, but in most cases this will not result in any negative effects other than a possible tendency to foul plugs more quickly, however, if the engine is in good condition and does not use excessive fuel or oil, plug heat range is equivalent to AC "5", and unleaded gasoline is used, and use is not limited to short trips, plug fouling should not be an issue.

Heat dissipated in a circuit is usually expressed as I-squared times R, but V-squared divided by R is also correct. This is why a ballast is used in 12 volt ignition circuits. Otherwise, the coil can overheat, and average primary current in excess of about 4 amps will tend to burn points.

The reason why the ballast resistor is mounted in a ceramic block is due to the high temperature that the resistance wire can attain, and coils and coil mountings must be designed to dissipate the current generated heat.

Duke

[Jack H.]

Re: Fried points

"Ballast resistor limits CURRENT, not voltage"

Not quite... It's a series circuit: battery to ballast resistor, ballast resistor to coil primary, coil primary to the distributor's points & condensor to ground.

When the points are open, there is no path to ground, no DC current flows and the potential at both the ballast resistor and coil primary are pegged to B+ potential.

When the points are closed, the path to ground is completed and current flows. Current flowing through the ballast will produce a voltage drop across the ballast (V=IR) dissipating heat in the ballast AND dropping the voltage presented to the downstream coil primary (B+ less the IR dissipation of the ballast).

So, the ballast resistor DOES reduce the voltage seen by other, downstream, components of the ignition...

[Jack H.]

Re: Fried points

Two other candidate explanations:

(1) Bad points/excessive spring pressure.

(2) Point contact surface(s) contaminated by stray oil/grease causing them to 'weld' together.

[Jim L.]

Re: Fried points

"Ballast resistor limits CURRENT, not voltage"

When the points are closed, the path to ground is completed and current flows. Current flowing through the ballast will produce a voltage drop across the ballast (V=IR) dissipating heat in the ballast AND dropping the voltage presented to the downstream coil primary (B+ less the IR dissipation of the ballast).

So, the ballast resistor DOES reduce the voltage seen by other, downstream, components of the ignition...

The purpose of the ballast resistor is to limit current flow through the coil primary and through the points, extending the life of both.

The fact that there is a Voltage drop across the ballast resistor is merely a by-product of current flowing through its resistance, not its main purpose.

And to clarify an earlier comment I made, power is expressed in units of Watts. The number of Watts given off by a circuit can be calculated in one of three ways, depending on what circuit parameter you know:

1. P = V^2/R
2 P= V * I
3 P = I^2*R

There is no typical or preferred way of doing this calculation. You choose the equation based on what you know about the circuit.

Jim

[Ray C.]

Re: Fried points

Thanks for the Correction!

.3 ohms from now on.

Ray

It's 0.3 ohms. The P&A catalog lists it as .03, but that is an error that has been in the book for over 40 years.

It would help if the OP would post the coil mfg./part number, and use an ohmmeter to measure coil and ballast resistance. There may be a compatibility issue. Also an idle current reading with an inductive ammeter would be useful.

It was not uncommon for early Corvettes equipped with the 0.3 ohm ballast to burn points, which is why it was dropped from production by 1965.

In most cases, point life can be extended by using the conventional 1.8 ohm ballast to reduce primary current. It will reduce ignition energy, but in most cases this will not result in any negative effects other than a possible tendency to foul plugs more quickly, however, if the engine is in good condition and does not use excessive fuel or oil, plug heat range is equivalent to AC "5", and unleaded gasoline is used, and use is not limited to short trips, plug fouling should not be an issue.

Heat dissipated in a circuit is usually expressed as I-squared times R, but V-squared divided by R is also correct. This is why a ballast is used in 12 volt ignition circuits. Otherwise, the coil can overheat, and average primary current in excess of about 4 amps will tend to burn points.

The reason why the ballast resistor is mounted in a ceramic block is due to the high temperature that the resistance wire can attain, and coils and coil mountings must be designed to dissipate the current generated heat.

Duke

[Jack H.]

Re: Fried points

"The purpose of the ballast resistor is to limit current flow through the coil primary and through the points, extending the life of both."

My interjection was intended to 'clarify' the statement Bill Caldwell made. He said the ballast resistor reduced current and not voltage.

Every coin has two sides and that's akin to saying it's the heads side of the coin that's important to a coin flip with the tails side having nothing to do with the outcome. UNTRUE! It takes both sides, evenly weighted, for the outcome of a coin flip to be correct (truly random).

Electrically, you have two ways to view energy (static-voltage, and kinetic-current), but energy is energy and around the closed loop of a passive circuit energy is conserved.

The purpose of the ballast resistor is to increase the effective life of ignition components downstream of the ballast resistor by reducing the energy they're asked to dissipate.

From a static energy viewpoint, the energy the points dissipate is V**2/R. That equates to the voltage impressed across them (squared) divided by their intrinsic contact resistance.

From a kinetic energy viewpoint, the energy the points dissipate is I**2xR. That equates to the current flowing through them (squared) times their intrinsic contact resistance.

BOTH are true! You can't reduce the current flowing through a series circuit component in a passive circuit without ALSO dropping the effective voltage that's impressed across the part. They happen hand in hand, silmutaneously.

If they didn't you'd violate the universal law of conservation of energy. So, when one guy says the ballast resistor reduces the voltage the downstream ignition components see, he's RIGHT.

And, at the same time, when another guys says, "No, the current flow through ignition components downstream of the ballast resistor is lowered." He's right too!

From an electrical engineering point of view (I are one), it's INSANE to argue that one side of the equation (say voltage aspect) is true while the other side of the equation (ergo, the current aspect) is false! They are BOTH true...

[Jack H.]

Re: Fried points

Actually, the drawing for ballast resistor 1931385 calls for an allowable range of 0.3 to 0.38 ohms...

[Duke W.]

Re: Fried points

The late Dale Pearman described the ballast as an "analog computer", and he was right.

The resistance wire in the ballast has an increasing resistance with temperature function, so the hotter it gets the lower the current in the primary ignition circuit, which reduces the amount of I-squared-R heat generated by the coil, which protects the coil from thermal runaway.

The 0.3 ohm is the nominal room temperature resistance. If you measure it on a hot day after a few minutes of idling it should be higher, and the resistor wire will likely be hot enough to burn you if you touch it.

I believe the only coils used with the 0.3 ohm ballast were the 107 and 091. I don't recall if they have higher primary resistance than other coils used with the 1.8 ohm ballast, but they may be designed to tolerate higher operating temperature.

Duke