When I restored my engine compartment,I replaced my ballast resistor with a new one, with the blue stripe. A couple 100 miles later, my old coil failed. I bought a new Standard Brands coil and the car ran fine for about 200 miles,and now that coil has failed. The coil has written on it"use with a resistor." According to the catalogs,a 65 L79 is suppose to take a ballast resistor with the blue stripe,which is what I have. Seems rather suspicious that two coils failed within 500 miles after I installed that new resistor. Any ideas?

 

How much voltage is going to the coil coming from that resistor?

 

I tried to check that Mike,and the batteries in my multimeter are goners. Going to pick up some new ones in the morning. I have a feeling that the aftermarket resistor is defective. Do you know how much resistance I should measure across the resistor?

 

Here is some notes I have on ballast resistors:

The only two external ballast resistors ever used on Corvette were 1931385 (0.3 ohm Black Dot) and 1957154 (1.8 ohm Blue Stripe). All 1965 used 1957154

Not 100% sure of origin or validly.

George

 

Thanks Magic,I'll check that tomorrow when I get new batteries for my multimeter.

 

All V8 Corvette coils run off 12 volts. Period. (I'm guessing the Blue flame 6 cars were six volt systems.) The resistor is there to limit the current (not voltage) through the coil and obviously also the points. When in doubt about what coil/resistor combo will work on most engines, just measure the individual resistances, and shoot for about three ohms total. This will limit the peak coil primary current to around 4-4.5 amps, enough to fire the plugs without burning up the points. The coil will generally handle a bit more than that.

 

You should not be reading 12 volts on the + post of the coil with your volt meter. Should be somewhere around 10-10.5 volts with the points open. Less with the points closed.

 

a proper working ballast resister should drop VOLTAGE to a 5-7 range.easy to check but to do it right unhook the wire leading to the coil usually at the bottom of the resistor, and leave the wire from the ign. switch to the resistor intact. turn on the key and measure the difference from one post to the other. also check that the coil is properly grounded- the bracket surface should be on a clean surface-scrape any paint on the mount, and if equipped with a tightening screw-be sure the bracket and the coil are clean. also do not over tighten the screw where you are actually denting the coil body. also the wire leading to the points should be a resistor wire aka "pigtail" not a regular one...good luck

 

Not necessarily correct. With the points open, there should be zero current flowing through the coil primary circuit. With no current flow, there is no voltage drop across the ballast resistor (V= IxR) or any voltage dividing action between the ballast resistance and the coil resistance. The meter should read basically the same voltage in all points of the primary circuit, equal to the supply voltage (battery or fuse panel), assuming no current leakage due to connector or wiring shorts.

 

 

Guys, there appears to be some misunderstanding on how the ignition coil primary circuit works. The ballast resistor limits the current through the coil and points. It does NOT produce a regulated voltage for the coil. The coil primary windings run off of 12 volts (always), and the ballast resistance and coil primary winding resistance limit the charging (primary) current.
With the points open, there should be essentially 12 volts at all points in the primary circuit (coil + and -) and the points connection. When the points are closed, current flows, and there will be a voltage divider seen between the coil winding and the ballast resistance. The exact voltages seen are dependent upon the supply voltage (battery or alternator supplied), the coil winding resistance, and the ballast resistor resistance. Electrical engineering 101.

 

The coil doesn't need to be grounded - none of the internal components are in contact with the case; they're all electrically isolated from it, and there is no continuity from either of the three posts to the case. The only ground circuit in the coil is from the (-) terminal through the points to ground.

 

im not sure what you are saying my friend or if we are both saying the same thing. but a ballast resistor does indeed drop 12.4 volts to 5 to 7 volts. they are all calibrated that way to only provide 5 to 7 volts to the coil . this is right from a motors auto repair manual or a snap on tool diagnostic system. waay back in the late 60's . we used to always have to check the voltage on the br as a usual part of a tune up. ive always been of the opinion that this whole voltage drop system, was designed to prolong points life by dropping the voltage to in turn drop the current and save the points from excess heat, also the primary wire or "pigtail" wire is an integral part of the system. to get 7 to 8 thousand miles on a set of points back then was a challenge.

 

true..... my emphasis here was on the coil bracket should be clean especially between the coil and bracket where the clamping screw closes and the bracket mounting bolts are clean because sometimes they are used for grounding lugs......but no the coil does not have to be grounded to work...sorry.....

 

Contrary to what I said above, I agree.

 

I'm not about to contribute to the confusion by adding anything technical to this thread.

I've measured the voltage at the coil positive terminal while an old Corvette was cranking and idling with an original coil, alternator, regulator, and ballast resister. I've also measured the voltage at either end of a stock ballast resister while at idle.

Maybe you guys are just quoting something you read, or maybe you use a voltmeter very differently from me. But, for some reason, none of the numbers posted so far reflect my observations of measured voltage at the coil or ballast resister.

 

Basically, you are using a voltmeter (set on DC volts) to measure a rapidly changing voltage signal with a large AC component. You are not going to accurately see what's going on this way.

 

I respectfully disagree with your description above. I've tried to explain the theory of how the system works (and I also apologize for my poor performance explaining it), but there appears to be a few old shadetree myths about what's happening. Again, no ill will intended, I'm just trying to explain the physics of what's going on. Please bear with me as some of the descriptions are rather dry.
The purpose of the ballast resistor is to limit the current through the coil windings and the points. (Voltage is not an issue as the points and coil primary windings have to regularly deal with voltages of several hundreds of volts.) A resistor is not an intelligent electrical component, and has no way of producing any value of regulated electrical voltage. It merely is an obstruction/restriction to the current that the system voltage (12 volts) tries to force through the coil and points. The coil doesn't care what system voltage is attached to it, all it cares about is how much current (and the resulting power dissipation and heating) is going through it. For example, it's conceivable that you could put a toggle switch across the ballast resistor and short out the resistor entirely above 3000 RPM. This will remove any issues relating to the effect of the ballast, and the reduced dwell time at higher RPM (though same dwell angle) will keep the coil current within a safe margin. (It will also give you a bit more spark energy to the plug.) Additionally, I've been running my stock coil without a ballast resistor for the last 25 years, with great success and performance. I retrofitted HEI electronics into the distributor system, but I kept the stock coil. The coil survives quite fine without any ballast resistor "voltage reducing", as the HEI module allows full 12 volts across the coil while also limiting the current once the coil has charged up to about 5 to 5.5 amps.
Regarding the descriptions in some of the repair manuals, they're more geared to quick diagnostics, rather than accurate technical reasoning.
There's more about the system to describe yet, but I've got a scheduled appointment I have to leave for. I welcome any additional technical discussions with you, and I ask for your continuing patience in my descriptions here.
Thanks again.
Mike

 

The bottom end of the coil secondary is connected to the case. The secondary circuit is from the top contact through the distributor to the spark plug through the engine block ground and back to the coil. I believe the coil must be grounded to work.
CUL Jim

 

At the Wells Manufacturing website, GM part # 1957154 crosses over to their part #DR794 , which they claim has a resistance value of 1.6 to 1.9 ohms.

GM part # 1931385 crosses over to their part #AL795, which they say has a resistance of .5 to .6 ohms.