I've got a '66 with a 70's style Delco electronic distributor (why I posted in the C3 forum). It started burning out control modules (the 4-pin boomerang-shaped unit under the rotor). The modules are only good for a few starts before they burn out. Has anyone encountered this before? It must be something in the electrical system, but maybe someone can give me an idea where to look. Thanks!!

 

Check for proper voltage going in to the dist. also make sure there is dilectric grease under the moduel and check the wires at the magnetic pick-up coil,sometimes they will break at the point where they go into the pickup.

 

Ttt

 

Thanks for the suggestions. Voltage is OK and I used dielectric grease. Wires appear to be OK (not loose) but I can't see where the wires enter the pickup because they enter underneath it and I'm reluctant to pull the pickup because I may not get it all back together correctly.

 

What is the voltage at the dist.?

 

The voltage question is important because on a points system, there is a resistance required in the power circuit which drops the voltage from battery level down to about 9V. This is normally done using a piece of resistive wire (You'd never know it's there because it looks like regular wire except it usually has a cloth covering on it) but if that wire has been replaced with standard wire you may find a ballast resistor in the circuit. Cars designed to run HEI or other electronic systems don't have this 'special' wire in the circuit. Check the voltage at the distributor end and ensure you have full battery voltage.

 

100%

 

Something odd is going on here. The four terminal modules are usually very durable. Several things come to mind, only one might be the cause.

1) Are you sure of the integrity of the pickup coil wires? These are usually white and green, I believe. Over the years the continued flexing of these wires due to the action of the vacuum advance will cause the wires to break. This will cause a no-start condition if one of the wires open-circuits. Sometimes disconnecting the vacuum advance hose will allow a restart due to the "breaker plate" not rotating and opening up the wires again.
2) Related to #1, if the pickup wires are intermittent they may, and I say may, cause the module to turn the coil on and off several times during the intermittent connection. This causes extra heat in the output transistor, possibly causing damage.
3) Is the grease you applied to the back of the module a heat conducting grease? Some have mistakenly applied a heat barrier grease to the module, allowing it to overheat.
4) Is there any possibility that there is a pathway from the high voltage rotor arc to the white/green pickup leads, or to the C- lead? These leads are designed for a few hundred volts, not continuous hits from much more than that.
5) Do the "failed" modules work on another vehicle? Are you certain that it is a failed module, rather than an intermittent connection that gets temporarily fixed when you plug in the new module?
6) I assume you have a clean 12v source for the module. Regardless, these modules are designed to start the car down to around five volts, so I wouldn't expect any issues if the voltage to the module isn't a perfect 12 volts, as long as it's not getting hit regularly with leakage from the high voltage wires.

There's a couple other items that come to mind, but they seem so unlikely I won't wear out my typing finger mentioning them. Keep us updated on your investigation here. Good luck!

 

Sorry, but that's incorrect. The points system runs off of 12v, not anything else. It is a decades old myth that it runs off of nine volts. (Several previous threads explained this.) The ballast resistor or resistance wire limits the current passing through the coil and points to a safe level, it does not transform the system voltage (12-13 volts) into any other voltage. An oscilloscope reading of the ignition system voltages will verify this.
Not trying to be difficult here, just trying to clear up a misconception.

 

Interesting thought. I know a lot of folks (me included) have always believed this. I also recalled a document I saw while looking into retro-fitting my HEI. It is in the Crane Cams web site and talks about how to determine if you have a ballast resistor. It talks about checking for approximately 8 volts at the coil + when the coil - is shorted to ground (another article in an Olds 442 forum FAQ section I read talks about taking the measurement with the points closed which I believe is essentially the same thing). In both instances, they clearly state that if the voltage is around 8V, you have a resistance (ballast resistor or resistive wire). If it's 12V, you do not. Here is the link to the Crane Cams site. The info is in the darkened box on page 1. http://www.cranecams.com/pdf/90001700a.pdf. I did a forum search and found this info from Accel which also seems to point to a voltage drop with ballast resistance http://forums.corvetteforum.com/show....php?t=1433513

Then I read this in an issue of Chevy Performance on performance HEI installations
"One of the misunderstood aspects of points ignition systems is that the ballast resistor (or resistor wire as used in most GM points-type ignitions) just limits voltage. While voltage is reduced, the resistor’s main function is to reduce current, since points cannot handle more than about 2 amps of primary current without drastically reducing operating life."

Now I also saw a couple of your posts on distributors and clearly you seem to be very knowledgeable on the subject too so at this point, I'm just curious about how we are seeing so many commercial sources telling folks to look for a voltage drop. I now believe that it is because most people have access to a volt meter and can measure that. Not many folks know how to/have the equipment to measure the current on the circuit. Now, I'm going to apply my very limited understanding of Ohms law which in very general terms says that voltage, current, resistance and power (watts) are all inter-related. It now seems as if both sides of the argument are technically correct, but that the reason for the ballast resistor is not the voltage drop, but rather the current limiting. If you place the resistance in front of the load, you affect the current delivery capacty, and the resistor's consumption of power drops the voltage to the levels measured at the + side of the coil. Does this make more sense????

Sorry for the length of this, but I know based on the various debates I read when I searched for threads, there are quite a few people who want to know. Plus, we were asking Webfoot about the voltage at his distributor based on what we believed to be valid science, backed up with corporate instructions.

 

I'm going to add a little (bad?) science to the froth...

The old style ignitions were designed to provide full voltage to the coil at startup, and reduced voltage during run-time.

This is because when an engine & combustion chambers et al are cold, it takes more energy to ignite the fuel.

Okay.... so that's the supposed myth.

Here's the science...

Ohms law in series circuits...

E = I * R, or
Volts = Amps (current) * Ohms (resistance)

Yes... I know this is not a purely DC series resistive circuit due to inductance... but the rule still applies when the primary coil is saturated.

At start, the ignition switch provides voltage to the coil via a wire that goes back to the coil from the starter. This is also the same "potential" (i.e.- voltage) as the battery, since the connection point at the starter (solenoid) is full battery voltage.

So, full battery voltage is applied to the coil when the ignition switch is in the "start" position.

When the ignition switch is relaxed (springs back) to the run position, the starter solenoid de-energizes, and the full 12 volts from the starter to the coil is discontinued. At this point, the ignition switch routes 12 volts to the ballast resistor, which then (in series) goes to the coil.

Hence, there is now a resistor in series with the coils primary winding.

In order for the engine to run, the current must flow through both the resistor and the coils primary winding.

Last time I checked, when current flows through a resistor, there is a voltage drop across it. This voltage drop is small, because the resistance of the ballast resistor is small (a couple ohms). But... it is still a voltage drop.

This means during run, the coil will get a lower voltage than when it did without the resistor in the circuit (start).

Hence, the 12V vs 9V difference.
In the run position, the current through the resistor & coil primary is lowered due to the increase in total circuit resistance. So yes, the resistor does act as a "current limiter", but it also drops the voltage down across the coils primary winding.... because some "potential" or voltage was lost across the resistor.

You cannot insert a resistor into a series circuit and not have a voltage drop across it, unless the resistance is zero ohms... in which case, it would cease to be a resistor.

Now... you cannot measure this unless there is current flow in the circuit. So if the points are open, there is no current flow, and any point between the battery and the points terminal will measure 12 volts... because current is -0- and resistance is infinity.

So... do we keep ohms law, or have we found a new cosmic relationship among voltage, current and resistance?

Waiting to be educated...

 

68vertible and Tom454:

Gentlemen,

I see light bulbs turning on above our heads. It's always a pleasure to converse with individuals who look at things logically and with good scientific principles.
There were a lot of items covered in your responses, but I'll try to respond to a couple that come to mind at the moment.
Because ignition systems involve an inductance (the coil), rather than a pure resistance we need to use additional formulas other than E=I*R. We also need to use V=LdI/dt, which is just a fancy way of describing that it takes time (t) for the coil (L) to charge up. While it's charging up, the current (I) through it is changing (increasing). This means that the individual voltages across the coil and any ballast resistance is continually changing (unless the RPM is slow enough for the coil to finally fully charge). Distilled down, it means that you can't measure what's happening in a dynamic (inductive) circuit with a hand held voltmeter. The meter will just average out the voltage it sees, and will give you a mistaken impression of what's going on. (I used to live in the midwest. It would be zero degrees in the winter, and 100 degrees in the summer. A slow reading thermometer would say it was 50 degrees year round.)
The ignition system/starter relationship is mainly due to limitations in battery capability. When cranking, particularly in the winter, the starter will pull so much current out of the battery that the system voltage would drop to 6-8 volts. With the ballast resistor in the circuit there would be insufficient current entering the coil to "charge" it up sufficiently to make a decent spark. If the battery could supply great deals of current without the attending voltage drop there would be no need to have the starter shunt circuit. The ignition system would always have 12 volts available to power it.
Regarding why some technical sources continue to use the nine volt supply claim, I can only speculate. It might be a lack of in-depth knowledge of ignition systems, or perhaps they just got worn down from explaining things, and it was easier to just promote the "old wive's tale". Who knows? I'm an engineer. I prefer the scientific description of what's happening.
Thanks again for your questions and comments. They were intelligent and courteous.

Mike

 

Maybe someone covered this but if my memory serves me the reason for a ballast resistor is to keep you from toasting your points if you are setting in the drive in with your sweetheart,and you want to listen to the radio and you turn the key to the run position rather than the acc. position.
Too Simple???

 

Simple, but also partly correct. If the points happen to be open while you're at the movie there is nothing to worry about. If they're closed then there will be some power dissipation and heating if the points are slightly resistive. This could cause some pitting, but I'll bet the main source of pitting is the slight arcing that occurs when the points open at the end of the dwell period.

 

Your still at it !!!

 

I'm not an EE, but I too am an engineer and a full member of SAE; most of my clients are vehicle manufacturers and their suppliers. I too have faith in & rely upon Ohm's Law. As a sideline I've built, owned or maintained circle track cars. I've also wired entire Busch cars from brake blowers to ignition to oil heaters. I have a few bits of specialty equipment including a Superflow 110 bench & a SUN distributor machine; setup & sell a few distributors too.

If tomorrow's time permits ... and if I remember to ... I'll try to verify both ignition supply and battery voltages on a bone stock survivor MY1971 C3 having original breaker-point ignition system. I'll use a typical VOM; I don't need a 'scope. Subject C3's points & condenser are not OE ... but its coil and all primary wiring are OE. Once tests complete, I'll post results along w/ a comment; in this thread. I encourage others to perform same tests & to post their results here.

 

Before you guys get into with 69427 you might want to check out this thread http://forums.corvetteforum.com/show...ighlight=69427 (go to page 1)****He is really smart and knows what he is talking about and in my opinion "too" technical for the purposes of this forum.We are working on Corvettes on this forum and I believe we should go by Chevrolet's "language" and discriptions so we can comunicate with each other without confusion.Especially the people that are "asking for help" because they are confused.69427 is probably right about a resistor not reducing voltage it only "resists" it.But cant we just keep it simple and go by what GM says for the purposes of this Forum? Mike- I really did learn from our last encounter and the research I did to "try" to prove you wrong but for the sake of these guys trying to fix their vette can we just go by what GM says the resistor does?

 

"The ignition system/starter relationship is mainly due to limitations in battery capability. When cranking, particularly in the winter, the starter will pull so much current out of the battery that the system voltage would drop to 6-8 volts. With the ballast resistor in the circuit there would be insufficient current entering the coil to "charge" it up sufficiently to make a decent spark. " - 69427


So the engineers designed the coil to produce a healthy spark when cranking (i.e. battery 6-8 volts) and used the resistor to reduce the average current and hence average voltage at the coil +terminal when the motor is running and the battery voltage is more like 12-14volts. This ensures the coil does not overheat as the average voltage at the +terminal is more like 9 volts.

PS: I have a stock 69 427 with original points distributor and have measured all of this with a multimeter and oscilloscope.

Joe

 

Let me re-phrase this:
What is the voltage at the dist.?

 

ignition supply and battery voltages on a bone stock survivor MY1971 C3 having original breaker-point ignition system. Subject C3's points & condenser are not OE ... but its coil and all primary wiring are OE. Resistance wire to coil + is black with pink nomenclature. Testing w/typical VOM. Nothing was disconnected. Nothing was jumpered.
------------------------------------------------------

OE Points Ignition: key on but motor not running
Coil + = 7.3 vdc
Battery = 12.7 vdc

So ... with key "ON" but motor not running ... the ignition is supplied with about 7-7.5 vdc to coil + because the resistance wire passes about 55-60% of battery voltage.
-----------------------------------------------------

OE Points Ignition: key "ON" & motor running & alternator charging
Coil + = 12.2 vdc
Battery = 14.4 vdc

And ... with motor running-alt charging ... the ignition is supplied with about 8-8.5 vdc to coil + because the resistance wire passes about 55-60% of battery voltage.
-----------------------------------------------------

Also true, (when running) the coil primary voltage rises & falls as coil charges-discharges. This rise-fall ("average") can be measured w/ a 'scope ... but cannot be accurately measured with a VOM (multimeter). AFAIK, most car enthusiasts do not have ready access to a 'scope ... but most either have or can easily get their hands on an inexpensive multimeter. Except in the rarest instance, I suggest a 'scope is not necessary to verify C3 proper primary ignition supply voltage.