My 74 L48 gets really bad gas mileage. I replace the points setup with a Pertronic electronic and when I measure voltage between the coil terminals with the key on, I'll I get is 5-6 volts.

Am I missing something? What should the voltage be with the key on?

ochsy

 

I'll confess some unfamiliarity on the technical details of the Pertronics unit, so I'll give you the basics of what should be there in a properly designed system.

With the key on, and the engine not running, you should be see zero volts across the coil. The only way you should be seeing any voltage across the coil is if there is current passing through it. This could be caused by a leaky switching transistor in the module (not a good thing), or the module is not sophisticated enough to know that you are at zero RPM (not a good thing either), and continues to drain current through the coil. This is analogous to the points being closed. That's fine for a crude system like points, but not a good selling point for an electronic system. In this case you will see a voltage divider effect where you'll see 5 or 6 volts dropped across the ballast resistance, and the remaining battery voltage will be dropped across the coil. (There will be a small voltage drop in the module when it's conducting, but a properly built part should not cause a significant voltage drop.)
What happens when you bump the engine to a different (distributor)position? Do the readings change?
Also, unless you've got an oscilloscope, don't bother trying to measure the coil voltage while it's running. You'll just get a garbage reading.

 

Voltage from positive coil terminal to engine block should be aound 9.5-10VDC. The Pertronix Ignitor has to have a good ground from the points plate to the engine block for good performance. There is a grounding wire on the underside of the points plate to the distributor housing and on many cars it is broken, the Pertronix unit may not perform well if this wire is bad. The best test to see if it is bad is to run a ground wire through the points window of the Dist Cap and ground the points plate to the engine block from there. Hope this helps. You can go to a full 12vdc to the + side of the coil with an Ignitor II set-up but no the std Ignitor unit. Hope this helps.

 

Both setups must, and do, withstand 12 volts. That's the system (battery/alternator) voltage in the car. This is also minor compared to the couple hundred volts that the module experiences regularly at the end of each dwell period.

 

Interesting. As I recall, my Mallory Unilite system required that I install a ballast resistor to bring the voltage down to approx 6 volts from 12V. Been ~10 years since I put it in, so I'll have to dig up the manual to check the reason.

 

69427, I am wondering if you are missing the point of my post. I am not saying something that will lead the poster into a "wild goose chase" yes there is considerable voltage at the Ignitor trigger/point contact set (thats why the condensor is on the points set to prevent their arcing and buring when this voltage is present.) The high voltage spike is only present for a VERY SHORT period of time (mili or microseconds) that would not be able to be measured by quite a few of the voltmeters that most enthuisiasts posses (NOR would it be much help in diagnosing the problem he is trying to find) when the secondary current flow is interuppted (this causes a magnetic feild collapse in the ignition coil and creates the secondary ignition voltage to fire the spark plug in it's HELLISH enviroment.) Enough with the Electrical Engineering BS lets get back to the basic problem and how to do some basic "shadetree" troubleshooting to fix this poor guy's problem: the voltage at the positive terminal of the coil to ground (NOT across the coil terminals as the poster is asking as different coil types will exhibit different voltage drops due to winding configurations, coil ratios, core construction and other factors) SHOULD READ between 9.5-10 Volts DC if the built-in factory wiring harness resistor wire is in place and in good shape. The Ignitor unit (or a points-set) STILL has to ground the negative side of the coil to get the current flow through the ignition coil to build up enough magnetic FLUX to get a dececent seconday voltage output from the ignition coil. There is a grounding wire below the points plate in the distributor (that grounds the plate to the distributor housing) and IT IS QUITE COMMON to find this wire BROKEN due to age which, then causes the secondary current to flow through the distributor rotating parts to get to ground CAUSING a significantly REDUCED current flow RESULTING in a SIGNIFICANTLY REDUCED secondary voltage output from the ignition coil. Like I said, try running a ground wire from the points plate through the distributor points adjusting window to the engine block and see if your ignition performance improves. If performance improves, you can either leave the "Semi Bubba" fix in place or remove your distributor and tear it down to replace the points plate grounding wire (wire connects from the underside of the points plate to a terminal by the Vacuum advance module in most Delco distributors.) I think what I have posted is MUCH more help the the average Corvette enthusiast than what you have given in your post if they want to use my basic advice for troubleshooting the circuit. You could also go out and buy a used Allen Test Systems secondary ignition scope for several hudred dollars and learn how to read ignition scope patterns to diagnose the same problem with the aid of a oscilloscope which is a practice that was quite common back in the 60's throught the 80's before America was DUMMED DOWN by our STUPID politicans and school systems (my local school district dosen't offer Auto Shop anymore but, they do have Auto Body Repair training curriculum so everyone can learn to slather Bondo on a car and sand it down to primer it.) I hope my postings help people who really work on their Vettes and are looking for some good basic common sense solutions to their problems and not just trying to flout their air of superiorority

 

I'm perplexed by your "really work on their Vettes" comment. I build my own engines (and run them hard), rebuilt my (5spd) trans and rear ends, suspension work, including the trailing arm spindle bearings, the glass work and paint on the car, redid the interior, and am currently putting a C4 suspension on my spare '69 frame. I think I qualify as a hands on Corvette guy. Oh, and I've also converted several distributors to electronic ignition.
I am however, not comfortable when inaccuarcies or old wive's tales are repeated and repeated. Thankfully, no one still strives for "road hugging weight", or talks about putting a "3/4" or "full race" cam in their car anymore. We've progressed past that in the automotive world. (Even in spite of the dumbing down, which I am in full agreement with you.)
In my initial thread I made the comments to the OP on a very basic level. I even asked for more diagnostic feedback. Nothing I mentiond should have been over his head.
I am troubled by the perpetuation of the myth that the ballast resistor somehow regulates the coil supply voltage. It does not. It is a current limiting device. Very different function. I even specifically mentioned that he should measure things at zero RPM. At zero RPM there should be zero current through the coil and module. He should see 12 volts at the + terminal. Not complicated, and very easy to accurately measure to see if the module is faulty and leaking current. Using a DC voltmeter when the engine is running tells you very little about a signal that varies by several volts in the span of a few milliseconds, nor about the condition of the parts. Too many variables going on.
Regarding an "air of superiority", I disagree with that label. I'm just a regular car guy. I am a stickler for accuracy, though.

 

"At zero RPM there will be no current flow in the system"

You are showing your true knowledge/ability in troubleshooting electrial circuits and arguing with the wrong guy here (I make a six figure salary doing just that: TROUBLESHOOTING ELECTRONICS and software engineering SCREW-UPS!) Also if the distributor is in a points closed or breakerless module enabled position THERE WILL BE CURRENT FLOW! in the ignition system EVEN AT ZERO RPM. I'll do you a favor and let you in on something that may save you some headaches in the future: if you leave the ignition key in the run position on long enough when this is happening you will overheat your ignition coil and BLOW the top of of it or do enough internal damage that the coil will be highly degraded in secondary voltage output so, do yourself a favor and keep the key off on your Vette while working on it. Why you would think otherwise shows your fundimental misunderstanding of the basic Kettering designed ignition system on our Vettes. I would hate to see you troubleshoot a K-66 TI amplifier system on a Vette.
Here is another really neat trick you can do with a systen at "ZERO RPM" when your firing off a new motor (since you said your a hands on guy like I am and I don't want you to flatten any more flat tappet cams by cranking and cranking and CRANKING you new motor), set your initial timing where you want it with the #1 cylinder on the firing stroke (I hope I don't have to go into "Otto cycle" engine theory with a smart guy like you) and turn your ignition key to the run position and rotate that distributor housing until you see a spark jump off that coil wire when it is held 1/4" off of a engine block surface (better yet for a guy like you, hold the coil wire in one hand and touch the engine block with the other hand until you feel the spark or get KILLED by a high output MSD ignition or other CDI type ignition unit.) For all you kids at home, don't try what I just described (holding the coil wire with one hand and touching the engine block with the other) as MOST modern day ignition systems use LEATHAL currnet levels and CAN KILL YOU OR CAUSE PERMANENT BODILY HARM to you

 

Your last comments went past sarcastic, to just plain rude, to put it very politely. I had expected a bit more cerebral content from a six figure guy.