I have an issue that's stumping me. I recently was diagnosing a dead miss in my 390 T-Bird (rebuilt brake booster leaking, by the way...the vacuum port is right on the #3 runner. Nice work, Ford), and at the same time I noticed that my coil lead was burned right in half at the distributor cap and there were burn marks just off to the side of every distributor cap terminal. The plug wires showed no signs of burning. When I bought the car three years ago, it burned points quickly, so I checked the coil primary voltage, and it was 12V with the engine running. I figured somebody bypassed the resistor wire, so I installed a Pertronix Ignitor I, and eventually, a Flamethrower coil. I know a lot of people have a hard time with Ignitors, but I've always had good luck. Here's my issue (aside from the burned ignition components). Now that I've noticed all of this, I dug a little more into my primary voltage. What's weird is that with the key on/engine off, the voltage is 6 or 7 volts. When the engine is running, it's 12 or a little more. The resistor seems to be hooked up, but just in case, I got a new resistor wire and hooked it up temporarily. That one had 10 volts engine off, 11-12 volts engine on. I have to admit, I'm not really sure why there's the difference with the engine off and engine running. I'm putting the car away for the winter in a few days, so I'll deal with the booster and the wiring when I get it out again, but I figured I'd run it by you all, and if you have any idea why I might have burned those components, I'd appreciate that too! Thanks! A
First I'm assuming that you are measuring the voltage at the coil + terminal. With the key on and the points closed, you should see about 6 -8 volts like you said. That's because you have a closed circuit from power to ground. If you bump the engine a bit to open the points, the voltage will go to 12 volts because you have an open circuit. Think about this; with the engine running you have a circuit that is closed part of the time and open part of the time. Your system voltage is probably between 13 and 15 volts. Seeing 12 volts at coil + terminal is normal with the engine running. It's sort of an average between the points open and points closed voltage.
Yep, I'm measuring at the coil positive. Your explanation makes perfect sense...thanks! It also might explain why I burned the coil wire and cap terminals - the Flamethrower coil apparently isn't for use with a resistor. The only thing I'm wondering now is why it burned points when I got it. Who knows, maybe I had a bad condenser or something.
Actually, at the risk of sounding like an idiot, why does the coil + see system voltage when the points are open instead of zero voltage? I've had a hard time wrapping my head around that.
Think of it as water in a pipe. Voltage is the pressure, you'll still have pressure with no flow through the pipe. Amperage would be the equivalent to amount of flow through the pipe.
Because you have an open circuit and no current is flowing. If you check the - side of the coil it will also read battery voltage. In fact the insulated terminal on the points will also read battery voltage. In order for the voltage to drop in a circuit, you have to have current flow. Close the points and you will see a difference in voltage between the + and - terminals of the coil. Resistors only work when current flows.
I run a 1.5 Flamethrower and Ignitor I in my Y-Block. You can use a resistor if you want, but there's something else going on there. What you really want to sort out in any ignition system is the primary circuit ohms resistance, the voltage will kinda take care of itself when that's straight. Pertronix makes several coils and it's important to use the right one, the overall ohms of the coil & wiring needs to be matched for performance and reliability. More resistance will tend to limit the voltage, but it also reduces current. A ballast resistor is generally "OK" to use with their. 1.5 ohm coil but it tends to limit the spark intensity. It will produce a good hot spark at the plugs running straight battery voltage. On the other hand the point style distributors, the cap and rotor especially were designed for older ignition coils that tended to limit the available output on the secondary side to around 20k volts. If there are any defects in the ignition components a high output ignition will find them. Ford used the same distributor rotor from '57 to '73 but somewhere in there changed to a "smog" version, I had a lot of trouble with arcing or kind of cross in high humidity and this took a while for me to figure out. The part # didn't change but the part did, they are shorter. The rotor/terminal gap has a similar effect as spark plug gap, increasing the firing voltage of the coil. I also was using "roll yer own" plug wires of the spiral wound type, it didn't like this combination of parts. I found a "long" pre-smog distributor rotor and installed carbon core plug wires and it settled down nice. Igniton is weird stuff, it all has to work together just right. Change one part, and it can get horked real bad.
You have that right! I've switched back to a coil designed for a resistor, kept the Ignitor (I carry a spare, so I threw that in, as I was working through one thing at a time yesterday), and I changed the rotor, wires, and plugs. I also did cold/hot compression tests before stumbling upon the leaking booster...It was a busy day in the garage yesterday. We'll see how it all works out after a few miles down the road. I just used a Blue Streak rotor on it, but I'll have to look for a NOS one. I have an OT 289-powered car that seems to use the same cap and rotor, and I haven't had any of these problems with that car, so like you said...one problem (whatever that is) can really muck things up.
I noticed a change in idle just from changing the coil wire and nothing else messing around once when troubleshooting and messing with a scope. I can't say one was better than the other, but it was slightly different. The two were quite a bit different in ohms, one is pretty aged. Supposedly coil wires wear out or break down a whole lot faster than the plug wires, they see a lot more pulses, so that makes sense. But the idea overall is that all the potential available coil juice makes it to the spark plugs to fire, electricity is "lazy" and will always try to find an easier path to ground. The engineers did all the work for us on ignition, I'm pretty sure it's easier to make it worse than it is to improve it, at least with some of the crapola parts out there.
If you're considering going back to the points distributor this tutorial may help you sort things out. A lot of it applies to electronic ignition systems as well. https://www.jalopyjournal.com/forum/threads/hot-rod-technical-library-basic-ignition-systems.983424/
Sorry to dredge this back up, but here's a picture of the distributor cap. Any idea what could cause this type of burning/carbon buildup?
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