First, let me se if I understand a basic theory. An ignition coil works by "charging" w/12 volts, then, by removing the ground it colapses the field and creates a spark...right? So you should be able to apply 12v to the + side of the coil, ground the other side, and when you remove the ground, it should throw a spark....right?

I can only get my coil to do this once in a while, and it only throws a weak looking spark. I changed the coil w/a new one and tried the same test, got the same results.

Also, if I hook a test light to 12v, then prob the "ground" side wire of the coil (the wire going to the module for the ground side of the coil), crank the engine, shouldn't the test light flash on and off as the dist rotates?

 

What color is your spark? blue is good orange is bad
Which coil are you talking about (ie HEI, Points, aftermarket)
Are you sure the coil is getting 12 volts?

I haven't ever tried that trick about cranking the engine with a test light, but it seems like the light would be off for the duration of the spark. But im not sure because electricity takes the path of least resistance, maybe instead of energizing the coil it may just run the test light full time, I would think it would do both, but your coil voltage would be comprimised due to the load of the bulb.

generally if im looking for spark i pull the plug wire and check with a dummy plug, or some people are big on hooking up a timing light.

 

Almost.

Don't forget the role of the cap.
The ignition capacitor serves two purposes.
One is to suppress the damage from the inductive discharge of the primary.
The other is to "resonate" with the coil, so that the collapsing field will "ring" the coil, and
cause a MUCH higher energy discharge in the secondary.

You can do what you're doing, but throw a cap across the ground your making, and I'll bet you see a very different result.

 

Where is this "ignition capacitor?"


I don't think my car has one. I did the motor swap, wired my whole harness, and never came across one on the donor car.
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Are you refering to the condensor on a points system?

 

What Cflik said. When working on old points-style ignitions I quickly elarned that it won't throw a stpark worth a damn without the capacitor hooked up. It's the difference between "did it make a spark??" and throwing a lightning bolt when you break the ground.

In an old large cap HEI the capacitor is next to the ignition module- a round metal canister. I don't have a small cap HEI laying around so I'm not sure where it is.

 

Points ignitions use the condensor to soften the voltage spike when the points close and open. This prevents arcing. Any sort of capacitor or condensor on a electronic ignition is to help with radio interference.

 

If you don't see one, it's likely built into the ignition module, but it's there somewhere, and it's probably a 0.22uf 400 volt cap.
The coil is around 100/1 step up ratio. You will usually get about 250 or 300 volt pulse at the primary when you break ground. Maybe 15 or 20 volts without the cap. See the difference ?
( for illustration purposes, not actual measured voltage numbers )

 

TRUE, but is only one of two reasons. ( see above post )

NOT true !
That's what resistance wires and plugs are for. ( explained in another thread )
Cap's in ignition circuits have NOTHING to do with radio interference.
In inductive discharge systems, they resonate the coil on field collapse.
In capacitive discharge systems, they resonate the coil on field build-up, and provide the charge pulse into the coil.

 

So....


Maybe I should add one. I have been going though gnition modules at an alarming rate. (Although, the lates one has lased a long while.... I cleaned up the bolts to get a better ground and it seems to be working)

 

are you using real thermal compound under the module and not dielectric grease?

 

You are correct sir.

 

\
Copy. I theorized about that, but wasn't sure. I knew the condenser helped save the points, but didn't know about it's other purpose/function.

Thanks!

-Tom

 

Just for kicks, today, set up a coil, spark plug ( gap .060 ) cap, and a AA battery !
Yup, a AA pen light batery.
With the cap, you'd think it was a full HEI system.
Without, nothing !

Never tried it before, but this thread got my curiosity up.
Worked with an AAA battery too !
I just don't have any batteries smaller than 1.3 volts, so I don't know how little voltage it would really take to spark a .060 plug gap directly off the coil tower WITH the cap.

If the original poster can't make it spark with a 9V battery, heck even a D-cell, I'd consider replacing the coil.

 

That's cool. I can't believe that 1.5 volts did it! lol.

My original flaw w/my test what that the condenser was not part of the original test. W/the condenser in the loop, it throws I nice fat spark, as it should.

Thanks again for straightening out my primary ignition system theory.

 

yes very little voltage is necessary to jump .060 with no pressure but add the 120+psi of and engine and you might find that the 1.3 volts wont due