No spark, been reading...need ideas

[Patcon]

I have read several of the threads on no spark issues. Even the recent one where there was a short in the dizzy. The car I am working on ran a year or so ago. Went to start it up but no spark. Pull off the coil wire try to ground, no spark. Get out volt meter and have 12v at both terminals on the coil with the switch in the acc position. I read in one posts maybe by Fastwoman that the coil sparks when the "-" terminal is grounded. I tried that with a direct ground and a manual push button ground. Thought the coil might be bad (MSD). Tried another coil (unknown condition) tried a third coil (unknown condition) No spark on any of them. Thoughts, anyone? Did I understand correctly that the coil should fire when tapping the "-" terminal to ground? What are the odds all three coils are bad? Could it be anything else? Where to from here?

Thanks, Charles

[Zed Head]

Year of car and type of ignition system (points or electronic) would help. Otherwise, you might get many random guesses.

I think that coil actually sparks when the ground is removed, breaking the primary circuit. The coil charges when the negative side has a circuit to ground, through the points or the ignition module, then the secondary circuit sparks when the primary circuit is broken. I have had it backwards myself in the past, I can't speak to what Fastwoman has said. The end result is the same, assuming there was no path to ground to begin with.

Either way, touching the coil negative to ground should charge the primary circuit (with 12 volts coming in through the coil + passing through the primary windings and to ground), then removing the wire from ground should break it, causing a spark at the main terminal. So you probably won't get a visible spark unless you break the coil negative path to ground.

[Patcon]

1970 240z l24 twin dgv's stock dizzy w/ points and MSD coil. I was thinking that the charging actually happened then ground is broken producing spark. Which is sort of what the points do? I had the switch on in the acc. position tested with a volt meter 12v on "+" side of all three coils I tried. 12v on the "-" terminal with or without the wire to the dizzy connected. Disconeccted the spade from the "-" terminal and added a seperate wire onto the terminal. When I touched that to ground I got no spark off of any of the three coils I tried. I figured if the coil charges when it has a ground the short period of time I touched it to ground should still be more time than the engine has at 5000 rpm to charge up. I guess I could have three bad coils. Is there a good way to test the coils? I have a good multi meter.

Thanks, Charles

[Zed Head]

As I understand it, "points" are actually short for contact breaker points. Current flows when the points are closed, when they open, breaking the circuit, the magnetic field collapses causing the secondary circuit to discharge. But I am not an expert in the field (pun intended!) and just writing those words made me uncomfortable.

The FSMs all have procedures for testing the coils. It involves just measuring resistance across the primary circuit and from the primary to secondary contact points. Resistance values are given.

You might check your points to be sure that when they're open no current will flow and vice versa. I don't think coils go completely bad that often, they are pretty tough unless overheated.

[StinkyCheese]

One thing that I did (don't know if was being dangerous or not..) - I left the key in the "on" position and opened up the dizzy. I loosened the points screw and played with the gap adjuster screw. When I got everything working I could open and close the gap and a spark would go across the points. Once I got that to spark, I put it all back together and it ran..

[Zed Head]

That actually sounds like a good way to check it as long as you don't short something to ground with your screwdriver. That's how they work in use, opening and closing about 9,000 time per minute (150 times per second - can't believe it myself but that's the math) at 3,000 rpm engine speed.

You should set dwell or point gap more precisely though once you know it works. The setting can affect your spark power, I believe.

[Patcon]

I have the factory service manual and I didn't see a method for testing the coils. I did see some resistance values, but I don't know how to use them.

Thanks, Charles

[FastWoman]

Tapping the (-) to ground should do exactly the same thing the points do. Of course you need to disconnect any wires other than your test wire from the (-) terminal first. I admit I've never done it, but I'd be surprised if this didn't produce a spark. I suppose the "tap" needs to be a solid, deliberate tap, not a casual brushing.

[240260280z]

For the archives, I had an intermittent spark on my 77z with electronic ignition and it turned out to be poor contacts on my ignition switch. When I turned it 100% of the way to "start" (i.e. to the mechanical stop), it would not spark but the starter cranked. If I turned the key ~ 85-95% of the way to "start", it would crank and spark.

[Patcon]

When I tried a seperate ground wire for testing the coils it was the only wire coming off the "-" terminal. On one coil when I touched it to ground you could see sparking as the test lead went to ground, but the coil still didn,t fire. I thought about the switch but I have good power at the coil and I have done all of my testing lately in the Acc position. I'm kind of stumped. I don't want to just throw parts at it before I am pretty sure of the problem.

Charles

[5thhorsemann]

You should be testing the car in the run key position. Eliminate the car from the coil, go to + batt (fixed) and - batt (touch) and hold the output lead to a good ground. if it doesn't spark up thatway the coil is toast.

[Zed Head]

Where are you looking for spark? You should have the main wire from the center of the coil disconnected from the distributor with either the cover pulled back so that you can see the metal end, or a bolt or something stuck in there. The end should be about a spark plug gap away from a good ground so that you can see the spark jump when you tap the coil negative to ground.

I might be stating the obvious but if you have power through the coil posts (the primary circuit) and you're seeing a spark when you tap the negative to ground, you should see a discharge from the main center wire of the coil if it has a gap it can jump to ground.