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1960s vintage technology - how to troubleshoot a no spark condition

I don’t need to solve this, just curious….

I have a 1969 Chevy with a six cylinder engine that hasn’t run in many years. It has no spark though it cranks just fine. Do coils and condensers and plug wires deteriorate over time when not used, or is this more likely just bad connections here and there? If I wanted to investigate (not sure I will), how do I evaluate the coil, distributor, and whatever else? Not sure it’s worth new plug wires or other parts, but might be interesting to hear the old beast run one more time if it’s not too much effort. Any quick things I should try? Yes, I know the fuel is long since worthless. Thanks for the education!

Check the ballast resistor/wire coming off the distributor. This happened with my 71 Buick Skylark back in the 80’s. I spliced in a Chrysler ballast resistor and the car started right up.

Ed B.

Could just be the points rotted after sitting for so many years. Pull the cap and rotor off and hae someone crank the engine. See if the points are actually opening and closing, you might just want to replace them. Grab a service manual and read up on how to set them properly.

I just had a similar situation on a 1970 boat that sat for too many years. Using a 12 volt test light (with the boat battery all hooked up), I figured out that electricity wasn’t getting across the points. Light sanding/filing the surface of the points and the boat was running great at $0 cost.

Thanks guys.
@asemaster: am I correct that if I have the points closed with in key on position and use a non-metallic object to open the points that I should see a 12 volt sized spark there? If there IS spark there at the points, does that say anything about the condenser? If this test is legit, then I may be able to isolate distributor issues from farther down the line (coil, plug wires). I vaguely remember being stuck long ago when the points were not properly grounded due to poor contact, and that may be a factor worth checking… if my more important chores today go quickly. I am grateful for the help.

First, check for 6 volts between the negative terminal of the coil and ground while the engine is cranking. If you get 12 volts, the points are not conducting when closed, if you get 0 volts, you are not getting any voltage to the coil or the points are not opening. As for the 12 volt spark, no, if everything was right, you would get about a 250 volt spark.

When you have current flowing through the coil, a magnetic field builds up around the coil. When you open the points, you disrupt the source of the current, so the fields collapse and generate a flow of current. Many people mistakenly believe that the voltage goes up because of the turns ratio between the primary windings and the secondary windings in the coil, but that is not true. The magnetic fields are developed by current in the primary windings, but when the field collapses, it generates voltage across both the primary and secondary windings of the coil. But the ratio is only 100:1 so that would only generate 1200 volts in the secondary.

The complete formula for the step up in voltage includes time difference. In a normal AC transformer, the ∆T (delta T) nulls out so only the ratio and applied voltage determine the secondary voltage, but the ∆T is different in the charge time and discharge time of a coil of a Kittering ignition system. The coils in the old Kittering systems were actually a tapped coil, so the full voltage was felt at the spark plug, but the tap, acting like a voltage divider only sends 1/100th of that voltage across the points, or somewhere between 160 and 250 volts depending on RPM.

Modern ignition systems work differently so the coils are actually wound as an Auto-transformer. That keeps the high voltage out of the computer that is triggering the spark. We just keep calling them coils though.

The simplest way to verify that the points are doing their job is to use a test light probe and place the probe tip on the minus side of the coil, which should tie to the points in the disty, and the clip lead of the probe to a good ground point. Then crank the engine and verify that the light flashes while the engine is cranking. If you see the light flashing then things are OK with the points and you need to check the rotor and the coil high tension lead to the disty for a problem along with the coil itself. If the light doesn’t flash then disconnect the minus lead and verify power is getting to that point on the coil. If you now have power then that means there is a short to ground on the wire lead to the points. Make sure the points are opening and closing while the engine is cranking. If you don’t have a test light probe to make these tests with I suggest you purchase one.

Thanks for the good explanation. The tip about checking voltage at the coil will be useful. Regarding the spark, I know about the collapsing field generating high voltage current resulting from the opening of the points. I think you may have misunderstood my question about causing a spark, or maybe I misunderstood your answer, or both.

The question I was asking was if I’d see a spark AT THE POINTS, ie, inside the distributor. I’m not sure I should expect a spark there, and wanted clarification on that. I had concluded that with the distributor cap off to look at the ignition points that there was no electrical connection to the spark plugs and thus no chance of creating a high voltage spark there in that scenario. Your suggestion about checking voltage at the coil may make this even simpler, I’ll try that first. Thank you.

Thanks Cougar. This is also very helpful. It’s been so long since I worked on this kind of stuff that I’m just rusty on it. I do have the test light and can follow your good explanation. I appreciate it.

A spark at the contact points might possibly be seen but not seeing a spark does not indicate a lack of current. The voltage at the points is minimal compared to the coil output.

Another possibility is corrosion on the rotor, or the contacts in the distributor cap.

Yup. Corrosion and deterioration is certain to be everywhere, with the usual problems that can cause.

Rod: thanks for the clarification.

Cougar’s advise was excellent…If your test light flashes when you crank the engine, remove the coil wire from the distributor (leave it in the coil) and hold the end about 1/2 inch away from any metal surface and crank the engine. You should get a nice fat spark jumping off the wire…

Tell us what you find…

Ok to answer your question, yes there is normally a small spark. Ideally the condenser would prevent this, and it does greatly reduce it, but typically there is a small spark.

I’d just check the voltage at the coil (with the coil grounded there should be 12VDC), check the impedence of the coil, and…whether those check good or not…chenge the points & condensor, all the ignition wires, and the sparkplugs. I wouldn’t even bother to try checking for spark until I did that.

You’re also going to want to evaluate the carburator. I’m willing to bet lunch that it’s varnish and dust.
Open the choke, peer down into the openin with a flashlight, operate the throttle linkage manually, and see if it sprays gas. You’'re probably going to end up rebuilding the carb and perhaps even draining and refilling the gas tank.

Thanks the clear instructions. Sorry for the delay, had some higher priorities come up.

Cougar wrote:

"If you see the light flashing then things are OK with the points and you need to check the rotor and the coil high tension lead to the disty for a problem along with the coil itself. If the light doesn't flash then disconnect the minus lead and verify power is getting to that point on the coil."

So here’s what I found with my 12v test probe (KO):

  1. bright light at both coil terminals.
  2. when cranking, both sides get significantly dimmer, about equally so. NO FLASHING.
  3. remove dist side wire, still get bright light on both coil posts.
  4. replace wire on neg dist post, open dist: bright light at points on the moving arm, but not on the stationary side.

To my eye, it looks like the points are closing when I crank the engine, showing that voltage is not flowing from the movable point to the fixed one. (test light probe on the stationary point, and clip lead to ground. So this is a problem with the points it seems. Is that correct?

I don’t remember the correct gap for this, but to my eye it looks too tight, but that wouldn’t explain the lack of flow between the points. Anyone know what this should be? Chevy 250 cu inch, 1969. Something like .030 or thereabouts?

After I have some lunch, I’ll clean the points and see if that changes anything. I’m not going to chase this very far, but if I can find an easy explanation, like dirty points, that would be worth the effort.

I’m grateful for the good help. @Mountainbike…as I was writing, I saw that you posted something. WIll check that now. Thanks to all.

Point gap should be about 0.016". 0.030" was the plug gap. If you have a dwell meter, the dwell should be 30°

@Mountainbike. Thanks. You are absolutely right about carb especially. If I were going to use this old rig, I’d do exactly what you said. However, this vehicle is history, I don’t need to make it run. But it’s a good chance for education, even if I won’t ever use it again.

BTW, I have found that an emery board for finger nails makes a good points file. A nail file, not so good. They used to sell a points file but haven’t seen one of those in years. You can also fold a piece of good aluminum oxide sand paper or emery cloth in half and use it in a pinch.

Thanks Keith. I have a simple dwell meter but the only time I’ve adjusted dwell was on distributors with that small window and an allen head screw to twist while the engine is running. This distributor is more primitive - is there a way to use the meter with the cap off? I know how to set the gap with a feeler gage, but that’s so frustrating.