The Stovebolt Site Forums The Shop Area The Electrical Bay spark at the coil, but not at the spark-plug wire

1951 Chevy 3100, all original (216 cid engine, 6 cylinders, 6 volts).

Hello. I’m posting this in Electrical Bay because I believe I have an ignition problem, and I believe it’s spark-related, not fuel-related. I’ve searched previous threads a little bit, but I’m under some time pressure, so I’m seeking a new discussion.

My wife wanted some topsoil, so I decided to do something useful with my seldom-used truck; the dirt place is 10 minutes away. I’ve driven the truck 3 or 4 times recently on short 15-minute drives, but because it’s seldom used, it always runs rough. I believe it’s because the spark plugs are very fouled. So today, before making the big leap to the dirt place, I changed the plugs. The truck started immediately and seemed to rev more smoothly when given gas in neutral.

I drove the 10 minutes to the dirt place, and when I got there, the truck died. This was fine, because I was about to turn it off. I assumed it died because, perhaps, the idle was too slow, or something about the drive had made it too hot, or something else that wouldn’t ruin the whole day. But when I tried to restart it, it cranked but would not start; wouldn’t even rumble. I added fuel to the tank, no luck. Tried my usual method of pouring gas in the carburetor and choking it (which it shouldn’t have needed, because it was well warmed up), but still not even a hint of ignition.

So then the dump-truck drivers came by and offered to take a look. One determined that there was spark at the coil wire to the distributor, but no spark at a spark-plug wire. So I went to NAPA and bought a condenser and points. Installed those, still the same situation: spark at the coil wire, no spark out of a spark-plug wire.

Had to leave the truck overnight at a remote dirt pit, hoping to get it running tomorrow.

Am I going down the right rabbit hole, the distributor and components? If not, what else should I check?
If I am in the right area, could I have installed things improperly?
The condenser clamp wouldn’t go in right, so I “modified” it, but the condenser is still grounded (if that’s important).
The shop manual says the points should be gapped to 0.022”, even though the ultimate gap should be 0.018”, because the cam follower will wear down. I did this.
There’s a plastic washer that separates the condenser “spade” from the points spring where they attach to the lug on the side of the distributor. It may be possible these two elements are touching each other (though I’ve taken pains to ensure they’re not, but it’s hard to see). Would this be a killer?

Any ideas? I’d really like to drive the truck home tomorrow, preferably with a load of dirt in the bed.

Thanks,
DJ

Spark from the coil wire but not at the spark plug would tell me that there's a problem with either the distributor cap or the rotor.
Check to be sure the contact at the center of the rotor is OK, as well as it's corresponding contact in the center of the distributor cap.
Is the rotor actually installed or sitting somewhere in the engine compartment? (I only ask that because I've done it ).

When you say "spark at the coil" do you mean the big high voltage lead that goes to the center of the distributor cap, or do you mean there is voltage at the two small terminals on either side of the center tower? If you're getting a high voltage spark from the center coil tower, you need to replace the rotor- - - -that's what sends the spark to individual plug wires. Rotors can short out sand send the spark to the distributor shaft instead of out to the plugs. If there's no spark from the center coil tower, recheck the wiring from the coil primary terminal to the points- - - -it's possible the plastic insulator block at the side of the distributor has shorted out.
Jerry

Kevin, good "point". The distributor cap and rotor look very new to the eye, but they're probably 20-30 years old, just seldom used. The rotor was definitely on the shaft. I'll check the contacts on the rotor and distributor cap again tomorrow. Even the dump-truck guys said "this thing (distributor cap) is brand new" (from appearance), but I haven't replaced it or the rotor since I've possessed this truck, and that's been 20-ish years.

Jerry, the dump-truck driver stuck a screw driver in the end of the coil wire that goes into the top of the distributor cap and had me crank the engine. He observed a spark. I'll check the other things you suggest. I need to re-read your post and digest exactly what you're saying.

If there are carbon tracks on the inside of the distributor cap, those can divert the spark voltage to ground instead of to the plugs.

Check the wire from the coil high tension term. to the dist cap center term. These can be open inside. Some early resistor wires were on the coil wire only. They were prone to burn out inside the insulation.
George

Hi everybody. Thanks for your suggestions. Here are my updates (short story is the truck still does not start).

First and foremost (for my wife), I got a load of dirt by another means.

1) I couldn't replace the coil wire to the distributor cap because nobody locally has one.
2) Ditto for the distributor cap and rotor, but I've ordered them. The existing cap and rotor look good to me, though.
3) I tried the trick of jiggling the points open with a screwdriver. Here are the results:
a) The first time I didn't realize I was supposed to disconnect the coil wire to the cap, so it was still plugged in. I accidentally touched the distributor shaft and the points arm at the same time, and they sparked (not at the points, but at the screwdriver).
b) After re-reading the suggestion, I disconnected the coil plug wire (black rubber) from the cap and held the coil plug wire terminal near a ground and jiggled the points open/closed. There was a spark at the end of the coil plug wire. (Does this mean my coil plug wire is good?)
c) Then I tried the trick of using a jumper directly from the battery to the input of the coil and jiggled the points. This also produced a spark at the end of the coil plug wire.
d) With this jumper in place, I tried starting it. Did not work.

So my current situation is I've ordered a dist. cap and rotor and am waiting for them. I think I'll tow the truck home and work on it here.

Any other suggestions? Keep in mind, the truck ran fine during my drive to the dirt place, then it died and won't restart. So it seems something happened suddenly, maybe due to jostling while driving on a bumpy road.

Thanks.

Is the distributor shaft turning when you crank the engine? It's a remote possibility, but I've seen sudden timing gear failures that stop the distributor from turning, which also kills the spark. You can open and close the points manually and get a spark, but if the distributor cam doesn't rotate, you'll get neither spark, nor valve operation. That usually happens with a fiber timing gear, if the engine was originally installed in a car. Trucks usually got a metal gear for more durability under heavy duty use.

It's definitely something to check, before you keep chasing electrical gremlins that aren't there!
Jerry

Thank you, Jerry. I'll double-check, but I know that yesterday, after I had installed a new breaker points and was trying to get the distributor cam onto the cam follower to gap the points, I was in the cab bumping the starter while my helpers watching the cam, and they said it was turning. So I think that's a "yes" to your question. But I'll check again tomorrow when I replace the distributor cap and rotor.

Try plugging one of the spark plug wires directly into the coil and bump the starter a few times. The engine should pop, snort, backfire, or do something that way. If a few cylinders will fire directly from the coil, the problem has to be in the cap and/or rotor.
Jerry

Now THAT is brilliant. Can't wait for tomorrow (but I will). Thanks!

It's also possible the timing gear has jumped a few teeth and the spark is so far out of time it won't run, but even then, there should be some fire at the plugs. You'll need to remove the valve cover and watch the valve action and the timing ball and pointer to see if the distributor and the crankshaft are in the proper phase with each other.
Jerry

SOLVED! I got the truck running again. Turns out it was very simple, and a few of you mentioned checking the rotor. Klhansen was the first, but there's an old post that I read years ago that says something like, "if your distributor cap is too tall, it won't contact the rotor". Well, my distributor cap isn't too tall, and in fact the engine was running at the time of the problem.

Turns out the problem was that the top of the rotor wasn't contacting the distributor cap. I bent upward the center contact on the rotor, and I got fire. Hotrod Lincoln's' suggestion of attaching a spark-plug wire directly to the terminal would've lead down this path.

The truck was very hard to start, possibly because in the course of this, I replaced the points and condenser, and maybe the points need fine tuning. But I was able to drive it back home (about 10 minutes, which is quite an excursion for this truck).

By the way, did you know a points and condenser and feeler gauge (now I have 3 of those), cost $50 these days? I was flabbergasted.

Thanks for all the suggestions!

Glad you got it running again.
Perseverance pays off.

LOL Last time I was down visiting my mom, I came across an invoice where my dad had taken the family 54 Chevy Station Wagon in for a tuneup. The whole bill, including parts, was right at $12. Those days are long gone.

Congrats on solving your issue. I do most of my shopping on line. Rockauto is a good company and the points, condenser, rotor and a feeler gauge for my ‘59 235 you could buy for less than $15 plus $7 shipping.

Thanks for the suggestion of Rock Auto. Because I was stranded at the topsoil place, I was somewhat desperate. This was at my local NAPA, which I'm happy to support, but I agree online would've been a better option, had I had the luxury of being patient.