The Stovebolt Site Forums The Shop Area The Electrical Bay ’40s positive ground ignition woes

Helping a friend with his late ‘40s GMC pickup. All stock 6 volt positive ground system. Was running fine 3 months ago now it has zero spark at the plugs. The starter spins the engine as it should. When the key is turned on, power (6 volts) is supplied to the ignition (-) side of the coil. When the starter turns with the key on, a test light will pulse at the points (+) side of the coil and at the terminal on the side of the distributor. The coil center wire will not spark nor will any of the plug wires. The distributor cap is crack free and the center button is under spring tension. The rotor appears damage free. The points are fairly new and set at 20 thousands as per my Chilton’s Manual. The coil is new. The condenser is one of Jon G’s that I have loaned out. I’ve pretty well exhausted my bag of tricks, so I’m looking for help. Weather here is on and off for outside work but I will do my best to try your suggestions and report back in a timely manner. Thanks

Try checking for a short inside the distributor. I once helped a guy on the side of the road who had a jeep that quit on him. When i pulled the distributor cap, I saw that the wire from the terminal to the points was chafed and grounding. Moved it away from ground and he was back on the road. The '40's distributor probably doesn't have a wire to the points, but something might be grounding out.

"New" doesn't necessarily mean "good" where the coil is concerned. Try checking the secondary windings for continuity with an ohmmeter. From either one of the primary terminals of the coil to the center tower, there should be a measureable amount of resistance, somewhere around 10-20 K-ohms. If there's an open circuit or extremely high resistance like hundreds of K-ohms, you've got an open secondary winding. Even a 12v coil should be able to produce a weak spark on 6 volts, particularly one that normally requires a ballast resistor, if you've got one handy to substitute for diagnostic purposes.
Jerry

Two things to check when I can get back to it. Thanks

Possibly an open on the coil center wire. Check with a meter or you can pull it and replace with any wire to see if the coil is putting out a spark to ground. You don't even need to spin the engine, just open the distributor and manually open and close the points with the ignition on.

Mark

Thanks Mark

Points too far apart. Try starting at .015 and see if any spark happens somewhere. Try a Chinese condenser....... or one from a known runner. Or try the JonG on another motor. Get copper plug wires.
I don't know anything about the JonG and I don't think this would matter.... but be darn sure it's OK with pos ground.

Thanks Bartamos, the Condenser made by John G is supposed to be positive ground compatible. Will try your suggestions.

Here is the results of my tests:

HRL, Coil showing approximately 6K resistance and leaking oil.

59fleet, coil wire shows continuity

bartamos, reset points to .016 still no spark.

Klhansen, ding ding ding found it when doing coil wire test. Was able to get an occasional spark by opening the points with a screwdriver but not consistent.

Seems that there may be more than one issue here. I have the distributor out and on the work bench to fix the bare wire.
The truck owner said he has another distributor that is supposed to be good but he was going to have to find where he had it stored. I’m going to fix this one anyway. Going to be a few days before I can get back out to where the truck is. The owner is elderly (yes older than me) and lives on a primitive farm about 15 miles away. No running water ( well water only) and house is heated with a wood stove. He uses the truck to haul feed to his few cows and pigs and now he is using his sons extra truck. So, I need to get after it. Keep the suggestions coming. Thanks

Post a picture of the truck when you get over there.

No can do, I’ve already asked and the answer was a resounding no!

Need to be sure the screwdriver is touching no metal (ie ground) other than the moving arm of the points when doing this test. Or use a small piece of wood like a 1/4" dowel or a plastic rod of some type.

Hey! You're helping him out. He owes you a picture.

Ball point pen tube.

Need to be sure the screwdriver is touching no metal (ie ground) other than the moving arm of the points when doing this test. Or use a small piece of wood like a 1/4" dowel or a plastic rod of some type.[/quote]


I have a small plastic/nylon screwdriver that I use for such testing.

I suggest you are better at this troubleshooting/fixing then we are............ You go cowboy!

I’m not so sure bartamos. The positive ground thing just goes against the way my brain flows. My ‘39 Dodge has positive ground and I try to not mess with it. Rest assured I will prevail but just how quickly remains a mystery.

electricity doesn't know the difference.

Well, the truck owner found his extra distributor and brought it to me. Already had the one currently in use out of the truck. He admitted he had been “messing” with the truck prior to all the problems, trying to get it to start faster. That explains a lot. I’ve known this gentleman since 1976 so I agreed to help him out. May have been a mistake on my part. I took both distributors apart and built one good one. Everything in the distributor tests out good. No shaft wobble, good points, Jon G condenser and no wiring shorts. Waiting on a good coil and new coil lead wire before I take everything back out to his place to install. Thanks everyone for the testing suggestions, they helped identify 3 problem areas.

Update: new coil and new coil wire installed. Still no spark. Got to digging around under the dash and found some mouse eaten wires in several places. The owner doesn’t like cats so the mice run wild in his barn where the truck is kept. I told him to get a new wiring harness installed and if it doesn’t start I would be back out to try again. So, for the time being I’m out!

Have you tried a temporary hot wire directly to the upstream side of the coil to see if you can isolate the open circuit to a specific area? It's about 5 minutes' worth of troubleshooting with a piece of primary wire that will narrow down the possibilities of where to look for the problem.

A 5 gallon plastic bucket, a beer can, a piece of stiff wire, and some peanut butter makes a self-resetting mousetrap that catches them by the dozen.

Jerry

Actually I have. Made up a jumper wire from a hot terminal on the voltage regulator to the (-) lead on the coil positive ground system. No spark. Jumper removed and there is full 6 volts at the (-) side of the coil with the ignition switch on. No spark. Ran through all the previously mentioned tests with the following results: coil wire has continuity. Ohm test between coil terminals with key off shows 0. Test between hot lead and coil center pegged the needle on my cheap tester. Points are opening and closing as they should. Tested for shorts in various components of the dizzy, no problems found. No problems found with the rotor or cap.
Odd thing is the truck was starting and running fine then one day no spark. Battery is good, ignition switch has not been left in the on position. Grounds are all clean and in the factory stock locations. As said I’m backing out and resetting.
Thanking everyone for the help.

Update: Just to bring some closing to the issue, the truck runs. New coil was dead right out of the box. Put a used one on that the owner had in his shop. New (used) condenser and a new coil wire. Repaired a couple of mouse chewed wires under the dash. Poured a dribble of real gas down the carburetor and hit the button. Fired right up, tuned the carburetor a bit to get a good idle and checked the timing. Turned it off and handed the key to the owner. He’s happy and I’m happy.