The Stovebolt Site Forums The Shop Area The Electrical Bay 12V Coils . . . Again!

1949 3600 w/mid-50's 235 and SM420, points ignition.

Diagnosing a rough running engine here. And I did a search on coil specs and there's a lot of posts with conflicting numbers. Plain old Google is even more divergent!

My coil is a 12 month old NAPA 905 12V with internal resistor. It reads 2 ohms on the primary and 5,000 ohms on the secondary. A few months ago the condenser died and I put in one of those solid state ones from the members here. I get no visible spark on the spark tester, but I get enough spark to start (easily) and run the engine, but it runs super rough. Takes almost full throttle and liberal pumping to get it to idle (I did overhaul the Zenith 228 today, no change in symptoms).

The truck was running beautifully for a long time and then slowly started developing a rough stumble under load or acceleration. That stumble got worse and worse until it suddenly started running rough in all throttle settings and overheating. Since I had recently replaced the coil and condenser I assumed an overly lean carburetor (it was new, off-the-shelf from 1970 so I thought ethanol had finally gotten to it after a year), and rebuilt the carb but didn't see anything out of whack or dirty inside (float fine, jets clean, etc.).

I'm thinking the coil is bad, but with an online google search, 5,000 ohms on the secondary is mentioned as OK in a few places. More seem to say 10,000-20,000 ohms though, so I thought I'd check in with you guys!

The combo of rough running and overheating is perplexing.

What is the terminal voltage at the battery posts with the engine running? In general, an internal resistor coil is a bad choice. It limits the voltage available during cranking when the battery terminal voltage drops due to the load of the current draw the starter creates. Also, check the voltage downstream of the coil, between the coil and the terminal at the side of the distributor with the engine stopped, ignition on, and the points closed. It should be less than 1/2 volt. It sounds like you've got some dirty or burned points that are limiting the current flow in the coil primary winding.
Jerry

2 ohms on the primary side of the coil seems pretty low to me as well. The internal resistor should be almost that much without taking the primary coil winding into effect.

As long as you have the voltmeter out, check coil + to ground with the key on, engine not running and points closed. It should read battery voltage.

Checked another NAPA coil at the store and it was also 2 ohms on the primary and 5,000 ohms on the secondary, so I guess mine is still good. Googled sources list everything from 0.8 - 3 ohms as o.k. for the primary and 6,000-20,000 ohms on the secondary for a 12V internal resistor coil. Surprisingly divergent views.

I changed the condenser, and retimed the distributor (static only, ran out of battery). The timing did look a bit off, not sure how that would change. Battery is charging now and we'll see in the morning. The points were a bit fat so I tightened them up, but they were clean and not at all burned or pitted.

"Looks good" doesn't make the cut- - - -check the voltage drop across the points- - - -engine stopped, points closed- - - -ignition on. It should be way less than 1/2 volt.
Jerry

The "extra" resisitance coil is not a good idea. Use Externally resisted coils with a firewall mounted ballast resistor. Use Rockauto 1956 Truck as a guide.
5 months ago you found a problem was a bad condensor.

This is way too simple to be measuring everything. Tune it up. It's cheap. You are in a stage where you don't know if it's spark or gas or timing or....
Put the meter away. Get to a boiler plate with replaced parts.

Points plugs wires condensor cap rotor coil ballast...........$70 plus ship

If still runs rough. Check spark at plugs, then at secondary tower and look at points spark in the dark. If all good, check timing. All good?.......gas. Vacuum leak. choke, fuel pump, filter, tank vent. Anyway, report what you find so we can learn from you..

Others are better at meters and diagnosis measurements.......that is another way. I can do that, you can do that with their help. That's OK. Cost effective analysis, time and new parts dictate my choice in these situations. Even though it's called throwing parts at it. You're not buying a motor or transmission or differential. Electrical is harder to trace, in a "run rough", than gas. So eliminate electrical ASAP.

Did Jerry's voltage check. Battery is 12V (analog meter) and with key on there's 12V at the + terminal of the coil. Downstream of the coil, at the distributor terminal block voltage is zero: key on, and points closed.

No visible breaks in the wire from coil to distributor. No visible faults in the points.

You mentioned it was running beautifully and then developed a rough stumble under load or acceleration which got worse and worse. Have you checked the resistance of your spark plug wires? Could you do that and let us know what you find? Also measure each wire and calculate your resistance per inch of wire.

Looking like it's the distributor insulator block. I'm getting continuity from the terminal on the outside of the distributor to the floor of the distributor and body of the distributor.

When points are closed, the wire from the coil neg to distributor will show ground. As in: continuity to distributor body/motor.

Open the points and check. Your floor is connected to ground (the body) and therefore so are the points.

Sorry Bart...I was typing while you were posting I suppose.

No problem Jon.

Commentary not aimed at anyone:
The most difficult cases are those where poster does not say "my truck is running rough, need help". But launches into "testing" and wanting to know how. Then we get involved teaching testing instead of getting motor ruinning good after questions and targeted tests or..... replacement of parts instead of giving a course which takes weeks of study and practice. Jerry gets frustrated at times. Don't blame him. He is tying to cram weeks of training he used to do into one post. Cutting to the chase and instructing a test procedure to narrow down a problem. Posters don't perform the tests or they side track. If a person is not up on ignition too much, I go with "get new ignition parts and call me in the morning" approach. We don't know if it's electrical or not. Sometimes assumed by posters in post 1. We don't do that but get sucked into it. Posters head off on their own during the diagnosis phase. It's good to want to fix it on your own and learn on the way, but it frustrates the methodical diagnosis continuity of helpers trying help from their house. If you are following Jerrys advice, test as he says, report the result and wait for next test.

Even though post title is "Coils again!" it does not phase us. Even though there are electrical test results. It does not phase us. We stick to finding out what the motor does and does not do. It is stated in this case that motor is slowly losing power, "Stumbling under load". That is what a bad fuel pump does or other gas related issues do. Carb dirt. Failing fuel pump diaphram or..... lines/filters/tank outlet slowly getting blocked off or..... building a vacuum lock in tank, bad gas, etc. etc. It can be ignition also or... loose wire, generator, battery... or worse. Forget checking ohms. Especially with an analog meter. If it runs it has spark. When you are ready, I would like to ask questions before you continue. Papa: We DO want to help you.

If you have the ability to check capacitance on your handheld meter, you might check the condenser you're using and also the one I sent you. Should be .22 uF within 5% or so. If however you're using one of the small round Chinese-made condensers, all bets might be off. I've seen some of those that would register sort of close to .22 uF held with one end pointing up and completely open with the other end pointing up. Not to say the condenser is your problem, but as the tv policemen say "we're just eliminating suspects, sir."

So the points were bad. Relatively new (500 miles) but the contact end popped off of one side and you can see the marks from it only having a partial connection to the underlying arm, which would explain the sudden change in running. Doesn't explain the overheat.

The multimeter was also showing some continuity through the distributor terminal insulator: about 30 ohms. Took those little blocks off and can't see any cracks in them.

Overheating is a separate discussion and probably should be directed to the Engine section.

Sheesh.

New coil, condenser, points, cleaned and gapped the plugs, rebuilt the carb, check timing . . . realized I'd forgotten the cam lube for the new points and took the distributor cap off to do so. Double checked timing and it was 180 degrees out! Pulled the distributor and the pin in the drive gear was shredding on the ends. I guess that's what the real trouble was.

Distributor is a Delco-Remy rebuild, just about a year old. They must have used a pretty soft metal in the pin.

These pins last about 3 days past forever, and don't have to be peened on the ends to stay put:

www.ebay.com/itm/155054334315?

Jerry

Running bad/overheating. Just for kicks.....Check oil for milky look. Check timing

Okay folks, please let's discuss problems in the right area.

Thanks Jerry. I was wondering what the right pin would be. What was in there looked like one of those thin, single wall roll pins. Nowhere near as substantial as the ones you pointed out.

I've used them successfully on the HEI distributors I modified for round track racing. Class rules for the division we ran prohibited using an aftermarket ignition system like Accell, Mallory, MSD, etc. The engine had to be "stock appearing". In racer-speak that means "Cheat all you want, just keep it hidden!" We drove Melling high volume oil pumps at 6K+ engine RPM and 50 PSI oil pressure, with bronze distributor gears and roll pins like the one I posted- - - - -with no failures!
Jerry