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Does anyone know of a good and acturate test for a conventional points ignition coil? Is there a resistance test? I'm suspecting a weak original "270 B-R" coil on my 1974 L-48. It's almost 34 years old.
Marty,
I test them by installing them on a running car. Bring it up to full temperature for 15 or more minutes and install the shielding too. That way the coil gets good and hot. If it passes that test you most likely have a good serviceable coil. Continuity tests are good but actual testing is much more positive.
Dave's advice is right on. Unfortunately, if you are at, say Carlisle, it is very difficult to do that. I recall that one of our esteemed members suggested putting an ohm meter across the terminals and checking for resistance. Iirc, an open field, or infinite resistance, is bad. I don't recall the initial numbers but I used this procedure to buy two correct coils at Carlisle and subsequent on-the-car testing proved them to be good.
DAVE-
Unfortunatly I have no test car. My 07 CTS, 04 Tahoe and 02 Cavalier won't work. Besides that my vette barely runs at this point and the plugs come out wet with fuel. Thanks for the try though.
GARY-
I checked the service manual and it sats something about a 210 volt test, what ever that is. It made interesting reading. There's no OHM reading spec'd out so I hope someone out there will have some test data.
Thanks for the input
Anyone have an old Motors Manual? They have the specs for the resistance on the windings. So does the shop manual. I am in the real estate office today and do not have access to my manuals.
There is no way to test a coil other than static resistance, which will tell you nothing about "under load" condition. Age & use time will render your old coil a problem & 34 years is a long time for a part that was designed to last 2 years. Coils are filled with oil which keeps the iron core insulated from the can. Over time "carbon tracks" develop in the oil and the electric current will intermitingly go to ground, especially at higher RPMs. So it may start & idle OK, but breakdown under load-causing you to wonder wht's going on. In the old days, when you received a "thorough tune up", the coil was routinely replaced, just like the plugs.
KEN-
My problem is poor, almost no idle, smells extremely rich and the plugs come out wet and rich. The spark I'm getting, to me, looks weak. I ran an OHM meter on it and I'm getting 2.3 OHMs of resistance.
THANKS for your input.
You can kill two birds with one stone. Buy a cheapo auto part store service replacement. Swap it in now to see if it cures your problem (although it does not sound like a bad coil to me). Even if your coil is good, you have an emergency spare to carry on road trips.
Testing resistance simply tells you the primary and secondary windings are intact at the LOW VOLTAGE probe conditions of your multi-meter. The ignition coil is a transformer. You want to know that it will actually pass energy from primary to secondary AND that it will do this at full power + when the coil is at operating temperature.
The winding resistance specs published in the service literature is a necessary but NOT a sufficient test! They were dealing with fresh/new parts and not 30-50 year old components.
Want to test your coil properly? Take the car to a shop with an engine diagnostic system and view the actual spark waveforms on an oscilloscope with the engine hot and running....
When I attempted to fire up my engine for the first time, one issue I faced was that there was no fire at all from the coil lead.
I ruled out the points, cap and plugs, so the next day I picked up a coil from Advance Auto for $18.00
I ohmed out the old coil against the new coil and found the readings were almost identical. I then installed the new coil and had a beautiful 3/4" long blue spark. All I could assume was that the old coil was somehow grounding out internally under load.
As a side note, I also noticed that the bottom of the old coil was dented in from resting right on the intake manifold. I made sure to hold the new coil up just a tad to prevent this type of damage from happening again and to also maybe help reduce any additional heat transfer from the intake to the coil casing.
Also, the Advance coil has a dull blue finish as opposed to the std black
CHUCK-
Thanks for your input. I replaced the coil last night with one from NAPA. It's alittle shorter than the original and also had a Echlin sticker on it (which I removed). I replaced the coil and like you, left it up off of the manifold approx. 1/8". This new coil by the way fixed my issues. I'll get a new "270 B-R" for judging purposes from Paragon later this year.
Ignition Coil Testing
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