VCCA Home Forums TECHNICAL - GENERAL Electrical - 6 volt 6 volt ignition coil resistance

I have some starting problems, car will only start with full battery charge. I now removed my 6 volt coil for measuring of the the resistance.
On the primary side it measures about 1,26 ohms.
On the secondary side it measures about 3100 ohms. According to other posts, this seems to be too low.

What should the minimum ohm read at the secondary side for a 6 V coil??
And what will happen if the resistance is too low?

Good questions. I measured the resistances of the primary and secondary of the coil on my '53 Bel Air, out of curiosity. The coil is Delco PN 1115380 (age unknown). The primary measures 1 ohm, and the secondary right at 5000 ohms.

I don't really think your secondary measurement condemns your coil, although it is possible that it has some shorted secondary windings (reducing the indicated resistance). If that were the case, it could lower the output voltage, but generally, if there are shorted windings, the coil will not function at all, especially when hot.

So, I also consulted the old (late '40s to early 50s) Delco bulletins in my library, and found the info there a bit less than satisfying. Perhaps because there were so many variants on the coils depending on year, make and model, specs like resistance aren't in the bulletins that I looked through. The Delco recommended diagnostic procedure was to take out the suspect coil, and replace temporarily with a known good unit. Hope that helps.

Thank you dreep for your reply and for measuring on your car.
I was surprised that nobody answered this quickly. And after «googling» this question, I am not so much wiser. But I have noticed this value varies from 5000 to 11000 ohms on other pages I have been reading.

Most common answer is to «replace with a good working one». But I will not buy a modern reproduction coil just for a try, I want to know the specifications.

If some more Chevrolet-friends here at VCCA kindly could measure their working 6 Volt coils, I will be happy, If the result is that my coil is lower than average, I will replace my coil with a NOS coil from ebay.

The reason you will only find rough estimates is that the resistance is not really what matters. 6 volt coils (and 12v coils that use a resistor) usually have a primary resistance of around 1.2-1.5 ohms. It should be around that to draw enough current to magnetize the core between firings, but not so much that the points fail right away. What makes some actual difference between ignition coils is the turns ratio between the primary and the secondary part of the winding.

If the secondary is wound in slightly heavier wire, the resistance will be lower, even if the turns ratio (the part that matters) is exactly the same. For this reason, ohm-testing coils it a shaky proposition at best. You would be mainly looking for an open coil (completely dead). An ohm test could also reveal shorted turns IF you knew EXACTLY what that particular coil measured when new. Usually you don't.


The first place I would look is the battery cables.

The important thing is to get enough voltage to the coil while cranking. Battery voltage while cranking is already quite a bit less than while running with the charging system working.

6 volt Chevrolets typically pull the hot wire to power the rest of the car from the starter, so any voltage that is lost in the cables while cranking is also lost to the ignition. Any loss in the ignition switch and wiring is added to that on the way to the coil.

Two things I might do if I had this trouble:

1) While cranking, measure the voltage at the battery posts, then, while cranking, measure the voltage at the starter from the hot post to the starter case or engine block. This will reveal how bad the loss is in the battery cables and connections while cranking.

2) Turn the engine until the points are closed, and the turn the ignition switch on. Measure the voltage from the starter positive post on the starter to the starter case or engine block (again, because it will be different while not cranking). Then, measure voltage at the coil, from the positive terminal on the coil to the distributor case or engine block. This will reveal how bad the loss in in the wiring and ignition switch.

It COULD be something wrong with the coil itself, but I'll bet it isn't. A bad condenser is another thing that can cause weak spark. If you have not tried a new one, it couldn't hurt. It doesn't cost much

Good luck.

Thank you bloo. Good ideas.
I found the positive +cable to bee a little worn. So I decided to renew both battery cables. I will also add an extra ground cable from bell housing to frame, not original on a 1939, but standard on later models.
I will also renew all ignition small parts and the spark cables.
Car is now parked for winter, so long time to check if this improvements will give expected results..

Get big fat cables. Ones for 12 volts aren't big enough. A parts outlet that caters to tractors will have big cables for 6v, as well as car/industrial battery stores, who can make some up from cable. 00 is a good size.

With a primary and secondary the coil acts like a transformer. Hard to test with an output from 30000 to 40000 volts. So if you put 10 volts AC on the secondary and measure the primary in millivolts (or microvolts) you can get the turns ration. Then use that figure to multiply 6 volt and you will find out if the coil will give the required output. One down side is it won't show any winding insulation breakdown. It will show shorted windings as a reduced output. A megohm tester may be needed to test for insulation integrity. Do you have any friends that work with old amplifiers, TVs or ham radio equipment? They'll have the gear and know how.
P.S. If you are looking for 30000 volts out with 6 v in the ratio is 30000/6 or 5000:1 hence 10 VAC on the secondary should give 0.002 volts on the primary. I mention low voltages to keep the testing safe but caution is still advised.

There is a fly in the ointment though. You can measure the turns ratio that way, and it will be accurate, but it wont make any sense. An ignition coil has not enough turns ratio to produce the voltage it actually produces. This is because when the points open, the magnetic field collapses faster than it was made. The fast collapse makes a higher voltage than you get with simple AC.

Since I posted this problem, the car went into winter storage. But now the problem is solved. I measured the coil with an old coil-measurer. It was OK. Then I renewed "everything else". New heavy-duty (1GA) batterycables. Extra ground-strap from starter to the frame. New distributor cover and rotor. But most important; new spark plug wires. Inside the old wires, I found a very bad connection between wire #3 and the distributor cover. I think that was the main problem.

Glad you got it fixed!