6v coil restistance

[1930artdeco]

Hi All,


Quick question to edumicate me. I have a stock electrical system and had a 6v coil. Well when she would get warm to really warm she would die and would be really hard to restart until she cooled down a bit. So I changed the condenser thinking that was the problem.

But in the meantime I got to thinking that the coil may be going out. So I checked my coils on my meter and got the following numbers:

post to post/post to center electrode
coil on car (6v oil filled): 2.3 ohm/ 8.1k ohm
new NAPA (6v oil filled): 2.5 ohm/6.7k ohm

I have read here that I should have 1.5 ohm resistance, what am I missing? Will an electrical guru spell it out for me please. Got a really good spark on my analyzer with the napa coil. Have not checked the other coil yet.



Mike

[Tom Endy]

Coil failure usually occurs at higher temperature. this is because the high voltage windings expand with temperature and if there is a break in the insulation it can short out. A number of years ago I was on a trip to Yellowstone from LA. North of Las Vegas we encountered heat over 100 degrees. Every time I pulled into a gas station to refuel the temp under the hood would go up and when I went to leave the car was hard to start and ran poorly for the next few miles until it cooled down under the hood. It wasn't until the coil failed completely that I understood what had been going on.

The only way to properly test a coil is with heat applied. For the average Model A person this is not practical. Best to just replace the coil with a new one.

Tom Endy

[Patrick L.]

Oil filled coils and Model As don't like each other. That is unless you mount it upside down to what the car is used to. Point the terminals up and it'll be OK.

[1930artdeco]

Ok, I will flip the new coil over. But I still don't understand the ohm's. People say it should be 1.5 but my new Napa coil is showing higher than that. Or, should I not worry as she runs fine?


Mike

[Big hammer]

Are you using a digital or analog meter? The battery in the digital when low show different ohms, analog you need to zero out the scale. The very best test is put it on and go for a drive! Take the old coil with you and some tools!

[30 Closed Cab PU]

Sponsored Links (Register now to hide all advertisements)

If you have a good 1.5 ohm coil - install it and space a spark plug lead away from its spark plug terminal. Gradually increase the gap until the motor starts missing/running rough. Install the 2.5 ohm coil and see if the gap where the motor starts missing/running rough is about the same.

If about the same then both coils deliver the same spark.

If you have to substantially decrease the gap on the 2.5 ohm coil so the motor does not run rough/miss then the coil has less spark capability (reserve).

With the 2.5 ohm coil installed reattach the spark plug lead and run it. If it runs OK, especially at low idle, it will probably be OK. But just in case, carry a spare 1.5 ohm coil. These are available from most Model A suppliers. I ran a Farm and Fleet generic 6 v tractor coil for a while that was at 2.2 ohms. Ran good for a year or 2, but noticed after that my motor was not running as good, so switched back to a 1.5 ohm coil.

It may be other design considerations in the 6v 2.5 ohm coil that makes it an equivalent. Higher ohms usually means higher amount of turns on the primary coil, or a more resistive wire is used, which can be a correct design with other considerations like # of windings on the secondary, gauge of wire, core material, etc.

I believe the original coils were speced for 1.5 ohms, not sure what the secondary was/is nominally speced.

[Jim/GA]

Originally Posted by 1930artdeco

Ok, I will flip the new coil over. But I still don't understand the ohm's. People say it should be 1.5 but my new Napa coil is showing higher than that. Or, should I not worry as she runs fine?


Mike

Mike-
Your ohmmeter is lieing to you. It is not easy to differentiate down at the 1.5 ohm level. You have a new 1.5 ohm coil from NAPA.

.

[rotorwrench]

The original model A coil was internally insulated with bitumin or tar for a better word. They didn't use oil like a big AC transformer does. I always wonder what some modern coils use since it is not one of the specifications that the manufacturer normally lists in their specs. Many use an epoxy resin but not all.

Most digital multi-meters don't have a way to zero them out like a old mechanical reading unit (D'arsenval meter movement). I have mine tested by a metrologist every 12-months with a decade resistor. This way I know it is accurate. A can type coil that is quality made to be used with a 6-volt system should be 1.5 ohms on the primary. The primary is what builds the magnetic field and it is the one that draws amperage to function. The secondary is a high tension step up coil and has a connection to the primary but it doesn't form a complete circuit.

If a 1.5 ohm can type coil is used, no ballast is necessary since it will run relatively cool in service. A coil with 2.5 Ohms may run hot or may have a weak spark on 6-volts and may need a ballast resistor mounted externally to keep it within normal amp draw specs. In any case, I don't know how well it will work on 6-volts. The 3-Ohm coils are made for 12-volt systems where no ballast resistor is required. The 2.5-ohm coils are pretty close to that and may run OK with a 12-volt system but it might run on the warm side or just not have as much inductive reactance as a 3-ohm would.

[1930artdeco]

Thank you for all of the reply's and info. I think for now I will run the coil 'upside down' and see how well she runs. Actually, she started better with the new coil


Mike

[Patrick L.]

It might be curious just to see what voltage is getting to the points. Full battery voltage or something a bit less. But, no matter, it should run fine.