6V coil issue

Smooth_One

Smooth_One

Senior Member
My car was idling in the driveway on Sunday for around 10 minutes to warm it up before going to a car show and it completely and abruptly shut off without sputtering or anything. I checked all the usual culprits and got to the coil which was showing no power and nobody had one in my town on Sunday so I had to wait till Monday to get one.
My question is somethings still does not seem right or maybe it is working as it is supposed to that when I use my test light with the voltage readout on the side everything has 6 volts when the key is on but when I test it at the coil output that goes to the top of the distributor cap it only has 2.3 V to 2.5 V Whether the key is on or off? Is that normal or is something wrong?
 
Isaiah, if you want to take voltage readings on the ignition system, you need to temporarily remove the rotor and slip something that does not conduct electricity between the points. I use a business card. Then you can read voltage at any point between the points and the other side of the coil and you should see 6 volts.

It is not normal to read voltage on the wire that plugs into the top of the distributor cap. But if the car runs OK, I would not worry about it.

Make sure you have the polarity of the new coil hooked up correctly, to match the ground that you are using on the battery. If you have the coil polarity backwards, the engine will run but not as well as it will when the coil is hooked up properly.
 
Jim/TX/GA said:
Isaiah, if you want to take voltage readings on the ignition system, you need to temporarily remove the rotor and slip something that does not conduct electricity between the points. I use a business card. Then you can read voltage at any point between the points and the other side of the coil and you should see 6 volts.

It is not normal to read voltage on the wire that plugs into the top of the distributor cap. But if the car runs OK, I would not worry about it.

Make sure you have the polarity of the new coil hooked up correctly, to match the ground that you are using on the battery. If you have the coil polarity backwards, the engine will run but not as well as it will when the coil is hooked up properly.
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Thanks Jim! I will check the polarity out again when I get home from work today but I am pretty sure it is correct.
And I will make sure I stick a business card in between the open points and check it all again.
 
Alternate way to test the coil. Using a multi-meter. This is the method I use.


Ignition off, remove fuse or disconnect battery.
Remove all 3 leads from the coil.
Measure across the coil small two terminals - 6v coil should be 1.5 ohms. 12 v - 3 ohms
Measure each terminal to the coil to distributor output socket - 10K to 14 K ohms.


With battery connected fuse installed, all leads connected to the coil.
Open the points as Jim suggested both sides of the coil should be 6v on the meter to ground.
Close points (or jumper across the points) and one side of the coil will be 6v, the other side is 0V.


Rotor installed, cap and body installed, nothing between the points, coil wire installed.. Remove coil wire at distributor, place free end 1/4 inch from a clean head nut. Place piece of cardboard under the free end of coil wire over to the stud to keep sneak spark path through paint from happening. With ignition on, crank starter, should see a nice strong blue spark. If real weak or yellow - check connections, try checking point gap, and clean points. Try again. If still not good spark possible bad/marginal connections or weak bad coil.
 
If a new repro coil, check if oil filled by shaking it. Some of these are recommended to be installed with the leads on top rather than downward for full internal windings coverage. The original tar/ paste or new epoxy coils can be mounted terminals- down like stock.
Also, 30closedcab test values may indicate an internal resistor if the packaging is unclear.
 
A coil used on a 12 volt vehicle that has a limiting resistor is actually a 6 volt coil. I have run Primary resistance checks to validate. Just FYI.
 
DHZIEMAN said:
A coil used on a 12 volt vehicle that has a limiting resistor is actually a 6 volt coil. I have run Primary resistance checks to validate. Just FYI.
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We run a 12 volt colt with an internal resistor which yields 3 ohms at each wired terminal.
 
Some use a 6v 1.5 ohm coil with a 1.5 ohm ballast resister to give total of 3 ohms when used on 12v systems. Configured this way the coil and resistor share the heat dissipation, so the 6v coil runs much cooler than a 12 v coil with a internal resistance of 3 ohms.


Theory is they are the same package and have the same ability to dissipate heat. Less chance of the 6v coil /ballast failing in comparison to the 12v coil.
 
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Umm. Both coils consume the same power (6V x 4A vs. 12V x 2A) which means they both dissipate the same amount of heat. The added resistor is an extra heater, consuming the same power as the 6v coil.
 
6 V, 6v coil, 1.5 ohms - current 6/1.5 = 4 Amps, Coil Wattage 4 x 6 =24 watts


12 V, 12 v coil, 3 ohms - current 12/3 = 4 amps, Coil Wattage 4x 12 = 48 watts


12 v, 6 v coil with 1.5 ohm ballast, - 6 volts drops across the coil of 1.5 ohms and 6v drops across the ballast 1.5 ohms, current is 4 amps 12/3. Wattage coil is 6 X 4 = 24 watts, Wattage ballast is 6 x4 = 24 watts.
 
Smooth_One said:
Thanks Jim! I will check the polarity out again when I get home from work today but I am pretty sure it is correct.
And I will make sure I stick a business card in between the open points and check it all again.
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Isaiah do you attend the Model 'A' garage meet at Charlie's barn in Sarasota? If so I'll be there next week and will be bringing in a coil / condenser tester I recently built. It's kind of like a Model 'A' ignition simulator.
 
jquinlan said:
Isaiah do you attend the Model 'A' garage meet at Charlie's barn in Sarasota? If so I'll be there next week and will be bringing in a coil / condenser tester I recently built. It's kind of like a Model 'A' ignition simulator.
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No sorry. I wish I could!
I still do that 5am to 5pm work stuff.
Al Leach invited me last weekend as well.
One of these days I promise I’ll make it.
 
30 Closed Cab PU said:
6 V, 6v coil, 1.5 ohms - current 6/1.5 = 4 Amps, Coil Wattage 4 x 6 =24 watts


12 V, 12 v coil, 3 ohms - current 12/3 = 4 amps, Coil Wattage 4x 12 = 48 watts


12 v, 6 v coil with 1.5 ohm ballast, - 6 volts drops across the coil of 1.5 ohms and 6v drops across the ballast 1.5 ohms, current is 4 amps 12/3. Wattage coil is 6 X 4 = 24 watts, Wattage ballast is 6 x4 = 24 watts.
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This is all based w/a non-running coil. The amperage of a coil on a running engine is a lot less.
 
katy said:
This is all based w/a non-running coil. The amperage of a coil on a running engine is a lot less.
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I agree, you are right since the points are only closed part time. I used this as an example. Still a 12v coil vs. the 6v coil with ballast comparison , the 12 v coil will run warmer.
 
30 Closed Cab PU said:
6 V, 6v coil, 1.5 ohms - current 6/1.5 = 4 Amps, Coil Wattage 4 x 6 =24 watts


12 V, 12 v coil, 3 ohms - current 12/3 = 4 amps, Coil Wattage 4x 12 = 48 watts


12 v, 6 v coil with 1.5 ohm ballast, - 6 volts drops across the coil of 1.5 ohms and 6v drops across the ballast 1.5 ohms, current is 4 amps 12/3. Wattage coil is 6 X 4 = 24 watts, Wattage ballast is 6 x4 = 24 watts.
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I don't have one to measure, but your former post said 12v, 6 ohms, which seems more reasonable than 3 ohms.
 
I have used a three OHM Pertronix flame thrower coil on my 12 volt converted model A roadster for 12 years with excellent results . This is a performance coil that gives instant starts and performance that can be felt . Pertronix also offers a 1.5 version of this coil for 6 volt use . I haven't seen any three OHM coils offered by model A venders . It will no doubt be easier to get a good three OHM direct than just walking in a local parts house and asking for a three OHM coil . Usually the counter man won't know the difference and will ask what model and year car the coil is for . If you say it is for a model A Ford that is probably as far as you will get with that .
 
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