The Ford Barn

WillC · 04-27-2019, 03:23 PM

Question on this Model A Iv been working on, I switched it back to 6 volt pos ground. I removed the 12 volt coil and replaced it with a 6 volt coil. I installed the hot wire on the neg side of the coil and the points side to the pos side and I checked my voltage to the points by removing the distributor and just using that large cable that screws into the side of distributor to test the voltage. The voltage on that cable is 6.48. Thinking maybe I have to the wires crossed I switched them on the coil and got the same reading. Shouldn't the voltage going to the points be more like 3 to 4 volts if the resistor is working right? If the resistor is blown I shouldn't any voltage right?

Charlie Stephens · 04-27-2019, 03:30 PM

Quote:

Originally Posted by WillC

Question on this Model A Iv been working on, I switched it back to 6 volt pos ground. I removed the 12 volt coil and replaced it with a 6 volt coil. I installed the hot wire on the neg side of the coil and the points side to the pos side and I checked my voltage to the points by removing the distributor and just using that large cable that screws into the side of distributor to test the voltage. The voltage on that cable is 6.48. Thinking maybe I have to the wires crossed I switched them on the coil and got the same reading. Shouldn't the voltage going to the points be more like 3 to 4 volts if the resistor is working right? If the resistor is blown I shouldn't any voltage right?

A stock Model A does not have a resister. Did you add one?

Charlie Stephens

WillC · 04-27-2019, 03:58 PM

There was a ballast resistor tied into the system when it was 12 volt, Should I remove it? Should there be a full 6 volts going to the points? I was always under the assumption that on a 6 volt system the power going to the points should be between 3 and 4 volts so the points don't burn. Am I wrong on this?

redmodelt · 04-27-2019, 04:24 PM

There will be a resistor in the coil. It would put out line voltage when cold and is it warms up the resistant drops the voltage. It should show line voltage when cold no matter witch terminal on the coil is used. It is just better for the coil to be wired right. The external ballast resistor would be redundant and would only be used with a coil that did not have one built in. You would be getting line voltage to start the car.

WillC · 04-27-2019, 04:42 PM

I understand, Just to be sure, because the system is now positive ground the (+) side of the coil goes to the points and the (-) side of the coil connects the power from the battery? Thank you. I keep going I just might be a full fledged Model A mechanic in 20 or 30 years!!!!!

Tom Endy · 04-27-2019, 04:49 PM

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There will not be a voltage drop unless there is current flow. When the cable is hooked up current will flow when the points close and there will be a voltage drop across the primary winding of the coil. The voltage you measured at the end of the pop-out cable is correct because there is no current flow.

Tom Endy

30 Closed Cab PU · 04-27-2019, 06:53 PM

ballast resister is only used in 12v systems using a 6 volt coil. Primary coil internal resistance for 6v coil = 1.5 ohms, resistance on 12v coil is 3 ohms.

On 6v coils in 12 v system a series ballast resistor of 1.5 ohms is added so the total is 3 ohms which is the requirement for a 12 v system.

Some like using a 6v coil with ballast since in this approach the heat dissipation for the 6 v coil is a lot less than running a 12v coil without the ballast..

Mike V. Florida · 04-27-2019, 09:41 PM

Quote:

Originally Posted by WillC

I understand, Just to be sure, because the system is now positive ground the (+) side of the coil goes to the points and the (-) side of the coil connects the power from the battery? Thank you. I keep going I just might be a full fledged Model A mechanic in 20 or 30 years!!!!!

Yes, follow the polarity of the coil through the wiring back to the battery. You have a positive ground car so the positive of the battery connects to the chassis. The positive of the coil goes through the cable to the points and when the points close it connects to the car ground and the positive of your battery. The neg of the coil goes through the wiring the other way and finally makes it to the Neg of the battery.

By the way ditch the resistor.

30 Closed Cab PU · 04-28-2019, 07:20 AM

You are on the right track being concerned about the polarity. I bought a spark tester offered by the Vendors that is placed in the top of the distributor cap that is placed in series with Coil Wire. When coil is wired correctly I got a reading of about 10KV. Swapped the primary wires on the coil and got 6 KV. My truck still seemed to run OK. Wired backwards, less energy available for spark at the plugs, plug spark probably not as strong. Possibly cause problems long term with fouling, plug life, gas mileage, have to run hotter plugs, other? I do not know.

04-27-2019, 03:23 PM	#1
WillC Member Join Date: Mar 2011 Posts: 50	power at the points question Question on this Model A Iv been working on, I switched it back to 6 volt pos ground. I removed the 12 volt coil and replaced it with a 6 volt coil. I installed the hot wire on the neg side of the coil and the points side to the pos side and I checked my voltage to the points by removing the distributor and just using that large cable that screws into the side of distributor to test the voltage. The voltage on that cable is 6.48. Thinking maybe I have to the wires crossed I switched them on the coil and got the same reading. Shouldn't the voltage going to the points be more like 3 to 4 volts if the resistor is working right? If the resistor is blown I shouldn't any voltage right?

04-27-2019, 03:58 PM	#3
WillC Member Join Date: Mar 2011 Posts: 50	Re: power at the points question There was a ballast resistor tied into the system when it was 12 volt, Should I remove it? Should there be a full 6 volts going to the points? I was always under the assumption that on a 6 volt system the power going to the points should be between 3 and 4 volts so the points don't burn. Am I wrong on this?

04-27-2019, 04:24 PM	#4
redmodelt Senior Member Join Date: May 2010 Location: Portland OR Posts: 6,340	Re: power at the points question There will be a resistor in the coil. It would put out line voltage when cold and is it warms up the resistant drops the voltage. It should show line voltage when cold no matter witch terminal on the coil is used. It is just better for the coil to be wired right. The external ballast resistor would be redundant and would only be used with a coil that did not have one built in. You would be getting line voltage to start the car. __________________ I know the voices aren't real but damn they have some good ideas!

04-27-2019, 04:42 PM	#5
WillC Member Join Date: Mar 2011 Posts: 50	Re: power at the points question I understand, Just to be sure, because the system is now positive ground the (+) side of the coil goes to the points and the (-) side of the coil connects the power from the battery? Thank you. I keep going I just might be a full fledged Model A mechanic in 20 or 30 years!!!!!

04-27-2019, 04:49 PM	#6
Tom Endy Senior Member Join Date: May 2010 Location: Southern California Posts: 3,131	Re: power at the points question Sponsored Links (Register now to hide all advertisements) There will not be a voltage drop unless there is current flow. When the cable is hooked up current will flow when the points close and there will be a voltage drop across the primary winding of the coil. The voltage you measured at the end of the pop-out cable is correct because there is no current flow. Tom Endy

04-27-2019, 06:53 PM	#7
30 Closed Cab PU Senior Member Join Date: Aug 2018 Posts: 2,332	Re: power at the points question ballast resister is only used in 12v systems using a 6 volt coil. Primary coil internal resistance for 6v coil = 1.5 ohms, resistance on 12v coil is 3 ohms. On 6v coils in 12 v system a series ballast resistor of 1.5 ohms is added so the total is 3 ohms which is the requirement for a 12 v system. Some like using a 6v coil with ballast since in this approach the heat dissipation for the 6 v coil is a lot less than running a 12v coil without the ballast..

04-28-2019, 07:20 AM	#9
30 Closed Cab PU Senior Member Join Date: Aug 2018 Posts: 2,332	Re: power at the points question You are on the right track being concerned about the polarity. I bought a spark tester offered by the Vendors that is placed in the top of the distributor cap that is placed in series with Coil Wire. When coil is wired correctly I got a reading of about 10KV. Swapped the primary wires on the coil and got 6 KV. My truck still seemed to run OK. Wired backwards, less energy available for spark at the plugs, plug spark probably not as strong. Possibly cause problems long term with fouling, plug life, gas mileage, have to run hotter plugs, other? I do not know.