The Ford Barn

AnthonyG · 10-25-2015, 06:40 PM

I was reading a couple of threads recently the resistor in the ignition circuit reduces the voltage to 3 volts (Ford rational was to save the points ). It tweaked my memory that I hooked up the stock resistor when installing new harnesses then months later installed engine with Mallory e-dist and only now am realizing I have the ignition starter circuit running through two resistors. The stock one in the original harness configuration and then down the line the ballast resistor supplied with engine in front of the coil. Will both resistors in series cause a weak spark? Probably should disconnect one or the other?

JSeery · 10-25-2015, 06:58 PM

Yep, most likely to much resistance (voltage drop). Think you want more in the range of 3.5 to 4 volts at the coil. Surprised it runs at all if the voltage is 3v or less!

AnthonyG · 10-25-2015, 08:03 PM

Thx for confirming. Will bypass under dash resistor and just go with the ballast resistor mounted near the coil.

JSeery · 10-26-2015, 12:01 PM

What you are looking for is ~ 3.5A through the points. This is calculated by using Current (A) = Voltage (V) divided by the Resistance (R) in the circuit, A = V/R. Measure the coil resistance (I will use 1.0 ohm for an example) and add it to the ballast resister value (say 1.4 ohm), so total resistance would be 2.4 ohms. Battery voltage should be ~6.4V, it will be less when cranking the engine and higher when being charged by the generator, ~7V.

For this example, A = 6.4/2.4 or 2.67A, that is too low which indicates too much resistance in the circuit. At 7V the current would be 2.92A, so still on the low side. If you use the same coil the resistance would remain at 1.0 ohm (again, this is just an example you would need to measure your coil), then you need to change the ballast resister.

If you used a 1.0 ohm ballast resistor the current would be A = 6.4/2.0 or 3.2A and running with the generator charging A = 7.0/2.0 or 3.5A.

These are just example numbers, the higher the system current the hotter the spark but the quicker the points burn and the capacitors go. Like everything it is a compromise, 3.5 to 4.0A is a good range to shoot for.

rotorwrench · 10-26-2015, 12:19 PM

The ballast does control the voltage due to the drop across the resistor but the intent was to control the current flow to the coil. A stable current flow makes things work better and last longer.

AnthonyG · 10-26-2015, 12:51 PM

Sponsored Links (Register now to hide all advertisements)

Thx for all the great info.

10-25-2015, 06:40 PM	#1
AnthonyG Senior Member Join Date: May 2013 Location: Pa. Posts: 2,217	Starter circuit resistor. I was reading a couple of threads recently the resistor in the ignition circuit reduces the voltage to 3 volts (Ford rational was to save the points ). It tweaked my memory that I hooked up the stock resistor when installing new harnesses then months later installed engine with Mallory e-dist and only now am realizing I have the ignition starter circuit running through two resistors. The stock one in the original harness configuration and then down the line the ballast resistor supplied with engine in front of the coil. Will both resistors in series cause a weak spark? Probably should disconnect one or the other? __________________ Nomad

10-25-2015, 06:58 PM	#2
JSeery Member Emeritus Join Date: Nov 2012 Location: Wichita KS Posts: 16,132	Re: Starter circuit resistor. Yep, most likely to much resistance (voltage drop). Think you want more in the range of 3.5 to 4 volts at the coil. Surprised it runs at all if the voltage is 3v or less!

10-25-2015, 08:03 PM	#3
AnthonyG Senior Member Join Date: May 2013 Location: Pa. Posts: 2,217	Re: Starter circuit resistor. Thx for confirming. Will bypass under dash resistor and just go with the ballast resistor mounted near the coil. __________________ Nomad

10-26-2015, 12:01 PM	#4
JSeery Member Emeritus Join Date: Nov 2012 Location: Wichita KS Posts: 16,132	Re: Starter circuit resistor. What you are looking for is ~ 3.5A through the points. This is calculated by using Current (A) = Voltage (V) divided by the Resistance (R) in the circuit, A = V/R. Measure the coil resistance (I will use 1.0 ohm for an example) and add it to the ballast resister value (say 1.4 ohm), so total resistance would be 2.4 ohms. Battery voltage should be ~6.4V, it will be less when cranking the engine and higher when being charged by the generator, ~7V. For this example, A = 6.4/2.4 or 2.67A, that is too low which indicates too much resistance in the circuit. At 7V the current would be 2.92A, so still on the low side. If you use the same coil the resistance would remain at 1.0 ohm (again, this is just an example you would need to measure your coil), then you need to change the ballast resister. If you used a 1.0 ohm ballast resistor the current would be A = 6.4/2.0 or 3.2A and running with the generator charging A = 7.0/2.0 or 3.5A. These are just example numbers, the higher the system current the hotter the spark but the quicker the points burn and the capacitors go. Like everything it is a compromise, 3.5 to 4.0A is a good range to shoot for. Last edited by JSeery; 10-26-2015 at 06:27 PM.

10-26-2015, 12:19 PM	#5
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 17,410	Re: Starter circuit resistor. The ballast does control the voltage due to the drop across the resistor but the intent was to control the current flow to the coil. A stable current flow makes things work better and last longer.

10-26-2015, 12:51 PM	#6
AnthonyG Senior Member Join Date: May 2013 Location: Pa. Posts: 2,217	Re: Starter circuit resistor. Sponsored Links (Register now to hide all advertisements) Thx for all the great info. __________________ Nomad