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Old 04-09-2018, 08:32 AM   #1
W1964
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Default Voltage Readings

I am trying to determine the cause of low voltage at the coil. (1939 Deluxe, 6 volt, 85 hp) My coil voltage is 3.1 and it should be 4.2 What should the voltage be at the following points:
Ignition switch- battery side?
Ignition switch- coil to Resistor?
Resistor -wire from Ign switch?
Resistor - wire to coil?
Thanks for your help.
Bill
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Old 04-09-2018, 09:15 AM   #2
JSeery
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Default Re: Voltage Readings

There should be very little voltage loss between the battery and the ignition switch and again between the ignition switch and the ballast resistor. So, I would measure the voltage at the battery and/or generator output. The voltage at either side of the ignition switch should be slightly less that the battery/generator output. Ignition switches can be a source of voltage drop if the contact points need cleaning.
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Old 04-09-2018, 09:28 AM   #3
JSeery
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Default Re: Voltage Readings

From a recent Bubba post:

Ignition “ System” divided into 6 volts of electrical balance…….

In a perfect world using ohms law. The ignition coil winding would offer .6 ohms, the resistor would offer .6 ohms .
This would equal 1.2 ohms of resistance allowing a current flow of 5 amps.

The wattage ( volts x amps) of the circuit would be divided equally dividing the heat build up caused by the resistance.

This is fine until we start the engine , then the voltage becomes higher ( like 7-8 volts), lets use 7 volts.
7 volts divided by 1.2 ohms would be 5.8 amps .
This increase in amps must be taken up somewhere! The increase flow would increase the heat in each resistance and the small gauge wire to the distributor would take up the rest. This lead is a special resistance lead designed to take up the surges allowing good point life!!!

Testing with volt drop the drop across the coil would be .6 ohms x 5.8 amps= volt drop of 3.4 volts , the same would work for the resistor with a voltage drop of 3.4 volts…….the increase of resistor heat caused by flow would drop another 1 volts.
Using 7 as a spec , the distributor lead and the contacts could see approx. .2-.4 of a volt in this final voltage drop….

Moral of the story is use the factory designed set up and live happily ever after. A regular coil will supply 20-30,000 volts. A flathead with low compression and a richer than normal air fuel mixture would never need this much voltage from the coil.
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Old 04-09-2018, 09:33 AM   #4
W1964
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Default Re: Voltage Readings

Thank you for your reply. So the voltage drop to the desired 4.2 should occur at the Resistor, right?
Thanks again.
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Old 04-09-2018, 09:35 AM   #5
JSeery
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Default Re: Voltage Readings

Yes, but 4.2 may be a little high. Did you see the Bubba post? And remember, voltage out is going to depend on voltage in, so it is going to be different running vs non-running and at different rpms running.

Another older post:

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 = 6.4V/2.4 ohms 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 = 6.4V/2.0 ohms 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; 04-09-2018 at 09:49 AM.
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Old 04-09-2018, 09:58 AM   #6
G.M.
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Default Re: Voltage Readings

Quote:
Originally Posted by W1964 View Post
I am trying to determine the cause of low voltage at the coil. (1939 Deluxe, 6 volt, 85 hp) My coil voltage is 3.1 and it should be 4.2 What should the voltage be at the following points:
Ignition switch- battery side?
Ignition switch- coil to Resistor?
Resistor -wire from Ign switch?
Resistor - wire to coil?
Thanks for your help.
Bill
Battery 6.3
I.S. B.S. 6.3
I.S C toR 6.2 Less indicates dirty contacts in I.S
R.W. from I.S.Same as above
R.W. to coil Switch on points open 6.2 Points closed min. 3.7
Below 3.7 when starter is energized voltage starter current pulls
coil voltage down to where spark isn't enough to fire plugs. Weak
spark is blown out from compression. Some times will fire just as
starter button is released. Will also start when car is pushed and
jump started in gear. Bad or small battery cables or dirty connections
cause big voltage drops.
G.M.
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