Voltage at coil

[Turbobus]

Does anyone know what the voltage at the coil should be with ignition in on position?
I was thinking it should be around 9 volts, I’m getting 4.8 volts. Back story, just restored, all new wiring, ballast resistor, coil battery all of it!
Starts ok but once it warms up and I put my foot into it misses and runs like crap until I lower the RPMs
Thanks

 

[PurpleBeeper]

5 volts I believe (if you're using a ballast resistor). Somebody correct me if I'm wrong. As for your problem.... I have had a factory electronic ignition box do that when it got hot

 

[65-440]

Yes, usually 5/6V after the resistor...full battery voltage when cranking

 

[MoparLeo]

At least 8 volts.
Read this article all the way through.
https://documentcloud.adobe.com/link/file?uri=urn:aaid:sc:US:636dd57c-823b-48be-8d47-d7eb866a4076#
Page 4, second paragraph to answer your question on voltage.

 

[Turbobus]

5 volts I believe (if you're using a ballast resistor). Somebody correct me if I'm wrong. As for your problem.... I have had a factory electronic ignition box do that when it got hot

Thanks

 

[Canadian1968]

I would not be happy with anything under 8 .

I run a 1 ohm ballast, gives me 10V @ the coil. Running like this for over 4 years now

But to address your problem, I would check you system voltage while the car is running ( just check it at the battery) You should have at least 13.5V. If voltage is good, you could simply have a dud ballast. Your ignition box does not effect the input voltage to your coil.

 

[383man]

Checking the volts at the coil with the eng running is not the same as with the key on eng off. With the key on eng not running it should be from about 5 to 8 volts as it will depend on the ballast and the coil. Checking coil voltage with the eng running is just an average because everytime the coil fires and the ECU breaks the primary circuit the voltmeter would read 12 volts (battery volts) since at that point the primary circuit is not grounding through the ECU which will cause the volts to read battery volts since all the current is grounding through the voltmeter. Then when the circuit flows again the volts will drop to around 5 to 8 volts when running a ballast. Its easy to test as if you take a car with points and put your voltmeter on the coil pos terminal and turn the key on eng off and points closed it will read the 5 to 8 volts. Now open the points with the key still on eng off and voltmeter still on coil pos terminal and the voltmeter will read battery volts. Its because the voltmeter is now reading all the voltage drop since its the ground. So its not a true reading with the eng running plus you have coil oscillations that can shoot some voltage back into the coil. The only true way to read the coil pos volts is with the circuit grounded and current flowing through the circuit which is with the key on eng off and points closed on point ign. Ron

 

[Nacho-RT74]

voltage changes with temp and current going through the ballast, depending on sparks/coil requirement per RPMs too

 

[HALIFAXHOPS]

Also depends on the ballast resistors value also there are a bunch of different ohms. Specs should be in section 8 of the FSM Not sure this a points ignition we are talking about here?

 

[Bee1971]

The single type ballast resistor is pretty much easy to find all types of different OHM Resistance

However

The dual ballast resistor is pretty much limited to finding the OEM Chrysler / Mopar
3656199 or early version of 3874767 with the open resistor element on the backside - Both had .55 OHM resistance on the positive side for coil and will bee marked .55 on the connector itself

Later production version of the Mopar 3874767 went to 1.2 OHM resistance on the positive side of coil

All aftermarket are in the 1.2 / 1.5 OHM for the dual or higher on the positive side of coil
And when they get hot really limits the voltage during engine run

 

[RJRENTON]

Checking the volts at the coil with the eng running is not the same as with the key on eng off. With the key on eng not running it should be from about 5 to 8 volts as it will depend on the ballast and the coil. Checking coil voltage with the eng running is just an average because everytime the coil fires and the ECU breaks the primary circuit the voltmeter would read 12 volts (battery volts) since at that point the primary circuit is not grounding through the ECU which will cause the volts to read battery volts since all the current is grounding through the voltmeter. Then when the circuit flows again the volts will drop to around 5 to 8 volts when running a ballast. Its easy to test as if you take a car with points and put your voltmeter on the coil pos terminal and turn the key on eng off and points closed it will read the 5 to 8 volts. Now open the points with the key still on eng off and voltmeter still on coil pos terminal and the voltmeter will read battery volts. Its because the voltmeter is now reading all the voltage drop since its the ground. So its not a true reading with the eng running plus you have coil oscillations that can shoot some voltage back into the coil. The only true way to read the coil pos volts is with the circuit grounded and current flowing through the circuit which is with the key on eng off and points closed on point ign. Ron

YES.....GOOD EXPLANATION....assuming that the switching is via the ECU. (The same phenomenon will occur when using points). The coil will produce a primary voltage spike, reflected back to the supply, every time the transistor on the ECU turns off to produce the spark. This is fundamental to the coil, regardless of the innitial primary voltage. Just my opinion of course.
BOB RENTON

 

[Canadian1968]

He never said how he is testing the voltage .

If he is placing a lead on the positive side of coil and then grounded to frame he will get one reading . But if he us putting a lead on the positive and negative of the coil terminals , obviously a different result.

I'm assuming that he is testing for his constant voltage coming from the ballast

 

[Turbobus]

5 volts I believe (if you're using a ballast resistor). Somebody correct me if I'm wrong. As for your problem.... I have had a factory electronic ignition box do that when it got hot

Thanks!

 

[Turbobus]

I would not be happy with anything under 8 .

I run a 1 ohm ballast, gives me 10V @ the coil. Running like this for over 4 years now

But to address your problem, I would check you system voltage while the car is running ( just check it at the battery) You should have at least 13.5V. If voltage is good, you could simply have a dud ballast. Your ignition box does not effect the input voltage to your coil.

Thanks!

 

[Turbobus]

Checking the volts at the coil with the eng running is not the same as with the key on eng off. With the key on eng not running it should be from about 5 to 8 volts as it will depend on the ballast and the coil. Checking coil voltage with the eng running is just an average because everytime the coil fires and the ECU breaks the primary circuit the voltmeter would read 12 volts (battery volts) since at that point the primary circuit is not grounding through the ECU which will cause the volts to read battery volts since all the current is grounding through the voltmeter. Then when the circuit flows again the volts will drop to around 5 to 8 volts when running a ballast. Its easy to test as if you take a car with points and put your voltmeter on the coil pos terminal and turn the key on eng off and points closed it will read the 5 to 8 volts. Now open the points with the key still on eng off and voltmeter still on coil pos terminal and the voltmeter will read battery volts. Its because the voltmeter is now reading all the voltage drop since its the ground. So its not a true reading with the eng running plus you have coil oscillations that can shoot some voltage back into the coil. The only true way to read the coil pos volts is with the circuit grounded and current flowing through the circuit which is with the key on eng off and points closed on point ign. Ron

Thanks

 

[Turbobus]

voltage changes with temp and current going through the ballast, depending on sparks/coil requirement per RPMs too

Thanks

 

[Turbobus]

He never said how he is testing the voltage .

If he is placing a lead on the positive side of coil and then grounded to frame he will get one reading . But if he us putting a lead on the positive and negative of the coil terminals , obviously a different result.

I'm assuming that he is testing for his constant voltage coming from the ballast

Thanks

 

[Turbobus]

Checking the volts at the coil with the eng running is not the same as with the key on eng off. With the key on eng not running it should be from about 5 to 8 volts as it will depend on the ballast and the coil. Checking coil voltage with the eng running is just an average because everytime the coil fires and the ECU breaks the primary circuit the voltmeter would read 12 volts (battery volts) since at that point the primary circuit is not grounding through the ECU which will cause the volts to read battery volts since all the current is grounding through the voltmeter. Then when the circuit flows again the volts will drop to around 5 to 8 volts when running a ballast. Its easy to test as if you take a car with points and put your voltmeter on the coil pos terminal and turn the key on eng off and points closed it will read the 5 to 8 volts. Now open the points with the key still on eng off and voltmeter still on coil pos terminal and the voltmeter will read battery volts. Its because the voltmeter is now reading all the voltage drop since its the ground. So its not a true reading with the eng running plus you have coil oscillations that can shoot some voltage back into the coil. The only true way to read the coil pos volts is with the circuit grounded and current flowing through the circuit which is with the key on eng off and points closed on point ign. Ron

Thanks