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EFV-8 Club Forum / General Ford Discussion / 3 volts to the coil

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Posted By Discussion Topic: 3 volts to the coil -- page: 1 2     JayChicago 09-02-2016 @ 7:32 AM reply profile send p.m. Member Posts: 480 Joined: Jan 2016 40 Ford, all stock I have read on this and other forums that you need 3+ volts at the coil primary terminal to get proper performance out of the ignition system. Is that to be measured with engine running or stopped? ken ct. 09-02-2016 @ 7:57 AM reply profile send p.m. Senior Posts: 1513 Joined: Jan 2010 It can be measured with the motor stopped but will depend wether the points are open or closed. Will either be 3+ or 6+ depending. ken ct. JayChicago 09-02-2016 @ 8:03 PM reply profile send p.m. Member Posts: 480 Joined: Jan 2016 I get distinctly different voltage readings at the coil depending if engine is running or stopped. But I have never heard that mentioned in discussions, just that I need 3+volts at the coil. I am reading 3.6 volts engine running, but only 2.7 volts engine stopped. So my question is: is this normal? GK1918 09-03-2016 @ 12:31 AM reply profile send p.m. New Member Posts: 127 Joined: Aug 2016 For years this has baffled-ed me why Ford used a resistor to drop coil voltage? I can see that for 12vt. My reason is I grew up with a ration of Plymouths Dodges of the 1930's 40's 50's and Jeeps, trucks, buses and none use a resistor. I have worked on many intercity buses of the 1940's that were 12 volts again none used a resistor. And then I have several F6's and several F8's 6volt pos grd. again no resistor. Only thing I can think of is that maybe something to do with duel points used up till 1948? I don't use a resistor so I get 6vts at the coil and I don't see any problem. sam TomO 09-03-2016 @ 7:48 AM reply profile send p.m. Senior Posts: 7253 Joined: Oct 2009 Jay, Your readings are in the normal range, but slightly on the low side. Check the voltage going to the ignition resister, it should be at least 6.2 volts with the engine stopped. If it is lower, check the battery voltage. If the battery voltage is more than 0.1 higher than the input to the resister, you have excessive resistance in the circuit. To find the point of resistance, turn on the ignition switch and tap the starter until the points are closed , connect the COM lead of your meter to the NEG post of the battery and probe the connection points with the + lead of your meter. Turn off the ignition switch and let the coil coll between probe points. Tom ford38v8 09-03-2016 @ 9:07 AM reply profile send p.m. Senior Posts: 2759 Joined: Oct 2009 Jay, as Tom says, your readings are low. The difference between the two readings reflect the state of charge on the battery alone, against the output of the generator while running. Alan JayChicago 09-03-2016 @ 1:37 PM reply profile send p.m. Member Posts: 480 Joined: Jan 2016 Tom and Allan Thanks. I have done the voltage drop tests, following the procedure posted by Tom in the past. I am getting .2 volt drop to the resistor, .1 to the switch, and another .1 thru the switch to the resistor. But I would think those tenths of a volt are almost negligible compared to the 3.2 volt drop I am getting across the resistor. (with engine stopped, battery voltage reading 6.1 volt) pauls39coupe 09-04-2016 @ 9:46 AM reply profile send p.m. Member Posts: 205 Joined: Jul 2014 Disconnect the wire at the coil and run your volt meter from the wire to a ground. You should see near battery voltage. If not your resister may be bad. Replace it with one correct for your year and model Ford. Voltage readings with the wire connected should be between 3 and 6 volts, depending on if the points are open or closed. Getting an accurate reading with the engine running is nearly impossible, especially with a digital volt meter. My cheap analog meter will hover around 3 volts with the engine running. The resister is a ballast resister which increases resistance as it heats up. You may see the darn things actually glow. The purpose is to level the voltage to the coil as the generator produces higher voltage, thus saving the coil. If the engine is off, and the coil disconnected you will see very little voltage drop across the resister. Many modern round can coils have an internal resister, thus they don't need the resister Ford used under the dash. I suspect this may have been the case with the early cars another poster mentioned. Clean up all your connections take the ignition switch apart, to lower voltage drop between connections. JayChicago 09-04-2016 @ 1:33 PM reply profile send p.m. Member Posts: 480 Joined: Jan 2016 Thanks for your input Paul "Disconnect the wire at the coil and run your volt meter from the wire to a ground. You should see near battery voltage. If not your resister may be bad." I believe this test will not test resistance in the circuit, you will always see full battery voltage this way. Resistance will only show itself with current running thru the circuit. "The resister is a ballast resister which increases resistance as it heats up. You may see the darn things actually glow." Yes, this is what I have discovered the hard way, did not see mentioned before. I have checked 6 different resistors the past few weeks: my original, an Eichlin replacement, and four used originals from VanPelt. On the bench they all measure .4 ohms, which theoretically should produce a voltage drop of 2 volts. (V=A x R) But on the car, with +/-5 amps running thru them and getting hot, they all produced a 3.0 to 3.4 voltage drop. This leads me to believe the resistance value of the ballast resistor is increasing about 50% when hot. I'm not an electronics engineer...would love to hear from someone if I am all wet, if I am missing something here. pauls39coupe 09-05-2016 @ 1:59 PM reply profile send p.m. Member Posts: 205 Joined: Jul 2014 Jay, Back to your original questions, in my experience this is normal. As Ken said 3 to 6+ volt depending on the points being open or closed with the engine stopped. Checking the voltage at the coil with the wire disconnected will tell you if the resister has failed (broken) or if there may be a problem down stream. It would be interesting to hear a electrical engineer weigh in on the subject. Are you having drivability or starting issues? Or just curious about the voltage values?

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