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EFV-8 Club Forum / General Ford Discussion / Coil Resistor

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Discussion Topic: Coil Resistor

bo8109b

11-16-2019 @ 8:10 AM

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Had an old original coil resistor lying around. Out of curiosity, applied battery 6.4 volts to one side and a volt meter read 6.4 coming out the other side. Thought these reduced voltage to the coil. How does that work?

TomO

11-16-2019 @ 9:06 AM

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In order to reduce the voltage, you must have current flowing. Reading the voltage with a meter does not create current flow.

Think of current as water in a pipe and the voltmeter as a pressure gauge. As long as the water is not flowing the pressure gauge will read the same pressure any where on the pipe. Once the water starts flowing, the pressure at the source will stay the same, but the pressure at the outlet will be reduced by any restriction in the flow, such as a crimped pipe or a half open valve.

Tom

supereal

11-22-2019 @ 9:43 AM

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In addition to my friend TomO's explanation, it should be noted that the coil circuit resistor is not a fixed resistance unit. When the vehicle is off, and the current flow ceases, the resistor cools, and the resistance drops. As the engine is run, current flow increases proportional to the rise in the system voltage, and the nichrome wire of the
heats up, increasing resistance. This, in turn, lowers the voltage at the output of the resistor to protect the coil and ignition points. It is a somewhat crude voltage
regulator necessary to prolong the ignition . Some believe that bypassing the resistor will produce more engine power, but actually just reduces the life of the system.

bo8109b

11-22-2019 @ 2:54 PM

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Would like to add that after installing Skip's rebuilt coil, the voltage at the coil with points closed is 1.75 volts. Sounds a bit low, but the car starts and runs fine.

JayChicago

11-24-2019 @ 9:44 AM

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Yes, that is low. Should be 2.5-3.0 volts at coil with engine stopped, then jump up to around 3.5 or more when engine running. I would look for additional resistance somewhere. I found a full 1.0 volt drop thru my old ignition switch. And as TomO said, circuit must be live, current flowing, for resistance to show itself with a voltage drop.

supereal

12-05-2019 @ 12:47 PM

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If you are measuring voltage at the coil with the engine running, unless you are using a scope or RMS (root mean square) meter, the reading will not be accurate because the input to the coil is momentarily grounded as the points open and close.

JayChicago

12-06-2019 @ 8:40 AM

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I don't doubt that using a simple voltmeter may not be scientifically accurate, but I believe it is close enough for our purposes because in real-life it works. Apparently these simple voltmeters are able to read-out an average as the voltage fluctuates with points opening and closing.

BTW, the input to the coil is never grounded, the output side is grounded thru the points while the points are closed, providing the path to ground for current flow thru the coil.

bo8109b

12-06-2019 @ 1:33 PM

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It turns out with my second post, the low voltage reading came from using a new coil resistor. Put the old original rusty resistor back in and the voltage at the coil was 2.9 volts. Lesson learned. Carl

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