| 51woodie | -- 04-13-2019 @ 6:38 AM |
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Thanks to one of our members, I got a NOS ignition switch. Now I want to test for voltage drop in the ignition circuit. As I understand it, a circuit has to be loaded to get an accurate reading of voltage drop. The plan is to: disconnect the coil to distributor wire, connect my remote start switch to the distributor terminal on the coil, connect a voltmeter to the ignition terminal on the coil, push the button on the remote switch for the time it takes to get a volt reading. As I understand it, I should get about 3 volts? Depending on the reading, I'll work my way back through the connectors, and resistor, to the ignition switch, each time activating the remote switch to load the circuit. Does all this make sense, or is there an easier/better way? This is a late '46 manufactured car, with the resistor up under the dash, and a stock "Skip" coil.
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| TomO | -- 04-13-2019 @ 7:06 PM |
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You are correct in the statement that current has to be flowing in the circuit in order to measure the voltage drop in the circuit. A voltage drop test is used to determine the resistance in the circuit and is done by connecting the voltmeter across 2 points in the circuit, not between one point and ground. An example of this would be the black wire on the NEG battery post and the other on the input to the coil. This reading would be the total drop between the battery and the input to the coil when the points are closed. What you are describing is measuring the input voltage to the coil. This is done by connecting the voltmeter between the input of the coil and ground. read the voltage on the meter and then tap the starter to turn the engine over enough to open or close the points. The reading will be lower when the points are closed, The reading you are looking for is when the points are closed. The reading will be different from what you will get from cranking the engine, because when you crank the engine the points open and close and you are reading the average of the 2 conditions. Do not leave the ignition switch on for long periods of time while making these tests, you can damage the coil. Tom
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| len47merc | -- 04-13-2019 @ 7:18 PM |
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In addition to TomO's comments on the coil, leaving the ignition switch on for excessive periods of time may/will burn the points as well. Steve
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| JayChicago | -- 04-14-2019 @ 8:51 AM |
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I'll ad my thoughts to what has already been said: While your method will work, I would not do it that way. Taking the distributor out of the circuit while taking readings will not give you a "true" reading, eliminates any additional resistance that may be present through the distributor. I would leave the distributor wire connected and use the ignition switch as your on-off switch. I don't think any harm will be done to the points in the minute or so that it will take you to take a reading and get back to the switch to turn it off. Don't be surprised if you find less than 3 volts at the coil with engine stopped. I tested several resistors and found they all produced a voltage drop of 3-3.3 volts in themselves. (this type of resistor's voltage drop actually varies somewhat, increases as it gets hot) Add another few tenths of a volt drop through the ignition switch and the rest of the circuit, and the total voltage drop ahead of the coil will be about 3.5 volts. So I think 2.5 to 3 volts at the coil should be considered normal, good, with engine stopped. Then later with engine running, points opening and closing, the averaged voltage should jump up to around 3.5 volts. A big variable to consider when measuring down to tenths-of-a-volt is the status of your battery. A good 6 volt battery can be anywhere between 6.0 to 6.5 volts. I would start by taking a reading of the battery by itself, to give you the base-line you are starting with.
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