Analyzing the data.

[azazzera]

Just took some ohm and voltage reading today trying to analyze coil. Now I'm sure I'm missing some large factors in order to get a full picture of ignition system but I hope some conclusions can be made. Here they are:

Coil ohms with wires off, cold. 3.4. Did not get coil hot, hot and tired at end of sail.

Coil (MMI gotten 3, 4 years ago )volts, engine running but not at full temp, within 5 or 10 min after start. 14.2 volts @ 1700 rpm. Engine fully warmed up temp at gauge and infrared at sender on engine approximately 170 F, 13.97 volts. I think it's strange that it's less than engine cold but not drastically out of parameters. Have some ideas why it would be lower. Would heat from engine cause voltage regulator to give slitly different voltage?. Discrepancy Could of course be coming from meter I suppose.

Alternator volts taken right from back of alternator: cold, 14.15 @ 1100rpm. Fully warm 14.08 @ 700 rpm. 14.17 @ 2000 rpm.

Don't necessarily have a question, engine has been running fine. Starting well. Has been giving me no problems. But I am running right at 4 amps at coil and a bit over. Coil temp has been 190-195 F. Taken at top of coil after two hour run time. No ballast befor coil. Can't find coil advertised at 4 ohms other than duel secondary for motorcycle. Every thing I have read here suggest that if coil was purchased through MMI and it was that one dosnt not need to worry about coil problems from excessive dwell time. No need for ballast resister. Yes I'm running electronic ignition.

Just curious about my set up. Trying to be as proactive as possible with PM. Grateful as always for any feedback.

[ndutton]

Every thing I have read here suggest that if coil was purchased through MMI and it was that one does not need to worry about coil problems from excessive dwell time. No need for ballast resister. Yes I'm running electronic ignition.

Actually we have advised about checking any coil you buy no matter where it came from, Moyer included. Please read this post from three years ago. As for your variance in voltage measurements, if you are measuring in different locations, system voltage drop is likely a factor. If measuring at the same point, probe contact can be a factor (I like to 'drill' the probe into the metal) but know that poor measurement technique can result in lower values, never higher. Therefore it's best to do your current calculation at the highest measured voltage. It looks to me like you're running at ~4.2 amps which indicates you would benefit from a resistor of around 0.75Ω according to the research we did seven years ago.

Just curious about my set up. Trying to be as proactive as possible with PM

That's about the smartest thing I've read on this forum in a while. Good on ya.

[tenders]

Those voltage fluctuations don't seem unusual to me.
* cold alternator output intentionally regulated higher on batteries before they are fully charged, to speed charging
* warmer alternator output might be lower as equipment temperature increases
* small changes in RPM at low RPMs can have significant effect on max output voltage
* noise from variations in testing conditions, locations tested, etc.

[azazzera]

Proper probe use.

I'll do some searching on the web but I thought it might be helpful to have a discution about how to use the meter specifically how to place the probes. I have always held the thought that if you have the most surface contact it would give you the best reading . I try to use the side of the probe on the side of the stud or contact I'm taking the reading off of. I've just learned that perhaps that's not so correct. using the very tip of the probe is best? and to use moderate amount of pressure. I assume on a flat surface. I've been known to use the tip between the threads of the stud. And whatever the way to be consistent with how you take the reading, correct?

I do have a question still about heat and how it may effect ohms, volts, amps..I think I'm correct in my understanding that as heat increases resistance also increases.. And amps falls with heat? How if at all is voltage effected by heat? If we assume consistent and proper technique that I freely admit was lacking with my readings, can the alternator output vary with heat? Then voltage drop is discussed with long wire runs. Can heat, expanding and contracting wire, effect voltages. Especially if you add poor terminal contacts and or corrosion.

Lots of thoughts I have in my head this morning.

[Don Moyer]

Azazzera. Since you asked, here's a link to an online calculator that will help you determine the way resistance varies with temperature: http://www.endmemo.com/physics/resistt.php You basically enter .0039 as the resistivity coefficient of copper. Then use the cold resistance measurement of your coil as the "original resistivity", and the temperature of your coil when hot as the "Final temperature". The calculator will then give you the theoretical resistance of your coil at that (hot) temperature. The calculator checks out quite close when I play around with coils in Brenda's kitchen oven. Don

[ndutton]

In both variances (resistance and voltage) there are physical properties that are expected and predictable and other factors that are not, corrosion and poor connections being the most common.

Don's post addresses the physical property of a copper conductor and the effect of temperature on resistance. Rounded off, the resistance of copper will increase 0.4% for every °C in temperature rise. Assuming a coil at rest measures at ambient temperature of 21°C (70°F) and 77°C (170°F) in use, it can be expected the pure copper wire component of the primary winding will experience a 22% increase in resistance. Note that there are other components creating resistance inside our coil so this figure represents only the copper wire contribution.

As an off the wall example (not suggesting these are real world figures), if a 4Ω coil's primary resistance was comprised of 1Ω coil windings and 3Ω internal resistor, the aforementioned 22% increase would be applied to only the 1Ω from the wire. The end result of primary resistance increase for the entire 4Ω coil at 170°F would be +0.22Ω for a total measurement of 4.22Ω. Beyond a certain temperature the insulation on the wire in the coil windings breaks down and internal shorts develop, a textbook damaged coil. No way will it function properly.

Voltage measurements
In the simplest terms, voltage regulation in alternators attempts to maintain a constant voltage by varying the output amperage to meet a connected battery's needs, fancy multistage regulation notwithstanding. To do this the alternator must be turning fast enough to meet those needs so measurements taken below that threshold will be less.

Voltage drop also has physical property and outside influence aspects. The predictable physical property component is resistance per length of copper wire. We've discussed it at length (unintended, sorry) on this forum and it is managed by wire gauge for given current and length. Outside influences are anything that affects the circuit resistance such as corrosion within the wire stranding and poor connections.

Voltmeter probe technique
We aren't working in a clean lab. The effect of moisture in the marine environment produces corrosion on all metals. We will not get an accurate voltage reading by laying a tester probe on a layer of corrosion or paint. That's why I advocate using the point of the probe 'drilled' into the metal being tested, a push and a twist is all it takes to dig into clean metal.